PhD theses

Roberto Fumagalli
Cuprate high-Tc superconductors studied by polarization-resolved RIXS (2020)

This thesis work presents some of the results obtained on one- and two-dimensional cuprates with resonant inelastic X-ray scattering (RIXS) during my three years activity in the group held by Prof. G. Ghiringhelli of the Physics Department of the Politecnico di Milano (Italy). In the last two decades the group has largely contributed in the development of this technique, especially in terms of realization of dedicated RIXS instrumentations. In particular, they designed three spectrometer: AXES (Advanced X-ray Emission Spectrometer) for the ID08 beamline at the ESRF – the European Synchrotron in Grenoble (France), which has been operational since 1995, and SAXES (Super-Advanced X-ray Emission Spectrometer), an evolution of AXES, installed in 2006 at the ADDRESS beamline at the Swiss Light Source (SLS) in Villigen (Switzerland). With the continuous improvements in their performances, accompanied by the need to improve the overall energy resolution, the group joined the ESRF staff for the realization of the first ultra-high-resolution ERIXS (European-RIXS) spectrometer, which holds nowadays the world record of energy resolution with a combined (beamline + spectrometer) resolving power of 30,000 at the Cu L3edge. Moreover, this activity has been followed by several achievements also from the scientific point of view that have been made possible thanks to the unprecedented performances offered by the dedicated RIXS end-stations available nowadays. Here I will present some of the results achieved with the ERIXS spectrometer in the last three years on insulating and superconducting cuprates, especially focusing the attention on the possibility to perform polarization-resolved RIXS at the Cu L3-edge. RIXS is a synchrotron-based spectroscopy technique used to study the properties of correlated electrons systems such as cuprates, through the analysis of the elementary excitations probed by X-rays. In particular, the excitations accessible by RIXS carry information about the degrees of freedom which characterize the superconducting cuprates. In fact, with RIXS we can simultaneously study the energy, momentum and polarization of the electronic, magnetic and charge excitations of the CuO2 planes. The latter are at the basis of the superconducting behaviour and they represent the core of all the copper-based compounds. In fact, cuprates are layered materials characterized by a stacking of superconducting CuO2 layers, consisting of Cu2+ ions alternated to O2- ions, intercalated by so-called blocking layers that act as charge reservoirs. Due to the strong electron-electron interaction typical of the transition metal oxides, the parent compounds of the high-Tc superconducting cuprates are characterized by a strong antiferromagnetic (AFM) correlation which persists upon doping. As a consequence, undoped cuprates are usually described as Mott insulators where the valence d electrons are localized on the Cu atomic sites. Here, the Cu2+ ions are in a 3d9 electronic configuration with the unpaired spin 1/2 per Cu site coupled via superexchange (which is exceptionally high in cuprates), that is mediated by the surrounding oxygen ions. The net result of this interaction is reflected in the two-dimensional (2D) nature of the electronic and magnetic physical properties. A system can become a superconductor with an increasing of the critical temperature by adding holes or electrons in the CuO2 planes of insulating parent compound: in this way Tc has a maximum in correspondence of the so-called optimal doping. Superconductivity in cuprates is usually achieved by altering the number of mobile carriers (holes or electrons) in the parent compounds via chemical substitution or by varying the oxygen content. The transition from the insulating to the superconducting regime is triggered by the dopant charges in the CuO2 planes which alter the local physical properties. In this context, long-range orders are rapidly destroyed giving way to new intriguing scenarios High-Tc superconductivity is a fascinating phenomenon, which still needs to be understood. In the last thirty or more years, physicists deployed all sorts of experimental and theoretical efforts to shed light on the mechanism behind it. In recent years, RIXS contributed enormously to the debate by providing information complementary to existing techniques (including inelastic neutron scattering, resonant X-ray diffraction, angle-resolved photoemission, etc.). RIXS is a spectroscopic technique capable of probing orbital, charge and spin excitations and the possibility of measuring the polarization of scattered photons gives access to further crucial information that is usually unavailable in polarization-integrated RIXS spectra, such as the interplay between charge, spin and orbital excitations. The ability to disentangle the spectral features of complex RIXS spectra is crucial to pinpoint the various contributions to the lattice, magnetic and electronic dynamics at play in high-Tc systems. Nowadays, only at the ID32 beamline of the ESRF it is possible to perform polarization resolved RIXS measurements which have played a crucial role in many cases, such as in the confirmation of the charge nature of the ordering discovered in the overdoped region of (Bi,Pb)2(Sr,La)2CuO6+x, in the assignment to spin excitations the enhanced dynamic response at the charge order vector of Nd2-xCexCuO4 and finally in the confirmation of the charge nature of the zone-center fast-dispersing excitations in La2-xCexCuO4. The polarization analysis has been applied also to systems other than cuprates, that is the case of CeRh2Si2. All these works will be discussed as an example of the potentialities offered by polarization-resolved RIXS. The main objective of this thesis work regards the study of insulating and superconducting layered cuprates, using resonant inelastic X-rays scattering technique, with special emphasis on the possibility to infer about the polarization state of the scattered photons. With regard to this, we present a systematic RIXS study of orbital, magnetic and vibrational excitations in a prototypical cuprate system belonging to the “123” family (NdBa2Cu3O7-x). We measured Cu L3-edge RIXS spectra of undoped, underdoped and optimally doped NdBa2Cu3O7-x thin films at different in-plane momenta with both perpendicular and parallel (with respect to the scattering plane) polarization of the incident X-rays. The experimental dataset allows to unequivocally determine the polarization dependence and the evolution of electronic dd excitations as a function of doping and scattering geometry. Moreover, we show that the polarimetric device, a polarization-selective optical element based on a multilayer, can provide crucial insights to disentangle the different contributions in the low energy scale due to spin and phonon excitations. Finally, we discuss the interpretation of the experimental data within the framework of the single-ion model and in terms of Stokes parameters. Our results will be of interest to the large scientific community interested in correlated electron systems because they provide important insights into the interpretation of polarization resolved RIXS data of cuprates. Besides the polarimetric study of the excitations probed by RIXS in cuprates, we present a detailed study of the dispersing behaviour of orbital excitations in quasi-one dimensional (1D) systems. It is known that in correlated oxides orbital excitations tend to have a localized character and are usually described by orbital and spin quantum numbers in a symmetry adapted atomic picture: thus orbital excitations do not disperse, irrespective of their spin character. However, theory predicts that at low dimensionality, dd excitations can split their orbital and spin components, giving rise to complex dispersion in momentum space. Indeed, orbital excitations with sizable dispersion, called “orbitons” due to their collective character, were observed with RIXS on quasi-1D cuprates. Our results on the quasi-1D spin = 1/2 AFM Heisenberg chain Ca2CuO3 are consistent with those of Sr2CuO3 but they are analyzed with an improved theoretical model. In particular, we verify how the Hund’s exchange affects the observed spectra and whether a predicted interaction between spinon and orbiton can be observed. Moreover, our RIXS dataset on the insulating 2D infinite layer CaCuO2 suggests that dd excitations may disperse also in 2D systems. Unfortunately, the theoretical model adopted for the interpretation of the dispersing orbital excitations in quasi-1D cuprate cannot takes into account the the sizable dispersion measured in the 2D CaCuO2. It is worth mentioning that CaCuO2, differently from all the other well-known 2D cuprates, it is characterized by the absence of apical oxygens. It has been demonstrated that the apical oxygens largely influence the possibility of valence electrons to `jump’ from site to site in the CuO2planes. This affects the magnetic spectra of the insulating parent compounds, where the in-plane hopping is larger in compounds where there are no apical oxygens, that is the case of CaCuO2. Regarding the dispersing behaviour of orbital excitations in this material, and without a theoretical model that explain this phenomenon, we believe that the role of the apical oxygens could be relevant also in the orbital excitations physics. Finally, motivated by the recent synthesis of the high temperature superconducting cuprate with putative 3z2-r2 ground state symmetry Ba2CuO3+x (BCO), we investigated its electronic structure by means of X-ray absorption (XAS) and RIXS techniques on a polycrystalline sample. We show that the XAS profile of BCO is characterized by two peaks that we ascribe to inequivalent Cu sites, a scenario that is reminiscent of the double Cu sites in YBa2Cu3O6+x. By tuning the incident X-rays energy to the peak ascribable to the Cu ions in the planes, the RIXS response features a single, sharp peak associated to crystal-field excitations, and argue that these observations are incompatible with the previously proposed crystal structure of BCO. We thus propose an alternative structure, which accounts for our results and previous powder X-ray diffractions experiments. Based on this, we analyze the low-energy spectral range of the RIXS spectra and estimate the magnitude of the magnetic interactions in BCO.

Greta Dellea
Collective excitations in high temperature superconducting cuprates studied by resonant inelastic soft X-ray scattering (2016)

Since the discovery of high temperature superconductors (HTS), more than a quarter of century ago, increasing efforts have been devoted to the search of the basic mechanism leading to superconductivity, but a conclusive and generally agreed explanation is still missing. In this scenario, a better understanding of collective excitations in layered cuprates, either competing or coexisting with the superconducting state, has become a fundamental issue. Cuprates are characterized by the presence of CuO2 planes – Cu2+ ions alternated to O2− ions – separated each other by blocking layers; much of the physics of cuprates takes place in these planes. The parent compounds have one hole per Cu site and the strong electron correlation, typical of transition-element oxides, leads to insulating behavior. These localized holes order antiferromagnetically (AF) and the resulting spin dynamics is well described in terms of spin wave, or magnon, excitations within the bidimensional spin ½ Heisenberg model. When the insulating parent compounds are doped, the additional dopant charges rapidly destroy the Néel ordering and trigger the superconducting transition in correspondence to a critical doping range. If the doping is further increased, the system reaches a non-superconducting metallic phase where low energy electronic excitations have Landau Fermi liquid-like properties. A boost in the interest in collective spin excitation and their evolution with doping came from recent experimental results and theoretical calculations, suggesting a paring by exchange of magnetic excitations, in concomitance with the development of resonant inelastic x-ray scattering (RIXS) at Cu-L3 edge, which has been proved to be the optimal technique to study magnetic, orbital and charge fluctuations in the CuO2 planes. RIXS in the soft x-ray regime is an energy loss spectroscopy, in which the incoming photons energy is tuned at an absorption edge. The signal enhancement at the resonance can be very large, making the measured inelastic signal strong enough to be detected. The choice of the absorption edge also provides chemical selectivity and stringent selection rules on the type of excited states created in the scattering process, mainly if the polarization of the photons is known and controlled. Finally, the sizable momentum carried by x-ray photons can be taken into account to measure the energy vs momentum dispersion relation of the excited states. This thesis presents some of the results obtained with Cu-L3 RIXS on superconducting and insulating cuprates during my activity in the group of Prof. G. Ghiringhelli and Prof. L. Braicovich at the Physics Department of Politecnico di Milano (Italy). The group has a well-established experience in synchrotron-based spectroscopies for the study of magnetic and electronic properties of transition-elements and rare-earth compounds. Recently they focused their activity on RIXS, contributing to the development of the technique, both from the point of view of science and instrumentation. Starting from the experimental evidence that optimally doped high-Tc super-conductors exhibit high-energy damped spin excitations (paramagnons) with dis-persions and spectral weights closely similar to those of magnons in undoped cu-prates, we extended our analysis to a large family of cuprates; spin excitations have been detected in a wide class of samples and dopings, from the well know bulk crystals and thin films, to more complex superconductors, obtained by superlattices and heterostructures, down to a few unit cells layers and nanopaterned structures, giving a further confirmation of the robustness of magnetic excitations and providing an ubiquitous ingredient for the superconductivity. We also used RIXS to measure the evolution of (para)magnons across the entire phase diagram of hole-doped cuprates, from the superconducting underdoped and optimally doped, to the non-superconducting highly overdoped samples. These results suggest a more complex explanation of the pairing mechanism, which could include the influence of the low-energy magnetic excitations, and other ordering phenomena. Superconductivity could be achieved by doping with both holes and electrons. The e-doped region of the cuprates phase diagram has been less investigated so far, mainly due to technical limitations in sample growing and experimental tech-niques. Changing the sign of the doping carriers has strong implications for the shape of the corresponding phase diagram and some important physical proprie-ties, such as pseudogap, stripe order and maximum critical temperature (Tc), dra-matically change. In that sense, the asymmetry between electron- and hole-doping in high-Tc cuprates is fundamental in understanding the processes at the basis of the superconducting transition. In this thesis we show RIXS spectra measured from the archetype e-doped Nd2-xCexCuO4 (NCCO) crystal and from the more exotic Sr1-xLaxCuO2/GdScO3 infinite layer cuprate heterostructure. Our data show a magnetic excitation hardening under e-doping in stark contrast with h-doping; this result is counterintuitive and interpreted in terms of a strongly itinerant character compared to the more localized spin dynamics found in h-doped cuprates. It is also interesting to notice that both artificial h- and e-doped superconduct-ing cuprates perfectly mimic the collective excitation behavior of the correspond-ing bulk crystals, envisaging the possibility to explore general properties of HTS physics on a broad range of conditions, by means of artificial compounds not con-strained to the thermodynamic limitations governing chemical stability of bulk materials. Our results suggest that any successful theory for HTS should require a detailed understanding not only of the magnetic excitation spectrum, but also of the combined effect of electron-phonon coupling and charge-order in the normal state from which superconductivity emerges. We found increasing general evidence that, in cuprates, spin excitations get coupled to both lattice modes and charge order. A better clarification of this three-actor scenario for the superconductivity pairing mechanisms will require further systematic use of high resolution resonant elastic and inelastic x-ray scattering. All the results discussed so far have been acquired with two dedicated high resolution RIXS spectrometer, both designed and build by the group of Prof. G. Ghiringhelli and Prof. L. Braicovich: AXES (Advanced X-ray Emission Spectrometer) working since 1995 at the beamline ID08 of the European Synchrotron Radiation Facility (ESRF) and now dismissed, and SAXES (Super-AXES) which is the evolution of AXES, and has been installed in 2006 at the ADRESS beamline of the Swiss Light Source (SLS). At present, the scientific output of soft-RIXS is reaching its limits due to tech-nical limitations in terms of energy resolution, signal intensity, outgoing polariza-tion analysis and sample orientation control. In order to overcome all these limitations, the new ERIXS (European-RIXS) instrument at the new ID32 beamline of the ESRF has been designed and commissioned and it is now ready for the first user experiments run. The final part of my Ph.D. work has been mainly devoted to the commissioning and performances characterization of the new spectrometer, which now holds the world record of resolving power with a total instrumental resolution of 35 meV at Cu-L3 edge (930 eV). This thesis reports a detailed instrumentation session, with preliminary experiments on antiferromagnetic and superconducting layered cuprates, and test measurements at different edges: Ni-L3, Mn-L3, Ti-L3, Ce-M5, Gd-M5, Eu-M5, extending the class of materials that can be investigated by RIXS, from the more studied cuprates, to a wide range of systems.

Matteo Minola
Magnetic, orbital and charge fluctuations in lavered cuprates studied by resonant soft X-ray scattering (2013)

This thesis presents the results achieved on cuprates with resonant soft x-ray scattering (RXS) during my activity in the group of Prof. G. Ghiringhelli and Prof. L. Braicovich of the Physics Department of Politecnico di Milano (Italy). The group has a well-established experience in synchrotron-based spectroscopies for the study of magnetic and electronic properties of transition-elements and rare earth compounds. In the last years they focused their activity especially on resonant inelastic x-ray scattering (RIXS), contributing substantially to the development of the technique, both from the point of view of science and instrumentation. They designed and built two high resolution spectrometers dedicated to RIXS: AXES (Advanced X-ray Emission Spectrometer) and SAXES (Super-AXES). AXES is working since 1995 at the beamline ID08 of the European Synchrotron Radiation Facility in Grenoble, France. SAXES, which is the evolution of AXES, has been installed in 2006 at the ADRESS beamline at the Swiss Light Source in Villigen, Switzerland. It is working since July 2007 and by now it holds the world record of resolving power with a combined resolution of 130 meV at Cu L3 edge (930 eV). In this thesis I present some of the results obtained with these spectrometers on insulating and superconducting layered cuprates using Cu L3 edge RIXS. This technique is shown to be the optimal probe to study magnetic, orbital and charge fluctuations in the CuO2 planes of these compounds, allowing energy and momentum-resolved measurements and adding crucial pieces to the puzzle of high temperature superconducitivity. CuO2 planes are the common feature of all layered cuprates and the set where high-Tc superconductivity emerges: these planes consist of Cu2+ ions alternated to O2− ions and they are separated each other by “blocking layers”. Although it has one hole per Cu site, each CuO2 plane is originally insulating, due to the large electron correlation typical of transition-element oxides. The Cu2+ ions (3d9 configuration) have one unpaired spin-1/2 per site and they are coupled via superexchange interaction J, i.e. the exchange mediated by oxygen, so to produce a bidimensional antiferromagnetic (AF) lattice. The hybridization with the oxygen ions is so strong that the superexchange is exceptionally high in cuprates (J > 100 meV), allowing the study of the associated magnetic excitations without the need of a few meV resolution. When the insulating parent compounds are doped, the additional degrees of freedom from dopant charges further complicate the electronic situation. A critical doping is required to destroy the various long range orders and superconductivity emerges when charges coming from the blocking layers dope the CuO2 sheets in a number that alters the situation and triggers the transition. Despite more than 25 years of studies, the origin of the superconducting state in cuprates is still unclear and remains the subject of intense scrutiny. In particular one of the central unanswered questions concerns the nature of the normal-state spin fluctuations that may be responsible for the pairing. Because of technical limitations, the experimental investigation of doped cuprates has been until now largely focused on low-energy excitations in a small range of momentum space. In this thesis we used high resolution RIXS to show that a large family of high-Tc superconductors (HTS), i.e. (Y,Nd)Ba2Cu3O6+x (RBCO), exhibits high-energy damped spin excitations (paramagnons) over a wide range of doping, with dispersions and spectral weights closely similar to those of magnons in undoped cuprates. The comprehensive experimental description that comes out from our systematic data acquisition enables quantitative tests of magnetic Cooper pairing models and supports the paramagnons as strong candidates to cover the role of glue for the Cooper’s pairs. Subsequently we have exploited the capability of RIXS to work very well on thin films, in order to study both insulating and superconducting cuprate-based heterostructures. Recently the technical progress in epitaxial growth has lead to the discovery of a panoply of exceptional magnetic and transport properties in artificial heterostructures of 3d transition metal oxides in general. Electronic, lattice and orbital reconstruction occurring at the interfaces can in fact influence the charge transfer between the oxides, while the modified dimensionality can affect the magnetic properties of the oxides. Among these heterostructures cuprate-based superlattices (SLs) are particularly interesting since they can be considered as new, artificial HTS, offering the opportunity of freely choosing the two building blocks i.e., the superconducting CuO2 planes and the charge reservoir blocking layers. We have carried out Cu L3 RIXS measurements on both insulating and superconducting (CaCuO2)m/(SrTiO3)n SLs and compared the results with those on a 14 nm thick CaCuO2 film, in order to understand what happens to magnons when the CuO2 planes are at the interfaces of a SL and if a (para)magnon-mediated superconductivity could still be possible. In all insulating samples spin excitations are in the form of dispersing magnons and in the SLs magnons have similar spectral intensity but reduced dynamics with respect to pure CaCuO2. This is the demonstration that the AF order is preserved in the insulating SLs, down to very small cuprate layer thickness and despite the chemical and structural alterations at the interfaces. On the other hand the superconducting SLs exhibit dispersing paramagnons, similarly to the case of superconducting RBCO. Moreover the orbital excitations, visible in RIXS spectra together with magnons and due to the ligand field felt by Cu2+ ions, have revealed a pyramidal coordination of copper atoms at the CaCuO2/SrTiO3 interfaces. These findings open the way to the production of new, artificial HTS based on cuprate/noncuprate SLs where the charge reservoir layer is constituted by the interface itself. Any successful theory for HTS should require a detailed understanding not only of the spin but also of the charge correlations in the normal state from which superconductivity emerges. Therefore we have studied charge fluctuations in the CuO2 planes by means of RXS. Despite intense efforts, to the present date only two clear ordering phenomena have been reported for correlations in the copper oxide sheets of cuprates: the above cited uniform AF in undoped cuprates and a uniaxially modulated AF, combined with charge order, in the so-called “214” family [with chemical composition La2−x−y(Sr,Ba)x(Nd,Eu)yCuO4]. The latter is known as “stripe order”, with a commensurate charge modulation with a period 4 lattice units, which greatly reduces the superconducting transition temperature of 214 materials at a doping level p ≈ 1/8 per planar Cu atom. Incommensurate spin and charge fluctuations in 214 materials with p ≠ 1/8 have been interpreted as evidence of fluctuating stripes. These findings have generated a long-standing debate around the questions of whether stripe order is a generic feature of the copper oxides and if stripe fluctuations are essential for superconductivity. We have used RXS to assess this issue and identify two-dimensional charge fluctuations with an incommensurate periodicity of 3.2 lattice units in the CuO2 planes of the superconductors RBCO, with hole concentrations p of 0.09 to 0.13 per planar Cu ion. The intensity and correlation length of the signal increase strongly upon cooling towards Tc, while further cooling below Tc abruptly reverses the divergence of the charge correlations. In combination with earlier observations, these data indicate an incipient charge density wave (CDW) instability that competes with superconductivity and, for the first time, we have the evidence that the anomalously low Tc found in underdoped cuprates is due to CDW, and not other phenomena.

Marco Moretti
Magnetic and orbital resonant inelastic X-ray scattering (2011)

This thesis deals with the resonant inelastic soft x-ray scattering activity I carried out within the group of Prof. G. Ghiringhelli and Prof. L. Braicovich of the Politecnico di Milano (Italy). The group has a long tradition in synchrotron-based spectroscopies for the study of magnetic properties and the electronic structure of transition-metal and rare earth compounds. In the last 15 years, the activity of the group has been concentrated in soft x-ray emission spectroscopies and resonant inelastic x-ray scattering (RIXS), in particular. Both on the instrumentation and the scientific side, the group contributed and still contributes substantially to develop this technique. In the last three years, results of fundamental interest and importance have been obtained, some of which are presented here. The largest effort regards the study of 3d transition-metal compounds, with special emphasisto insulating and superconducting layered cuprates, by means of Cu L3 edge RIXS. RIXS is shown to probe both orbital and magnetic excitations in these compounds, with the possibility of sampling different regions of the reciprocal space (finite transferred momentum, q). Often, RIXS data are integrated with soft x-ray absorption (XAS) measurements, including x-ray linear dichroism (XLD) and x-ray magnetic circular dichroism (XMCD), to get a deeper understanding of the problems. The presence of 2D CuO2 planes of Cu2+ alternated to O2− ions is a common feature to all layered cuprates. Cu2+ ions (3d9, one-hole system) have one unpaired spin-1/2 per site, which are antiferromagnetically coupled via super-exchange interaction. The strong hybridization with neighboring oxygen ions is such that super-exchnge interactions are exceptionally high, i.e. with coupling constants larger than 100 meV. These CuO2 planes with antiferromagnetic order constitute the background in which doped holes in the superconducting compounds have to move. Cu-O hybridization and pure crystal field effects give rise to the removal of the Cu-3d state degeneracy. The hole is thus characterized by its orbital and spin degrees of freedom. The lowest energy state, the ground state, is known to be the one with the hole in an orbital with x2 − y2 symmetry. Transitions of the hole to orbitals having different symmetries, possibly accompanied by spin-flip processes, are allowed and are called crystal field, or dd, excitations. The study of these excitations by means of Cu L3 edge RIXS is here demonstrated to be very effective. Thus, crystal field (dd) excitations were studied in several undoped layered cuprates, parent compounds of high-Tc superconductors, as a function of the scattering geometry and incoming polarization and are demonstrated to be local excitations with no or little dispersion with respect to the transferred momentum q. A single ion model is introduced to evaluate the RIXS crosssection of such excitations and to fit the experimental trends. It was possible to unambiguously determine the energy and symmetry of Cu-3d states in La2CuO4, Sr2CuO2Cl2, NdBa2Cu3O7, Sr0.5Ca0.5CuO2 and CaCuO2. dd excitations in superconducting systems, on the other hand, show a very little dependence on polarization and scattering geometry and the features that are remarkably separated in the insulating compounds merge into a unique asymmetric peak, of triangular shape. Note that, however, the energy of such dd excitation peak is always larger than 1 eV. A special case of dd excitations is that having the hole in a final state whose symmetry coincides to that of the ground state (x2 − y2), but with opposite spin, i.e. a pure spin-flip transition. Of fundamental importance is the theoretical demonstration and experimental evidence that such spin-flip processes are allowed in Cu L2,3 edge RIXS. Spin-flips perturb the antiferromagnetic order and trigger spin-waves within the CuO2 planes. Therefore, RIXS can be exploited to probe the dispersion of magnetic excitations, for instance magnons, of layered cuprates, both insulating and superconducting. These findings put RIXS as a technique on the same footing as inelastic neutron scattering (INS) in this field of research. Magnon dispersion in the antiferromagnetic insulating cuprates could be mapped in a large fraction of the Brillouin Zone, in perfect agreement with INS measurements. Results are interpreted with success combining linear spin-wave theory and the single ion model already mentioned. Discrepancies are found at large transferred momenta and high energy losses, hardly reachable with INS experiments. Moreover, the use of x rays allows the measurements of tiny samples or thin films, precluded to INS, because of the limited neutron cross-section. The application of Cu L3 edge RIXS to superconducting compounds revealed unexpected magnetic behaviors, which are discussed here. Magnetic excitations in doped layered cuprates are demonstrated to survive both in the normal and superconducting state. For underdoped La2−xSrxCuO4 (LSCO) the spin dynamics shows a branch dispersing up to 400 meV coexisting with a branch at lower energy. Note that only the latter has been already observed with neutrons. The presence of the high-energy branch indicates that underdoped LSCO is in a dynamic inhomogeneous spin state. Magnetic excitations are also seen in preliminary measurements on optimally doped LSCO, which, however, do not show any clear doubling of the magnetic features. RIXS spectra of PbyBi2−yLaxSr2−xCuO6+d are also presented, below and above the pseudogap phase transition temperature Tc. Magnetic interactions between neighboring spins are studied also by means of XMCD, which provides evidence of an out-of-plane spin moment both in undoped and doped cuprates. The results suggest that, together with shortrange in-plane antiferromagnetic correlations, the Dzyaloshinskii-Moriya interaction, which causes the canting of the spins out-of-plane, survives up to optimal doping and in the superconducting state. Finally, having clear the effectiveness of L2,3 edge RIXS in probing orbital excitations in 3d transition-metal oxides, we looked for orbiton dispersion in LaTiO3 and YTiO3, and YVO3 and GdVO3, i.e. we moved from one-hole to one- (Ti3+ in RTiO3 is 3d1) and two-electron (V3+ in RVO3 is 3d2) systems. These materials are magnetically and orbitally ordered. As spin-waves (magnons) perturb the magnetic order, orbital-waves (orbitons) perturbing the orbital order are expected to arise in the solid. These excitations differ from the local dd excitations in that they have a collective nature, which manifests in a dispersive-like behavior versus q. Although very little dispersion of orbital excitations was found in our measurements, theoretical calculations support the collective nature of these excitations in the studied compounds.

Valentina Bisogni
Local and collective excitations in cuprates investigated by high resolution resonant inelastic x-ray scattering (2010)

Thanks to the properties of synchrotron radiation sources and to recent improvements in the instrumentation, Resonant Inelastic X-ray Scattering (RIXS) has been established as a very powerful technique for the study of strongly correlated electron systems. This is due to the enormous progress in energy resolution in the soft X-ray range achieved first at the European Synchrotron Radiation Facility (ESRF) in Grenoble and later with further progress at Swiss Light Source (SLS) in Villigen. This happened in 2007 and after; thus high resolution RIXS can be regarded as a new emerging field. In this thesis we present one of the first applications to the doped high-Tc superconductors and to their undoped parent compounds. Most of the experimental work has been done at the soft X-ray beamline of the ESRF, ID08, using the AXES spectrometer. Further soft X-ray RIXS measurements have been done with the SAXES spectrometer at the ADRESS beamline of the SLS. We added also hard X-rays RIXS measurements at the ID16 beamline of the ESRF. The Resonant Inelastic X-ray scattering technique is presented by explaining its physical mechanism and by showing the importance of energy resolution in the study of the low energy scale excitations. The RIXS instrumentation used at the ID08 beamline together with the recent refocusing intervention done on the AXES spectrometer are illustrated as well. A general presentation of the cuprates introduces the systems under study together with their intriguing electronic and magnetic properties. Original results on these materials have been obtained. Two kind of excitations have been investigated by RIXS, having the same orbital character, Cu 3d, but with completely different behaviour: local for the dd-excitations and collective for the magnetic excitations. The dd-excitations have been measured by Cu-L3 RIXS on the undoped cuprate compounds and a single ion model is introduced to explain them within a crystal field approach. Magnetic excitations at lower energy than the dd have been accessed by soft X-ray RIXS for the first time in 2007 at the ESRF during this thesis work. Further work (at SLS) allowed different contributions to be distinguished in the undoped La2CuO4: the single magnon, as measured with Cu-L3 RIXS, and the bimagnon, as measured with RIXS at three edges, the Cu-L3, the Cu-K and the O-K. Also data on bimagnon seen with hard X-rays at the Cu-K edge are presented; the data were taken at ID16 of the ESRF with a new spectrometer developed by the ID16 staff (S. Huotari and G. Monaco). The most relevant result on parent compound is probably that RIXS results are in perfect agreement with neutron work; this allows a great extension of work to tiny samples that cannot be studied with neutrons. The data analysis is supported by a theory of magnetic excitations in RIXS due to J. van den Brink, L. Ament et al.; this theory is summarised here. Finally, magnetic excitations in the underdoped La2−xSrxCuO4 have been recently measured by means of Cu-L3 RIXS and discussed by comparing them to the case of the La2CuO4 parent compound. We present data suggesting phase separation in underdoped La2−xSrxCuO4; of course this is a work in progress.

Andrea Piazzalunga
High Resolution RIXS spectroscopy in the soft X-ray range: new instrument and results (2008)


MSc theses

2023 2022 2021 2020 2019 2018 2017 2016 2015 2014 2013 2012 2011

Davide Impelluso
Ligand field and magnetic excitations in High Entropy Oxides studied by RIXS (2023)

Maryia Zinouyeva

Piero Florio
Performance analysis and simulations of a laboratory x-ray absorption spectrometer (2022)
Francesco Rosa
RIXS study of magnetic excitations in nickelates (2022)

Giacomo Merzoni
Temperature dependence of charge excitations in RIXS spectra of superconducting cuprates (2021)


Marcello Maraschi
Electrical and spectroscopic characterization of polarization phenomena in CdTe X-ray detectors (2020)

The technological development of semiconductor detectors, with a significant improvement in front-end electronics, has given a major boost to the development of high-energy X-ray analyses. In the industrial sector, the possibility to have radiographies with spectroscopic information has paved the way for new possible methods of non-invasive analysis, such as the identification of possible contaminants through material recognition techniques. The main systems using this technique are based on detectors that use CdTe or CdZnTe crystals as semiconductor, due to their high efficiency in revealing X-rays (thanks to high atomic number) and the possibility of being used at room temperature with low noise (thanks to the wide band gap). This thesis studies the phenomenon of bias in CdTe detectors with Schottky contacts, in particular referring to the detectors developed by Xnext®. After illustrating the main theoretical models in the literature, we present a simulation showing how polarization is changing the spectroscopic response of the detector, in particular by assessing the variation over time of the position of a peak and of the average efficiency of collecting charges in the crystal. The experimental analysis is focused on two different types of measurement, one of the reverse current variation over time and one spectroscopic. With the former, it was possible to estimate the height of the Schottky barrier at the anode, the proportionality coefficient between the electric field at the anode and the change in the height of the barrier, and the energy level of the trap state in the gap. However, a long-term current measurement (7:30 h) made it necessary to assume the presence of two different trap states in the gap. The lack of further measures at different temperatures did not allow us to assess their energy. With spectroscopic analysis, we focused on assessing the energy level of the trap state by analyzing peak displacement and variation over time in the average collection efficiency of charges in the crystal. The results are in accordance with the literature. These results, to be completed in the experimental part with other measurements lasting several hours, suggest that a direct determination of the presence of more than one energy level for the trap states in CdTe is needed in order to better understand its intrinsic limits and the appearance of polarization in X-ray detectors based on CdTe.


Mattia Pagetti
Determination of electron phonon coupling from RIXS spectral intensity (2019)

I have worked on this thesis within the group of professor Giacomo Ghiringhelli, at the physics department of Politecnico di Milano, between the autumn of 2018 and the spring of 2019. The group has a long and consolidated tradition in synchrotron based spectroscopy, aimed at the study of magnetic and electronic properties of the materials. In the last twenty years, indeed, the activity of the group has been focused on X-rays emission spectroscopy, and in particular on the experimental technique denominated RIXS – Resonant Inelastic X-rays Scattering. The data obtained from it, possibly complemented with the ones obtained from other techniques (XAS, XLD, XMCD), allow us to study, and thus to characterize in energy and momentum, the material’s excitations – which are direct and unequivocal manifestations of the atomic and electronic structure. Participating in both the technical and theoretical development of RIXS, the intensive work of the group have, since the beginning of the new millennium, paved the way to a new and more sharp understanding of matter’s microscopic phenomena and dynamics, providing a new and complementary point of view (endued with many peculiarities) in the landscape of the already present experimental techniques (INS, Inelastic Neutron Scattering, most notably). Since 2009, more in particular, the group research has been focused on 3d transition-metal compounds, with greater emphasis on cuprates, exceptionally unique materials characterized by a quasi bidimensional structure (given by CuO2 layers), notable for being high temperature superconductors; and most importantly, puzzling (the true nature of high temperature superconductivity is nowadays still missing a coherent explanation) and suitable to be investigated through RIXS. My thesis can be placed in this contest – I will thus begin from a brief introduction on cuprates (paying particular attention to LCO and NBCO, and mentioning the most important characteristic of superconductors and strongly correlated systems), spending some words on the importance of phonons in the mechanism which is thought to be at the basis of high temperature superconductivity, to then dedicate a whole Chapter (the second one) to the RIXS technique, with a focus on theoretical and technical aspects, and incorporating some practical consideration. The third Chapter is the main body of the thesis: it is devoted to the phononic dynamics which can be characterized through a RIXS investigation. In first place, the theory is shown (following the tracks already given by the literature), and then RIXS spectra are simulated, accounting for the premises and for the assumptions – given the phononic excitations in the sample, those spectra are what we expect as a result from a RIXS experiment. Finally, the last chapter is an analysis of the results obtained during the experiment that we carried out at ESRF on the beamline ID32. Particular emphasis is given on the analysis method – this is a predominant aspect in RIXS technique, more than in othes experimental techniques.

Samir Jokar
All in-situ ultra-high vacuum ionic liquid gating study on oxide thin films (2019)

The study of how the properties of materials can be influenced, be they electronic, magnetic or even structural, is of enormous interest for both fundamental science and technological applications. An example is the use of electric fields to control the transport properties in semiconductors, where charge carrier accumulation is electro- statically induced. This mechanism is known as ’field effect’ and is the foundation of much of modern day electronics, being exploited in metal-oxide-semiconductor field-effect-transistors (MOSFET). The field effect has been proven to be very effective for the gate control of conventional semiconductors, but it is less effective when it comes to other materials. Materials such as transition-metal oxides, exhibit for example chemical diversity, superconductivity, ferroelectricity, magnetism, and are thus of contemporary interest. The electric field strength required to induce, for instance, a phase transition in these materials, can exceed by far the breakdown limit of a standard SiO2 dielectric gate. In this context, ionic liquid gating has been proven to be an alternative and broadly applicable approach, capable to induce higher electric fields and thus to drive sufficient carrier densities. In this approach the solid state gate dielectric is replaced with an ionic liquid. When a potential is applied to the gate electrode, a rearrangement of the ions leads to the formation of an electrical double layer at the interface with the material, which creates the electric field. In the last decade many experiments based on this approach have been conducted, allowing to study and manipulate the properties of various materials. Nonetheless, the mechanisms behind this process have not yet been fully understood, and contradictory results have been published. So far, in this type of experiments it is common that after its fabrication, the device is exposed to ambient conditions, in order to apply the ionic liquid. In this stage, for example, water can come in contact with the device, affecting the outcome of the measurements. In this thesis project, a new experimental setup that allows to apply the ionic liquid in-situ has been developed. This opens the possibility to conduct original studies under unique experimental conditions, potentially bringing new insights in this research area.

Paolo Distefano
Spectral correction and actual imaging capabilities of a CdTe detector for X-ray multienergy radiography (2019)

Multi-energy radiography is an innovative X-ray inspection technique based on counting X-ray photons in number and energy for each pixel. With such spectroscopic radiography, it is possible to obtain the attenuation spectrum of an inspected object and perform material identification. The information collected is richer with respect to the traditional techniques that purely operate a total absorption measurement. This means that, even if it is generally possible to locate extraneous materials inside an object, it is not possible to infer anything about the nature of the inclusions. In this thesis two main issues connected to material identification are discussed taking as reference XSpectra® technology developed by Xnext® and material identification algorithm developed by G. Bubba in Material identification and Compton scattering effects in X-ray multienergy radiography of homogeneous compounds. The first issue is to estimate the imaging capabilities of such a technique: assuming to have an ideal detector, these are mainly linked to the number of photons the detector must receive to reconstruct a reliable image. The number of photons reaching the detector is linked to the flux emitted by the source and to the velocity of the object, that is also important for its industrial applications. The second issue is connected to spectrum distortions introduced by a CdTe detector, with a particular attention to escape and incomplete charge collection phenomena. The study of distortions effects was treated through simulations based on the data collected at ESRF in June 2018 that allowed to build a detector response matrix. These measurements were also instrumental to the definition of a first correction procedure that, even if it must be optimized, looks promising.

Riccardo Marco Augusto Bona
Crystal field driven metamagnetic transitions in HoIr2Si2 analyzed by means of X-ray spectroscopy and full atomic multiplet calculations (2019)

Lanthanide materials exhibit exotic properties due to the concurrence of atomically localized and collective electronic interactions. A complete theory adequate to properly account for all the experimentally determined features has not been conceived yet. Still, in the framework set by full multiplet atomic theory, starting from a local Ansatz, most of the characteristics regarding rare earth compounds are properly described. The coexistence of anisotropies, set by crystalline environment, and electronic coupling effects between rare earth atoms leads to unconventional magnetic phase diagrams. X-ray absorption and Resonant Inelastic X-ray Scattering data have been collected for HoIr2Si2 sample, with high quality synchrotron light, at beamline ID32 of the European Synchrotron Radiation Facility (ESRF). A combined analysis of X-ray spectra and magnetic properties reported in literature yielded a complete set of parameters which binds the shape of the ground state 4f wave function of Ho3+ ion in HoIr2Si2. Metamagnetic transitions occurring in HoIr2Si2 can be well described combining crystal electric field model with exchange coupling interactions. The methodologies exploited in this thesis work can be fruitfully applied to the analysis of similar compounds, characterized by intriguing magnetic phase diagrams.

Riccardo Piergiacomi
Modern tools for X-ray spectroscopy simulations and data analysis applied to strongly correlated systems (2019)

Strongly correlated systems exhibit many exotic properties due to a strong Coulomb interaction among electrons. Several models try to describe these phenomena, but a complete theory taking into account all possible interactions has not been completed yet. Thus, further studies are needed to verify the correctness of models and uncover the physics still unexplained. X-ray absorption and Resonant Inelastic X-ray Scattering performed with synchrotron radiation are valid techniques to investigate strongly correlated materials. Some studies about intermediate valence in heavy fermion compounds reported disagreements with the theory, while others reported successful spectra simulation on strongly correlated oxides. In this thesis work, Multivariate Curve Resolution methods are employed as a tool to investigate intermediate valence in intermetallic ternary compounds based on Yb and Ce. In addition, starting from Density Functional Theory calculations, parametric spectra simulations of X-ray absorption and Resonant Inelastic X-ray Scattering performed on NiO and MnO are tested. The data analysis and spectra simulation methodologies presented and tested can be useful tools to better understand experimental results and shed some light on the theoretical models.

Sebastian Fava
Growth dynamics of graphene on molten copper (2019)

Since it’s discovery in 2006, Graphene has known no rivals in terms of number of applications that scientists from all over the globe have thought for him, ranging from spintronics to energy storage, from transistors to bio-compatible devices. However, what’s still hindering his big step from laboratories to industry is a cost-effective method to synthesize large-scale good-quality crystals. Over the past decade, great improvements have been made in this direction, and CVD consolidated as an excellent candidate for this arduous task. Among other methods, a novel technique consisting in the synthesis of crystals on transition metals in the liquid phase has proven to overcome many difficulties related to defect-inducing dislocations and low-diffusivity of solid substrates. Nevertheless, a clear physical insight over the processes involved during graphene nucleation and growth is still lacking, and many of its parameters are derived by post-process analyses, neglecting those crucial intermediate steps that may conceal key-factors involved in the process. The reason for this trend is that it’s technically difficult to combine different experimental set-ups, and an ad-hoc design is more than ever needed to conduct a complete and satisfying investigation. This is the reason behind the LMCat project, that developed a reactor suitable both for CVD growth at high temperature by hydrocarbon decomposition and for in-situ Raman and optical studies, in order to follow in real time the growth of graphene flakes and, at the same time, determine its physical properties. Additionally, it aims to prove X-ray techniques, such as GID and XRR, as an efficient tool for high temperature characterization, a feat never achieved before. This is the framework of this thesis work, which can of course cover it only partially and at a rather early stage. The focus has been put on the surprising high contrast showed by radiative optical microscopy at high temperatures (~1100 C°) and on the first, surprising results coming from X-ray analysis. The former has been proven as an effective tool for following the growth and derive kinematical parameters, the latter as a potential tool for quantitatively estimate its crystal structure at conditions prohibitive for standard probes.

Marco Magnaterra
Crystal field scheme of the heavy fermion system CeRh2As2 studied by means of X-ray absorption and X-ray Raman scattering spectroscopy (2019)

Heavy fermion systems are lanthanide- and actinide- based intermetallic compounds where the strong electronic correlation gives rise to a rich phase diagram and exotic physical phenomena. The description of these systems, after more than 80 years from their discovery, is still a big challenge. In this thesis the newly investigated heavy fermion system CeRh2As2 is analyzed. The goal of the research was to provide an independent determination of the crystal electric field (CEF) scheme and crystal electric field parameters acting on the rare earth Ce ion, starting with the mere knowledge of its crystal structure. The determination of the CEF scheme was carried out using x-ray absorption (XAS) at the M(4,5) edge of Ce and x-ray Raman spectroscopy (XRS) at the N(4,5) edge. The experiments were performed respectively at ID32 and ID20 of the ESRF. The symmetry reduction due to the presence of the neighbouring atoms causes the presence of a natural out of plane linear dichroism that is observed in the experimental spectra acquired with these techniques. The determination of the crystal field scheme was achieved via the comparison of the experimental spectra with atomic multiplet theory simulations performed with Quanty. Atomic models can be successfully applied in these systems due to the high localization of the 4f orbitals. A crystal field scheme Gamma71-Gamma6-Gamma72 is proposed, with a quasi-degenerate quartet ground state. In fact, the distance between the first excited state Gamma6 and the ground state Gamma71 is determined to be bounded between 0meV and 6meV.

Leonardo Martinelli
Effects of spatial confinement on charge order in YBa2Cu3O7-x studied by resonant X-ray scattering (2019)

In this thesis, I focus on the effects of spatial confinement on charge order in YBa2Cu3O7−x (YBCO), studied with Resonant X-Ray Scattering (RXS). YBCO belongs to the family of cuprate superconductors, among which it probably represents the most studied example. Cuprates are a class of ceramic materials, famous for exhibiting superconducting properties at temperatures above the limits imposed by BCS theory, formulated in the ’50s by Bardeen, Schrieffer and Cooper. This is, however, not the only peculiarity of these crystals, which display other electronic properties not explained by the standard band-theory: among them, indeed, the presence of spatial modulations of electronic charge with a period not commensurate to the underlying lattice. These can be detected by the use of resonant x-ray scattering at Copper L3 edge. The importance of such charge density waves resides in the fact that they exhibit a clear competition with superconductivity at low temperatures. This evidence encouraged the scientific community to perform a series of experiments devoted to understanding the nature and the characteristics of this charge order. The hope is to find, in this phenomenon, the key to explain the microscopic origin of superconductivity in this class of materials. This dissertation fits precisely in this context. The thesis work, prominently experimental, is based on two RXS experiments performed at beamline UE46-PGM1 at Bessy II synchrotron facility. The first of them, held in January 2019, probed nanopatterned samples of YBCO; the second one, in which I participated, used thin films of the same material grown along [100] direction. In both cases the CuO2 planes are then confined: in the former by the geometry of the structure, in the latter by the film thickness. Acquired data has been subsequently analysed and compared to some numerical simulations. They show a clear reduction in the charge order signal when the dimension of a-b planes is shrunk down to some hundreds of nanometers. This is interesting, since the correlation length of such modulations is lower than 10 nm. This result, which needs further confirmation, seems to suggest that charge order is sensitive to phenomena taking place at the mesoscale.

Andrea Marino
X-ray spectroscopy based investigations of the electronic states of CeCu2Si2 and YFe2Al10: absorption and photoemission (2019)

Strongly correlated electron systems are typically compounds consisting of transition metals or rare earths, with partially filled d or f bands. The electron-electron interaction within the d or f shell is far from being negligible and the one-electron approach fails at grasping the behaviour of these compounds. Hybridization may exist between localized d or f electrons and the electrons of other bands, such as the wide conduction bands in a metal. In cerium based intermetallic compounds the low-temperature physics is driven by the interaction of lattice of f electrons with the itinerant conduction electrons. Such materials show a plethora of different ground-states, ranging from antiferromagnetic to unconventionally superconducting and Fermi liquid behaviour with a huge enhancement of the electrons’ effective mass. This thesis focuses on the prototypical compound CeCu2Si2. It was the first unconventional superconductor to be discovered in 1979 and still arouses interest in the scientific community as theoretical and experimental efforts are trying to unveil the superconducting pairing mechanism. Since the interesting properties of the material ultimately stem from the interaction between the f and conduction electrons, knowledge of the f states is of great interest. Polarization dependent soft x-ray absorption spectroscopy has proven to be a powerful tool to obtain information about the symmetry of the 4f ground-state. In this work, we investigate the crystal-field ground-state wave-function of CeCu2Si2 looking at the linear dichroism of polarization dependent soft x-ray absorption spectroscopy at the Ce M4,5 edge. In particular, this is done in the temperature range from 250mK to 250K, i.e. from well below the superconducting temperature (Tc = 0.6K) to well above the Kondo temperature (TK ≈ 10−20K), in order to probe the ground-state wave-function in the mK regime and assess the impact of hybridization as a function of temperature. The experimental data are supported by full-multiplet calculations based on a single-ion crystal-field approach. The overall temperature trend of the linear dichroism is well explained in terms of thermal occupation of excited crystal-field states. Small deviations are discussed in terms of hybridization of f electrons and conduction bands. We also focus on another compound, YFe2Al10, where Kondo physics originating from the presence of f electrons is absent. The Fe, however, carries a magnetic moment that does not order down to 0.1K. This compound has recently attracted interest since it is naturally poised on the verge of ferromagnetic order happening at 0K and undisguised quantum critical behaviour has been observed. We report an explorative spectroscopical investigation of the role played by the Fe atoms in YFe2Al10. The valence band soft photoemission spectrum is measured and compared with preliminary density functional theory and dynamical mean-field theory calculations, performed by P. Hansmann at the Max Planck Institute for Chemical Physics of Solids (MPI CPfS), in order to evaluate the role played by correlations in the compound. The x-ray absorption spectrum at the Fe L2,3 edge is measured and compared with reference Fe oxide samples in order to assess the valence of Fe in YFe2Al10. The magnetic moments of the Fe atoms and the magnetic susceptibility are measured by x-ray magnetic circular dichroism at the Fe L2,3 edge. The valence band is better fitted by the calculations when including a modest amount of Coulomb interaction strength between the Fe 3d electrons. The difference between the valence of Fe predicted by dynamical mean-field theory and the one suggested by the comparison of the x-ray absorption spectra with oxide references can be interpreted in terms of quite large charge fluctuations. Furthermore, x-ray magnetic circular dichroism data confirm the saturation of the static susceptibility with increasing applied fields and the values of the effective moment of the Fe atoms, in accordance with what has previously been measured with a susceptometer.

Pietro Marabotti
Magnetic dynamics of CaIrO3 probed by resonant inelastic X-ray scattering (2019)

In this thesis, I focus on the investigation of CaIrO3 by means of resonant inelastic x-ray scattering (RIXS). CaIrO3 belongs to a new class of strongly correlated electron materials of the 5d series, namely iridates. In iridates, the electron correlation is expected to be weak, whereas spin-orbit coupling (SOC) is strong and alters the electronic structure of these compounds, leading to the so-called jeff =1/2 Mott insulating state. The crystal structure of CaIrO3 features both corner- and edge-sharing IrO6 octahedra along the c and a axes, respectively, which are theoretically predicted to support magnetic couplings of different strength and nature. Furthermore, the strong tetragonal distortion, whose energy scale is comparable to that of SOC, modifies the electronic structure of the jeff = 1/2 model and possibly introduces anisotropies in magnetic interactions of CaIrO3. Using iridium L3 edge RIXS, we are able to probe magnetic and electronic properties of CaIrO3 in view of the good energy resolution and the favorable cross section. I performed RIXS measurements at the ID20 beamline of the European Synchrotron Radiation Facility (ESRF). RIXS data show a characteristic temperature dependence, strongly correlated with the Néel temperature of the system. In addition, the momentum transfer dependence is very peculiar, with strong intensity and modulation of the shape of the RIXS spectra along the corner-sharing direction. By comparison with literature, I conclude that the low-energy dynamics of CaIrO3 is dominated by a two-spinon continuum, thus suggesting that the magnetism in this system has mostly one-dimensional character. I adopt a simple model based on the 1D s=1/2 Heisenberg chain to extract the relevant magnetic interactions in CaIrO3. In order to do so, I compute, calculate and compare the magnetic dynamic structure factor of a coupled spin chain to experimental data. I obtain values of the magnetic couplings which are consistent with that of literature.


Francesco Barantani
Phonons in a ferroelectric superconductor : optical spectroscopy on CaxSr1-xTiO3-d (2018)

Strontium titanate SrTiO_3 is a quantum paraelectric insulator. A ferroelectric order can be stabilized by chemical or isotopic substitution. Moreover, upon electron doping, the material can also be turned into a metal, which become superconducting at temperatures around 300 mK. So far, the superconducting pairing mechanism remains unknown. In 2017, the coexistence of a ferroelectric and superconducting phase was discovered in CaxSr1-xTiO3-d, accompanied by an enhancement of the critical temperature with respect to the non-ferroelectric SrTiO3. The aim of this work is to improve the knowledge of the phonons in this material in order to help to find new perspectives for the pairing mechanism. In this work, FTIR spectroscopy measurements on SrTiO3 and CaxSr1-xTiO3-d will be presented. Their temperature dependence will be analysed and changes arising upon Ca substitution will be discussed. From the experimental data, the optical conductivity of the samples will be computed using a Drude-Lorentz model for the dielectric function and Kramers-Kronig relations. This quantity gives new information on the phonons of this material and also microscopic information such as the effective mass of the free carriers in the metallic samples.

Susanna Boitano
Temperature dependent investigation of PTCDI-C8 thin film structure (2018)

Nowadays, the fields of electronic and opto-electronic devices are basing future applications on organic semiconductors and on the study of thin film morphology made up by those materials, in order to determine and tailor the performances and the efficiency of the devices. In particular, this thesis deals with thin films of PTCDI-C8, a ntype organic semiconductor made up by conjugated molecule which has proven to have potential in organic opto-electronics, such as organic solar cells, and organic electronics, such as organic field effect transistors. In this case, an important parameter that influences the performances and the efficiency of the devices is the charge carrier mobility. In order to better understand how this aspect can be tailored, studies on the thin film morphology are necessary. In the framework of this thesis an upstream study of the PTCDI-C8 thin film morphology evolution with temperature changes will be presented. In particular the focus will be on the evolution of the crystal structure at different temperatures, analyzing the unit cell parameters and their modifications. In the first part an introduction regarding organic semiconductors and PTCDI-C8 will be illustrated. Then an overview of the experimental techniques and methods used to study the thin film crystal structure will be provided: X-ray techniques and organic molecular beam deposition growth method. Afterwards, the experimental results will be discussed. It has been seen that PTCDI-C8 unit cell parameters follow a trend while changing the temperature, and at the temperature of T=200°C a different crystal structure appears from grazing incident X-ray diffraction measurements.

Giovanni Pirro
Application of scaled wave optics propagator to model synchrotron beamlines (2018)

The ESRF storage ring upgrade program (EBS) will make the X-ray beam up to 100 times more coherent and consequently increase the interference phenomena. This property is fundamental to techniques such as holography or phase contrast imaging. The quantitative characterization of the coherence allows to optimally design experimental setup and a reduce time and resources necessary for the analysis of data. The beamline computer simulation is the most effective tool for a first analysis of the beam characteristics on the various optical elements. The aim of this project was to implement in Python a scaled propagator to be integrated into the OASYS graphic environment for optical X-ray simulation and the study of the range of the values of the magnification factor to obtain a propagation without artifacts. This propagator is the first step towards the complete characterization of the X-ray beam and the design of a beamline. In particular, thanks to the future union with the COMSYL package, for the coherent mode decomposition of synchrotron radiation, this project is the basis of a software for the modeling of a whole beamline with the analysis of the characteristics of the partial coherence of the beam and the non-ideality of the optical elements.

Benedetta Arcaini
Characterization of the response of a CdTe x-ray pixel detector using monochromatic synchrotron radiation (2018)

The main scope of the XSpectra® multienergy radiography, a new X-ray inspection technology developed by Xnext®, is the unique identification of the materials composing an object and, hence, the correct value of its atomic effective number Zeff. To use correctly the identification method a calibrated system is needed and so it is necessary to know the detector behavior in order to eliminate undesired distortions of the spectrum due to the single photon detection in CdTe. Starting from previous studied from the literature, we have obtained our own detector response matrices due to the charge sharing, weighting potential and escape peak flux-independent phenomena. In principle the inversion of the total response matrix should lead to the correct spectrum, ie the spectrum that would be measured with an ideal detector. The largest distortion comes from the so-called escape peak, due to the loss of collected charge by the emission of fluorescence photons from the CdTe crystal before the full photon-charge conversion process is complete. We realized some measurements with sub-mm size monochromatic beam at ESRF in June 2018: here it was possible to test the XSpectra® detector under different conditions of photons energy and flux. We have observed the increment of the “hole tailing” effect as the incident energy increases because of a higher number of charge trapping events. This phenomenon causes a very important distortion at high energy, to be added to the effect of escape peaks. The same data provided an experimental estimate of the R value, the ratio between the escape peak intensity and the main peak one. Using spectra at 40 keV, 60 keV and 100 keV we found that the R parameter follows a trend with photon energy opposite to what is published in literature. Eventually we could fit peak with an asymmetric function whose shape depends on the energy. These results are providing the basis to construct a more realistic response function than what has been proposed up to now, and will allow Xnext® to apply advanced algorithms for the recognition of low density materials in X-ray images for industrial applications.

Yiones Aouadi
Simulation of Montel system (2018)

The simulation of x-ray beam properties during the transport along a beamline is important for the design, the optimization and the operation of the beamline. For reflection optical system single mirrors or particular combination of them are used to increase the focusing property of a beam. A typical example is the Kirkpatrick-Baez (KB) system, very popular at the ESRF because of its many good properties and so well studied. However, the extreme quality of the synchrotron beams that will be available with the ESRF upgraded storage ring pushes the requirement in optics to consider more and more perfect elements. During my trainership period at ESRF, I developed a python library in able to simulate a beam propagation along simple surface conic mirrors, and combination of them such as the already discussed KirckPatrick-Baez and another system, named Montel. The aim of the thesis was that to study the effect of Montel system using the builded library. xiv

Caterina Amendola
Implementation of a decoherer system at the ID17 beamline of the ESRF (2018)

Multilayer mirrors are widely used as X-rays monochromators. They represent an advantageous alternative to crystal monochromators because of the larger energy bandwidth, and consequent higher photon flux density they provide [1] . In the field of X-rays synchrotron radiation imaging the main disadvantage in the use of multilayers as X-rays monochromator is the intensity modulation of the reflected beam, due to the imperfection of the multilayer surface and to the high degree of spatial coherence of the source for large monochromator-source distances [2]. Because of beam instabilities this intensity pattern cannot be properly removed by flat field normalization and it is cause of artifacts in the images especially if acquired with Computed Tomography [2]. The use of a rotating random phase screen diffuser (referred as decoherer in this work) is sometimes employed to suppress the intensity modulation pattern [3]. However, the decoherer strongly affects the degree of spatial coherence of the beam, which is a fundamental requirement for certain X-rays Phase Contrast Imaging (PCI) techniques, widely employed in biomedical field [4]. In this thesis work, performed at ID17 of European Synchrotron Radiation Facility (ESRF), a rotating disk constituted of several sheets of commercial SiC (P400) abrasive paper was employed to reduce the intensity modulation generated by W/B_4 C multilayer monochromator. The decoherer’s effect on the optical system was studied by changing the number of decoherer’s sand paper, its speed of rotation and the object-decoherer distance. The above-mentioned parameters were optimized in order to smooth the intensity stripes pattern caused by multilayer reflection, and at the same time preserve the phase contrast signal. The experimental results were compared to the ones obtained using a Random Phase Screen model [5], implemented in simulations. The theoretical model applied in simulations show a good agreement with the experimental results, thus it allows estimating the decoherer’s effects also for different optical system configurations. Finally, Computed Tomography images of biological samples were acquired, and the images reconstructed. The results show a strong disappearance of ring artifacts when the decoherer was used with the optimized parameters.

Filippo Ghelfi
X-ray Raman scattering study of fluorinated graphites (2018)

Fluorinated graphites (CxF)n are an interesting family of materials for both fundamental and technological reasons. They are 3D-structures where the fluorine atoms are located above and below the graphite planes, alternating in different ways and presenting different type of interactions. Fluorinated Graphites are found in many applications in technology such as cathode materials in lithium batteries, in energy applications in which high energy density is required and as lubricants. These materials have been studied since the beginning of the XX century with various techniques, but the precise crystal structure, the nature of the C-F bond and the C sp2 – sp3 transition upon fluorination or physical compression are still unclear. The aim of this work is to apply X-ray Raman Scattering (XRS) to get a deeper insight on the evolution of the electronic and local atomic structure of these materials by changing the stoichiometry or by applying an external pressure. XRS is a non-resonant inelastic X-ray scattering technique that gives the unoccupied electronic density of state, giving an information comparable with X-Ray Absorption spectroscopy (XAS). In addition, the use of hard X-rays gives a real bulk sensitivity and allows to perform experiments in embedded conditions. We focused on the C and the F K-edge, at different stoichiometric quantity of F. In addition, we studied the effect of the application of static compression as a tool for tuning the electronic properties and promote the formation of new metastable phases, employing sapphire anvil cells up to a pressure of 10 GPa. Experimental data will be presented and discussed. In particular a critical comparison with previous XAS results will be given, together with details on the on-going works and future perspectives for completing the study on these materials.


Federico Meneghin
Image charge screening effect on EuO thin films (2017)

The aim of the thesis project is to study the change in the value of the Curie temperature in thin films of europium monoxide (EuO) interfaced with magnesium and magnesium oxide Interface effects can indeed dramatically modify the value of some characteristics parameters of materials, and in particular of the so-called correlated systems to which EuO belongs. A relevant part of the work was devoted to the growth and the analysis of the samples, that must be perfectly stoichiometric and of good crystalline quality. To achieve these conditions, we used molecular beam epitaxy (MBE) for the growth of the samples, and RHEED and XRR for the analysis of the crystal quality and the epitaxial growth. The chemical composition was checked with experiments of X-ray spectroscopy (XPS, HAXPES). By investigating the magnetic behaviour of the samples, with the superconducting quantum interference device (SQUID) technique, it was shown the influence of the image charge on the ferromagnetic ordering, and in the end on the Curie temperature, of the thin films of EuO In the last years an increasing interest in EuO emerged due to the technological demand of new performing magnetic materials mainly in the field of spintronics. Its large exchange splitting, which guarantees a spin-filtering of nearly 100%, its relatively simple crystal structure and the large magnetic moment of about 7 µB per functional unit, are promising features for the application of EuO as a spin-filter tunnel barrier in direct contact with Si or GaAs and with metal electrodes. Here, the properties of thin films and surfaces play a major role, an aspect still under investigation. In this context, a project like the one to which this thesis is devoted turns out to be a fundamental testing ground, and the results will enable the development of further studies also on different materials.

Giacomo Bubba
Material identification and Compton scattering effects in X-ray multienergy radiography of homogeneous compounds (2017)

Multienergy radiography is an innovative X-Ray inspection technique able to perform material identification by measuring attenuation spectrum of the inspected object. This sets it apart from conventional X-Ray techniques which either measure radiation intensity, retrieving then measurements integrated over energy, or at most divide radiation in two ranges -low energy and high energy- measuring the intensity separately in the two regions. In this thesis the physics relevant for multienergy radiography is discussed taking as reference the XSpectra technology developed at Xnext. In particular, the problem of material identification of homogeneous compounds is studied, and a method directly based on the physical processes of X-Ray interaction with matter is proposed. Furthermore, Compton effect from atomic bound electrons is studied in order to discuss the radiation scattered from an extended and homogeneous body, and see how it affects the measurements performed. In doing this, a simple model is derived and discussed. The approach is a theoretical discussion, as at the time this work was conducted there was not the possibility to perform direct measurements on XSpectra or more in general on systems which could be properly set to reduce undesired effects like pile-up. However simulations reported and discussed along the thesis support the discussion. Results obtained from simulations seem to indicate the method proposed for material identification performs better than a dual energy one taken as example of already existing inspection techniques. While the model derived for Compton-scattered radiation from an extended and homogeneous body seems to indicate its contribution to measurements depends both on problem geometry and material composition, being however at worst nearly one order of magnitude smaller than values of ideal measurements for the majority of the cases considered.

Edoardo Mariani
Lattice dynamics and phonon anomalies in Cerium compounds. The particular case of CeBi and CeSb (2017)

In this thesis work a study on lattice dynamics of Cerium Bismuth and phonon anomalies of Cerium Antimony by means of IXS and TDS is presented. The study of lattice dynamics allow to obtain the dispersion curves of the material, and from these it is possible to have access to a lot of physical properties such as sound velocities and elastic constants. The analysis of the phonon anomalies is performed at low temperature, 14 K, where it is possible to observe super-lattice structures made by a periodic succession of paramagnetic and ferromagnetic planes. At the end, in order to give an explanation to the experimental data, a possible alternative model is presented.

Manuel Apollo
Non resonant inelastic X-ray scattering investigation on a carbonate glass (2017)

Carbonate melts are ionic liquids consisting of carbonate CO3 2- molecular anions and metal cations (such as K+, Mg2+, Ca2+) that interact principally due to coulombic interactions. Natural evidences and laboratory experiments proved their existence inside the Earth, as reported by Jones et al., where they are considered important as participants in the mobility and long term storage of deep carbon, important agents of mantle metasomatism and in diamond formation; however, little is known about their physical and chemical properties at extreme conditions due to technical challenges in performing laboratory experiments at these extreme conditions. Due to the difficulties of directly studying carbonate melts at high pressures, this thesis will be focused on carbonate glasses, which are good analogues for carbonate melts as outlined by Genge and Jones and demonstrated by previous vibrational studies by Seifert, who suggested a high level of structural similitude. For example, the parameter NBO/T (non-bridging oxygen per tetrahedral cation) does not change between the molten and the glass state; for instance in silicate glasses 4

Giovanni De Vecchi
Charge order discommensurations and fluctuations in cuprate superconductors studied by resonant inelastic X-ray scattering (2017)

This thesis concentrates on the ordered phase, involving the charge degree of freedom, present in superconducting cuprates. Charge order appears, together with high temperature superconductivity, when the parent compounds are doped, both with electrons and with holes. These two ordered states compete, but at the same time they are intimately connected since experimental results demonstrate the ubiquitous character of charge order in superconducting cuprates. Therefore, it is of paramount importance the complete phenomenological description of electronic density modulations, to understand the real mechanism driving the pairing of electrons. The present work tackles the problem showing the results of simulations and of the analysis of experimental data. A brief introduction reports the main physical properties of cuprate parent compounds and the general features of charge order in this class of materials. The following chapter discusses the implications of a recent theory, which is based on the idea that the overall periodicity of the charge modulations, as measured by scattering probes, comes from the interplay at microscopic level of commensurate domains and localized phase defects: the “discommensurations”. In particular, we focus on the relationship between the charge order pattern in the real space and the peaks it generates in the reciprocal space, in order to shed light on the link between the outcomes of the STM real space measurements and the output of diffraction and scattering experiments (like Resonant Inelastic X-ray Scattering, RIXS). I also briefly present the physical properties of the resonant x-rays scattering mechanism and the instrumentation used to perform this kind of experiments, in connection with the results presented in the final chapter, obtained in a recent experiment at the synchrotron radiation facility (ESRF, Grenoble). In this final part of the thesis, we discuss the motivations for this experiment, the methods employed for the analysis of the data and the consequences of the results achieved. The main reason pushing this research is the precise measurement of the onset temperature for the charge order in NdBa2Cu3O6+δ, defined as the temperature at which the charge order becomes visible. The unequalled resolution of the instrumentation used, gave us the possibility to detect charge order at higher temperatures than those usually reported in literature, in particular higher than the pseudogap temperature T* (defined as the temperature at which a peculiar gap opens in the in the spectrum of superconductive cuprates). This experimental finding is of paramount importance, since it challenges the theories based on the correlation between charge order and the pseudogap regime. Moreover, in the underdoped sample measured, two different kinds of peaks related to charge order are clearly visible: one peak is narrow and temperature dependent, while the other (displaced in the reciprocal space) is broad and almost temperature independent. These novel experimental results are discussed following the theoretical framework provided by the group of Prof. Grilli.

Michele Tortora
Effects of uniaxial strain on charge-ordered YBa2Cu3O6.7 investigated by Raman and X-ray scattering (2017)

The following thesis presents the first intensive study on the effects of uniaxial strain on the structure, lattice vibrations and charge ordering phenomena (CDW) on YBa2Cu3O6.7. To this purpose it has been used an innovative piezoelectric-based device for uniaxial strain tuning. The efficiency of the device to deform the sample has been verified indirectly and directly by measuring lattice vibrations and distances with Raman spectroscopy and x-ray diffraction. At cryogenic temperatures the device shows a full capability to transfer the strain on the sample with a homogenous distribution. The efficiency of the device is considerably reduced at room temperature where only ¼ of the nominal strain is transferred to the sample with a much more inhomogeneous distribution. The reason has been identified in the higher plasticity of the used epoxy glue at elevated temperatures. The strain device has been succesfully used in non-resonant inelastic x-ray scattering measurements which allow to observe the evolution of the CDW signal and related phonon anomalies as a function of strain. The compression of the sample along one of the copper-oxide plane axes determine un considerable enhancement of the CDW peak, with an increase of 35 % at the critical temperature. Similar strain-induced enhancements have been observed also in the phonon anomalies around the CDW wave vector. The temperature dependence of the reported effect shows remarkable differences with respect to the magnetic-field induced enhancements of the CDW. Resonant inelastic x-ray scattering measurements confirm the previous observations and also show that the symmetry of CDW is characterized by an orthogonal distribution of uniaxial domains in the CuO2 planes. This supports previous RIXS studies and shows a common element of charge ordering observed in a large family of cuprates.

Alberto Fabrizio Riva
Expanding the accessible P/T domain of the Paris-Edinburgh Press : a versatile tool to study liquid network structures at extreme P/T conditions (2017)

The understanding of thermal profiles in high P/T devices such as the Paris-Edinburgh press (PEP) is important for ensuring the acquisition of high-quality in-situ data as well as for the identification of assembly parts that affect the overall heat performance. The PEP is one of the classical high P/T instruments used to study material properties in-situ up to 25 GPa and 2300 K. For example, it is commonly employed to determine properties of highly reactive liquids, such as their local structure, density and viscosity with X-ray absorption spectroscopy, X-ray or Neutron diffraction. However, the thermal distributions in the sample at such high T can only be obtained from calculations and so far only one study reported results on a very specific assembly type. This thesis was therefore dedicated to study the T distributions and heat performances in the three most employed PEP assemblies using finite element calculations. One of the main results of the present study are the rather small maximal $Delta T$ of all PEP assemblies (60 K at 2500 K for the most basic cell assembly), indicating that the PEP is a very well suited device for high P/T experiments. Moreover, we could reveal that T offsets between sample and calibrant material positions are lower than intrinsic uncertainties of the cross-calibration technique. The present calculations also allowed us to identify the graphite furnace and the molybdenum electrode as key assembly parts that highly affect the overall heat performance. The employment of materials with well-chosen thermal conductivities can therefore significantly increase the T performance. Finally, we conducted calculations on new optimized cell assemblies that allow to extend the current P/T domain of the PEP.

Ruggero Giampaoli
Temperature measurements at megabar pressures : direct comparison between reflective and refractive optics for the laser heated diamond anvil cell (2017)

During last decades, lots of efforts have been done to improve experimental setups aimed at studying matter at very high temperatures and pressures. Such a commitment has been most of all motivated by a growing interest in geophysical research and in possible applications in material science. Progresses in experimental methods and the increasing appeal of the topic have led many scientists to investigate different physical properties of materials at extreme conditions: phase transitions, magnetism, electronic structure and many more. The only static technique that allows to reach very high pressures (up to 300 GPa) and temperatures (thousands of Kelvin) is the laser-heated diamond anvil cell (LHDAC): the sample is squeezed in between two diamonds to achieve the needed pressure and heated with powerful lasers (≈ 100 W). Samples loaded in these kinds of cells are very small (< 1000 µm2 x 5 µm) and, therefore, performing in situ measurements is very challenging: great care should be paid in sample preparation and the experimental apparatus should guarantee stable and reproducible measurements. So far, results of different experiments do not always coincide. This is the case, for example, of melting temperatures: hundreds of K deviations have been found in different investigations of the same materials. Many melting studies are carried out in synchrotrons, where a high brilliance x-ray beam can be used to probe the sample and detecting the melting (XRD, XAS). In particular, at the European Synchrotron Radiation Facility (ESRF) two different beamlines, ID24 and ID27 have obtained quite different results in studying phase transitions. Several reasons may lie behind these discrepancies: chemical contaminations of the sample, possible misalignments between laser and X-rays, differences in melting criteria, inaccuracies in temperature measurements. In particular, high-temperature metrology has often been the object of long-standing controversies and this has been the subject of this thesis. In the LHDAC, temperature is measured spectroradiometrically by fitting the thermal radiation with Plank’s law using the grey body approximation. Since LHDAC temperature is not uniform and it could vary of about hundreds of degrees in few µm, the accuracy in this kind of measurement is ultimately limited by the resolution of the optics used to collect this radiation. The optics of the two beamlines are different: ID27 adopts reflective optics based on Schwarzschild objectives, while on ID24 temperature is detected using an infinity objective with two achromatic doublets. Both optics are used in many laboratories around the word which use laser heated diamond anvil cell to reach high temperatures and high pressures. The aim of this work has been to directly investigate the effect of optics isolating them from the other possible sources of inaccuracy in melting temperature measurements. To do so, an ad hoc experimental setup has been assembled using the beamline objectives. Many laser heating test on real W samples loaded in diamond anvil cell have been performed obtaining systematic temperature differences up to 200 K at temperature above 2500 K. The obtained spectra have been analysed using the two-colour technique and compared with the ones taken on the beamline. Furthermore, to understand the influence of optics resolution in temperature measurements, theoretical simulations have been carried out considering different point spread functions.

Alessandro Smareglia
Ab-initio XAS spectra of high pressure and high temperature metals (2017)

This work was done during my six month long experience in the Theory Group at the European Synchrotron Radiation Facility, in Grenoble, FR, under the supervision of Keith Gilmore. The instigated topic is the calculation of high quality ab initio xanes spectra of 3d metals, Fe and Co, across the melting curve in the pressure – temperature phase diagram in order to ultimately found the melting point of the cobalt at the Earth’s inner/outer core interface. The spectra were simulated using first molecular dynamics to build many atomic boxes then two different approaches were used to calculate the final spectrum: DFT + FMS and DFT + BSE.

Andrea Carlantuono
X-ray emission spectroscopy to retrieve the local spin moment in 3d transition metal compounds. The particular case of iron (2017)

A study on a large variety of iron compounds via X-ray Emission Spectroscopy (XES) will be presented in this work. In particular the Kβ and to less extent the Kα emission lines are evaluated as tools to extract quantitative information about the local 3d spin moment. Despite that the Kβ and Kα emission lines arise from core-to-core transitions, they have a strong sensitivity to the spin state which comes from the exchange interaction between the core-hole left in the 3p (Kβ) or 2p (Kα) states and the 3d electrons. The aim of this work was to carry out a systematic study on a wide range of iron compounds in order to understand the role of other factors different from the 3d shell spin like the degree of ionicity/covalency, metallicity, local coordination, etc in the K-emission spectra. We have found significant variations in the Kβ spectra for groups of samples with the same formal oxidation and spin state depending on the ligand, type of coordination or even the metallic character. These spectral changes can be interpreted in term of a reduced 3p-3d exchange interaction due to the delocalization of the 3d states. Using samples with very different nature to perform a calibration between the relevant parameters in the spectra and the nominal spin may lead to wrong conclusions. This result is of fundamental relevance to the scientific community applying K-emission to perform spin-state determination studies.

Edoardo Cappelli
Muller-Stokes calculus for X-ray crystal optics (2017)

The ability to precisely control the polarization state of X radiation is becoming more and more important in synchrotron beamlines which specialize in the study of magnetic materials. The use of crystal phase plates is one of the best alternatives to achieve this goal as they offer high yields, energy tunability, and fast reversal of helicity. The aim of this thesis work was to create a Python library for the simulation of diffractive birefringence effects in perfect crystal phase plates. This library was then to be integrated into the OASYS graphical environment for X-ray optics simulation through dedicated widgets, thereby offering both a user-friendly interface and the possibility to work hand in hand with other software and include the new functionalities into a full virtual experiment. The result of this work is the crystalpy library. It works in the Müller-Stokes formalism and uses Zachariasen theory of dynamical diffraction to compute how the diffraction from a crystal phase plate affects the polarization state of an X-ray beam. The crystalpy package in OASYS is now fully integrated with SHADOW and will in time replace and renew the crystal code in ShadowOui.


Roberto Fumagalli
Resonant inelastic X-ray scattering of iridium fluorides (2016)

In this work, we will study the electronic properties of Ir Fluorides by means of Resonant Inelastic X-ray Scattering (RIXS). The measurements were performed at ID20 beamline at the European Synchrotron Radiation Facility (ESRF), Grenoble. 5d transition metal oxides, iridates in particular, have recently been intensively explored as they display new fascinating phenomena, arising from the strong spin-orbit coupling to which they are subjected. The interest in iridium compounds is increased mostly following the theoretical prediction of high-temperature superconductivity, where the authors proposed this behaviour due to the similarity between the electronic structure of iridates and cuprates. The most known and well studied compound is Sr2IrO4, a Jeff = 1/2 Mott insulator. On the basis of electronic structure calculations, Birol et al. suggested that Rb2IrF6 may be characterised by a SU(2) symmetric ground state and therefore be a good candidate to host high-temperature superconductivity. These materials have not been extensively characterized yet, and the main goal of this work is to examine their electronic structure. We therefore carried out extensive RIXS measurements in Rb2IrF6 and other Ir Fluorides and developed a simple single-ion model for the interpretation of the data. Our results show that the eff ective trigonal/tetragonal crystal feld splitting of the 5d-t2g states is large. Contrarily to what the above-mentioned theoretical calculations predicted, we conclude that the ground state wavefunction in Ir fluorides is not close to the SU(2) symmetric case relevant for high-temperature superconductivity in iridates.

Michele Conni
Complete mapping of magnon dispersion in antiferromagnetic layered cuprates using RIXS (2016)

In the last decades, the high-temperature superconductivity phenomenon has been widely debated, and even now the physical mechanisms on which it is based are still unknown. However, its correlation with the magnetic properties of the materials in which it shows up has been experimentally probed multiple times; this suggests that a better understanding of the superconductive compounds magnetic interactions is necessary. To this purpose, the Resonant Inelastic X-ray Scattering (RIXS) technique has proven to be a fundamental method to measure the magnon dispersion relation in superconductive cuprate compounds. In particular, the ERIXS (European RIXS) soft X-ray spectrometer, set inside the ID32 beamline in the European Synchrotron Radiation Facility (ESRF), offers at the present time the best energy resolution possible (less than 55 meV at the Cu L3 edge). This instrument has started operations in 2015 and the very first experiment has consisted in the measurement of the magnon dispersion relation for three different layered cuprates compounds: NdBa2Cu3O6.1, Bi2Sr2CuO6+x and CaCuO2. The data analysis is a critical step for understanding the measured samples main features. This procedure however is not univocal, since the theoretical model of high-Tc superconductivity has not been fully developed yet. In this work, the use of the explicit t-J model has been taken into account, and it has been found to be quite insufficient in describing correctly the systems examined. A model based on effective exchange interactions is therefore proposed, which has been implemented in a fitting algorithm thanks to the SpinW Matlab toolbox (from Paul Scherrer Institute). Finally, a physical meaning to the results of the fitting is suggested, correlated to the contribute provided by the apical oxygens of copper ions to the magnetic behaviour of the inspected compounds.

Annalisa Tamborrino
Development of a GUI-based scientific software for the analysis of resonant inelastic X-ray scattering data acquired with 2D detectors (2016)

Resonant Inelastic X-ray Scattering (RIXS) is a “photon in-photon out” spectroscopic technique that probes elementary excitations in complex materials by measuring their energy, momentum and polarization dependence, with bulk sensitivity and chemical selectivity. Due to its unique features, RIXS has become one of the most innovative and powerful tools for the investigation of the magnetic and electronic structure of the systems under study. For this reason, several dedicated instruments have been built in many synchrotron facilities all over the world, exploiting the availability of high-brilliance X-ray radiation. During the last years, the instrumentation has achieved a great development, showing a continuously increasing energy resolution and improved sample position control. In fact, a new generation RIXS spectrometer for soft X-rays, called ERIXS, is now fully operative on the new ID32 Beamline at ESRF, in Grenoble. It has been designed and built by the group of Politecnico di Milano lead by prof. Giacomo Ghiringhelli and prof. Lucio Braicovich, jointly with the staff members of ID32. The group has previously designed the spectrometer AXES at the ID08 Beamline of the ESRF and SAXES at the ADDRESS Beamline of the SLS, giving a great contribution to the success of RIXS with soft X-rays. The application of RIXS in this energy range allows the study of the properties of transition elements and rare earth compounds, but the energy resolution has always been a limiting factor for a full characterization and interpretation of the experimental results. This is one of the reasons that led to the construction of a high resolution spectrometer. In effect, ERIXS features unprecedented key figures that have been achieved through an extreme optimization of the components of both the Beamline and the spectrometer. The optical layout of a RIXS spectrometer is very simple: the radiation coming from the Beamline goes to the sample and the scattered beam is dispersed by a diffraction grating, that separates the different energy components and focusses them onto a 2D position sensitive detector. Data analysis software is needed to convert these 2D images into energy loss spectra. The image on the detector is composed of isoenergetic lines along one direction and the energy dispersion direction perpendicular to these lines. The resulting RIXS spectrum can be obtained through an integration along these isoenergetic lines, which we call the Traditional (or Integrating) algorithm. In this context, the spatial resolution of the detector and the size of the charge cloud generated by the photons play a crucial role. For this reason, an innovative algorithm (Single Photon Counting, SPC) was developed in the past years, in order to obtain the photon impact position, through a calculation of the center of mass of a group of pixels containing the spots. These are the two algorithms currently in use at the Beamline to extract the spectrum from a raw image coming from the detector. The individual spectra extracted from each image then need further processing before they can be summed to the final RIXS spectrum which is usually obtained from several tens to a hundred individual spectra. In the framework of this thesis, a software was developed that is able to help with all these steps and the data analysis. In addition, some further tools were implemented to be able to determine parameters like the detector inclination or the pixel to energy conversion factor needed in the data analysis. The software comes with a graphical user interface (GUI) to make the RIXS data analysis easier and more intuitive to the users. This represents an added value for the Beamline, considering that many users from all over the world carry out their experiments with ERIXS and the available time is limited, thus a simpler and faster way to analyze the data becomes a very useful tool, both during and after the beamtime. The software is called RIXSToolbox and has been developed under the guidance of Dr. Kurt Kummer, Scientist at ID32. RIXSToolbox is written in Python, a high-level programming language, with a large standard library. This choice arises from the fact that it provides tools for many tasks but, most of all, it is completely free, thus it doesn’t require any license. It is now available online for download. A full characterization of the software will be provided in this thesis, describing all the functionalities in a simple way, supported by the real illustrations of the corresponding interfaces. The description will feature a step by step analysis on NiO real images acquired with ERIXS during the commissioning of the instrument. At the end, the resulting RIXS spectrum will be shown as the output of the last step of the procedure. The choice of illustrating this result has the dual purpose of showing the effective functioning of the software and the enhancement introduced by the utilization of the new spectrometer. NiO is a 3d transition-metal oxide and is classified as a strongly correlated electron system. These materials have received a great deal of attention over the years, because of their promising physical properties, but they are still far from being fully understood. RIXS is a suitable technique for the investigation of this type of materials. The spectrum shown in the last part of this work confirms that the new high resolution spectrometer ERIXS allows to obtain a real improvement as it can be clearly seen by comparing it with the definition and resolution of the spectral features in previous works. For the case of NiO, in fact, we will see that it is possible to distinguish the magnon peaks that otherwise would be partially covered by the elastic line. To summarize, the first half of this thesis work will introduce the new ID32 Beamline, focusing on the RIXS branch that hosts the now full operative ERIXS spectrometer. While the second one is dedicated to the presentation of the RIXSToolbox software, that is the central topic of the work, showing an example of utilization on a recently measured NiO RIXS spectrum that will be discussed in the last part.

Andrea Del Prete
An analysis of the damaging process in Europium doped BaAlMg10O17 (2016)

The present work has been carried out at the European Synchrotron Radiation Facility (ESRF) in the High-Brilliance X-Ray Spectroscopy beam-line ID26 under the supervision of Pieter Glatzel and Lucia Amidani. The topic is the study and simulation of spectroscopic data concerning the crystal phosphor BaMgAl10O17 doped with Europium or Manganese or both. Despite the high luminescence yield gained with the inclusion of the two metals, the material faces damaging when exposed to heating or irradiation. The aim is to identify the main physical phenomenon underlying the loss in luminescence starting from the two most cited in literature: oxidation of Europium and structural changes in the crystal. Two main techniques of analysis will be employed: a statistical tool, named Principal Component Analysis, and the first-principle simulation of spectral features. The first will be addressed to the study of an irradiation-related dataset, while the second will both try to calculate electronic structures and assess the reliability of the two software by comparing their results.

Anastasia Leone
Operando SXRD study of Ag(111) oxidation (2016)

The current study was done at the European Synchrotron Radiation Facility (ESRF) in the ID03 Surface Di_raction beamline, under the supervision of Dr. Maciej Jankowski. The topic of this research is the characterisation of the catalytic surface Ag(111). Silver is an important catalyst used for the selective oxidation of ethylene to ethylene oxide, a chemical whose derivatives are largely produced in industry. An optimization of this catalysis would have an huge impact from the economic point of view. For this purpose, the reactive surface species must be identi_ed. Several studies have been done on this topic, but until now this active species is unknown. The past studies have been limited by the condition of Ultra High Vacuum, necessary in surface science, that creates the so-called pressure gap between the experiment and the true working conditions of the catalyst. The aim of my work, was to _ll this gap by using the Surface X-Ray Di_raction technique to investigate the oxidation of the Ag(111)surface. Before the experiment, the sample was prepared by following typical cleaning surface procedures, such as sputtering and annealing. Then, the cleanliness and the crystallographic quality of the surface, have been checked by AES and LEED measurements. During the experiment we observed two oxygen-induced reconstructions: the well-known p-(4 x 4) and the bulk oxide. Our hypothesis, based on the observed evidences, is that the synchrotron radiation induces the oxidation process.


Stefano Grassi
Electrochemistry of graphene supported Pt nanoparticles (2015)

The data described in this thesis are from the experimental activities carried out during my stay at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). The work was supervised by Dr. Jakub Drnec from Surface Diffraction beamline ID03. We used an iridium single crystal cut perpendicular to a (111) crystallographic direction (Ir(111)) as a substrate. The clean surface was prepared under Ultra High Vacuum (UHV) condition by successive sputtering and annealing cycles and the quality of the surface was checked using Low Energy Electron Diffraction (LEED) and X-ray Photoelectron Spectroscopy (XPS). The clean Ir(111) was also further investigated with cyclic voltammetry (CV). After characterization of the clean surface, we deposited graphene (Gr) layer by physical vapor deposition exposing the surface to C2H4 at a pressure of 10-7 mbar while heating the sample up to 1300 K for 60 minutes. Again, the graphene preparation is followed under UHV condition using the same techniques as for the clean surface. Cyclic voltammetry performed after the deposition shows that the graphene layer deactivates the Ir surface reducing the electrochemical current flow. We also assessed the stability of the graphene and its interactions with the electrolyte by putting drops of different solutions on the surface (pure H2O, 0.5M HClO4, 0.25M H2SO4 and 0.5M NaClO4). The results show different behavior of the system depending on the acidity of the environment: when in contact with water it was easy to restore the sample to its original conditions, while when we used an acid solution it becomes harder to remove the water from the sample. In order to achieve a better understanding of the process, we imaged the surface topology using Atomic Force Microscopy (AFM). We again obtained different results in the two cases: in the first one, after the interaction with water, we get images similar to that of the Gr/Ir system, while when made to interact with an acid solution on the sample appeared numerous “bubble” of considerable dimension (around 100 nm of diameter). In the last step, we deposited Pt nanoparticles on top of the graphene sheet by thermal evaporation of a Pt rod, in order to investigate the possible implementation of this system into a fuel cell. From the CV we found that the graphene layer is not oxidizes during the measurement and so the Gr/Ir can be used as a catalyst support and the results obtained shows that the system could work in a fuel cell.

Paolo Pellicioli
Gafchromic film microdosimetry for microbeam radiation therapy (MRT) : comparison between a microdensitometer and an optical microscope (2015)


Luca Zanotti
Uniaxial strain on high Tc superconductor : YBCO (2015)

This project stems from the need to deepen the comprehension of the competition between the superconductive regime and Charge Density Wave (CDW) in the Cuprates, unconven- tional superconductor. Since several scattering experiments show a CDW peak in both directions (a* and b*) in the first Brillouin zone, it is still not clear whether the spatial charge order configuration is checkerboard-like or biaxial ordered. The project related to this thesis has been thought with the aim to introduce an uniaxial asymmetry (along both a or b) in YBa2Cu3O6+x (YBCO) in order to probe the charge arrangement response. The rig employed to accomplish such purpose is a piezo-electric device designed and built in Max planck Institute-CPfS by Dr. Clifford Hicks. Since all the available conventional cryostats employed in the optical measurements were not able to host the device, the initial experiments have been carried out at room temperature, and thought the course of the project several tests to characterize the device, establish the optimal preparation procedure of the samples and the pressure transmitting media have performed. Nonetheless, we have observed a change in the constant lattice of YBCO through XRD measurements and a shift in the Raman modes in Sr3Ru2O7 (SRO) (100) as strain response. In a second step, we have adapted the strain device to our optical cryostat, in order to carry out temperature dependent measurements. Encouraging results have been obtained result during the course of this project.

Fabio Zampieri
Electrochemical layer by layer growth of chalcogenide thin films on Ag(111) (2015)

This thesis project was conducted at the European Synchrotron Radiation Facility (ESRF) in Grenoble, in Surface Diffraction Beamline ID03. The project aim of the project was the deposition and the characterization of chalcogenide thin films on the surface (111) of a silver single crystal substrate. Chalcogenides are promising candidates for the fabrication of anodes for lithium-ion batteries. In fact chalcogenide based anodes show improved performances, both increasing the lifetime and the capacity of the batteries. In the experimental it is described the instrumentation used in the experimental work (the electrochemical system and the x-rays diffractometer) and an introduction to the methods used for the synthesis and the characterization of the samples (electrochemical atomic layer epitaxy, Bragg diffraction and reflectivity measurements). A brief overview on the state of the art of the deposition of antimony and zinc chalcogenides is reported as introduction to the result section. In this section methods and deposition parameters of each chalcogenide (Sb2S3, Sb2Te3, Sb2Se3 and ZnSe) are described as well as the outcomes of the electrochemical characterization. The results of an extensive structural investigation conducted on ZnSe thin films are also reported.

Andrea Carpi
Lattice dynamics of VO2 above the metal insulator transition (2015)

In this thesis a study of the lattice dynamics of vanadium dioxide by means of Inelastic X-ray Scattering (IXS) and Thermal Diffuse Scattering (TDS) is presented. The project was conducted at the European Synchrotron Radiation Facility in Grenoble. The main measurements were performed at beam line ID28 using the IXS technique, while the TDS results were obtained at beam line ID23. The study of lattice dynamics allows to obtain information about the collective excitations (phonons) in a periodic lattice. Thanks to quantum mechanics, a phonon can be seen as a quasiparticle, representing an excited state of the mode of vibrations of a crystal. Knowing the phonon dispersion allows to access various material properties, such as sound velocities, elastic constants, phonon-phonon interactions, electron phonon-interactions and dynamical instabilities. For example, phonons are responsible for the spectacular phenomenon of superconductivity: electron-phonon coupling is the driving mechanism of this process, allowing to explain the behaviour of BCS superconductors. Vanadium dioxide exhibits a Metal-Insulator Transition (MIT) at about 340, K, and this material has been widely studied along the past decades, trying to understand the mechanism driving this transformation. Here the low temperature phase is the insulating phase, while the high temperature one is metallic. The VO_{2} is neither the only vanadium oxide, nor the only transition metal oxide showing this kind of property, but it is the only one being characterized by a transition temperature so close to room temperature. In addition to being interesting from a fundamental point of view, VO_{2} has recently drawn a lot of attention since it offers a novel route to novel electronic and photonic applications. Together with the MIT, a Structural Phase Transition (SPT) takes place, suggesting that the lattice dynamics can play a crucial role across the transition. One possible model which allows to explain the MIT process is known as Peierls transition: this model attributes the radical transformation of this material to electron-lattice interactions. Nevertheless, the MIT process can also originate from electron-electron interactions, and this time the mechanism is referred to as Mott (or Mott-Hubbard) transition. Different theoretical models and a lot of experimental results have been presented to support both the explanations, but an accurate description of lattice dynamics was still missing. The aim of this work is to provide a thorough description of the lattice dynamics of VO_{2} in the metallic phase, thus above the transition temperature. Combining the IXS and TDS techniques, it is possible to obtain direct dispersion relation measurements and diffuse scattering maps. These results give a good picture of the phonon energy (and intensity) landscape, highlighting the role that lattice dynamics plays within the metal-insulator transition. The experimental results show the undeniable presence of lattice instabilities in the high temperature phase of vanadium dioxide. Indeed, low energy acoustic phonons are present in more than one high-symmetry direction of the Brillouin zone. From these results it is evident that electron-lattice interactions play an important role across the MIT, but, of course, the results themselves do not allow to exclude the presence of electron correlations.

Margherita Rezzaghi
Magnetic resonant inelastic X-ray scattering of layered perovskite iridates at high pressure and optical schemes for improving the energy resolution (2015)

This thesis work concerns a magnetic high pressure RIXS study of the Sr3Ir2O7 compound. The measurements were performed at beam line ID20 of the European Synchrotron Radiation Facility, Grenoble. The motivation behind this work is that 5d transition metal oxides have recently attracted attention due to their peculiar properties, such as their unexpected insulating behaviour. In fact the Hubbard model, which gives a successful explanation of the electronic structure of 3d metal oxides, can not be naively invoked to explain the ground state properties of these compounds, including the one of our interest, Sr3Ir2O7, the second member of the Ruddlesden-Popper series (Srn+1IrnO3n+1). The opening of the gap responsible for the insulating behaviour is due to the strong spin orbit coupling which forms the so-called Jeff = 1/2 state. However, even if a model for the ground state has now been developed, the magnetic behaviour of this material is still not well understood so far. Indeed at least two theories were elaborated to interpret the magnetic excitation spectrum of Sr3Ir2O7, but none of those satisfactorily accounts for all the experimental observations. In this thesis we intend to utilise the Ir L3 edge resonant inelastic X-ray scattering (RIXS) technique and, by applying pressure, we intend to study the response of magnetic excitations to external stimuli. This is necessary to provide new experimental evidences which pose further constraints on the theoretical models. However, those presented here are the first measurements of magnetic excitations performed with RIXS under high pressure, and unfortunately the acquired data are not sufficient to confirm any of the existing theoretical models. Anyway our study paves the way for future experiments at higher pressures, essential to develop a complete theory for magnetic excitations in iridates. This thesis also comprises a part devoted to the development of new instrumentation which allows us to improve the experimental energy resolution. The main motivation is three-fold: i) a better energy resolution would allow the separation of closely lying magnon branches or other types of excitations, which would otherwise be indistinguishable; ii) lattice vibrations, i.e. phonons, could then become accessible to RIXS. These are hardly accessible with the current set-up, but are extremely interesting as one could extract quantitative information on the electron-phonon coupling; iii) the width of magnetic excitations is nowadays energy resolution limited and a precise assessment of their intrinsic life time is precluded. Assessing the line width of excitations beyond the experimental energy resolution would shed light on possible decay mechanisms and couplings to other quasi-particles. Our calculations and simulations show that an experimental energy resolution as good as 5 meV can be achieved.

Cassiano Langini
Advances in the detection at the new XMCD and RIXS beamline of the ESRF (2015)

The European Synchrotron Radiation Facility is a synchrotron radiation source offering forefront experimental instrumentation for the investigation of the magnetic properties and the electronic structure of materials. Within a general upgrade programme of the facility, the new ID32 soft X-ray beamline has been built, offering two experimental end-stations: one for RIXS and the other for XMCD measurements. ERIXS, the new spectrometer of the RIXS branch, is designed to achieve an unprecedented resolving power, requiring the optimization of all its components. In the present work we explore the possibility of enhancing the spatial resolution of the CCD detector of the spectrometer, which can be done by calculating the centre of mass of the charge clouds generated by each photon. This approach introduces some artefacts in the reconstructed images that we were able to remove by using the so-called eta-algorithm and one of its possible extensions. In the meantime we carried out the very first experiment of the XMCD branch of ID32, studying the temperature dependence of weak ferromagnetism in samples of doped high-Tc superconducting cuprates.

Lorenzo Valzania
Strategies for the validation of a new algorithm for the local reconstruction of grains with diffraction contrast tomography (2015)

Three-dimensional synchrotron radiation X-ray diffraction imaging techniques offer the possibility for non-destructive bulk characterization of polycrystalline materials at the micrometer length scale. Minute changes in electron density (different crystallographic phases, cracks, porosities) can be detected using 3D imaging modes exploiting Fresnel diffraction and the coherence properties of third generation synchrotron beams. Three-dimensional X-ray diffraction techniques on the other hand explore the crystalline structure of materials. By exploiting the symmetry of Friedel pairs, X-ray diffraction contrast tomography (DCT) can image the 3D shape, orientation and elastic strain state of the grains in polycrystalline sample volumes containing up to one thousand grains or more. Previously employed reconstruction procedures used to assign one average orientation per grain, lacking sub-grain resolution and failing when intra-granular misorientations are larger than few tenth of a degree. In this work we explore the possibility of accessing the intra-granular crystallographic structure with a recently developed six-dimensional algorithm sampling the orientation space, which has been already successfully tested on synthetic data sets. For the first time its performances are validated on experimental data. We present a comparison between a grain map obtained from electron backscattering diffraction and the corresponding DCT reconstruction of a titanium sample, thus providing a first cross-validation of the method. Secondly, a comparison between results from the six-dimensional DCT and pinhole and section topography of a magnesium sample provides unprecedented insight into its crystallographic and sub-grain microstructure. Investigating the strong and weak points of the algorithm, the present work stimulates ideas for future improvements, which may be crucial in the study of deformation processes in structural metals employed in engineering.


Matteo Rossi
X-ray Raman spectroscopy on iridate perovskites (2014)

In this thesis a X-ray Raman scattering study of the electronic structure of the first two compounds of the Ruddlesden-Popper series Sr_(n+1)Ir_(n)O_(3n+1) (n = 1, 2) of iridates is presented. The measurements were performed at beam line ID20 at the European Synchrotron Radiation Facility, Grenoble. 5d transition metal oxides, iridates in particular, have recently been intensively explored as they display new fascinating phenomena, arising from the strong spin-orbit coupling to which they are subjected. Indeed, a simple Hubbard model, applied with great success to 3d transition metal oxides, would predict a metallic state for these 5d compounds, in view of the larger bandwidth and smaller electron correlation in the 5d orbitals; instead, some iridates, among which the samples studied, are insulators. The opening of a gap is due to the strong spin-orbit coupling which enhances the effect of correlation, narrows the effective bandwidth and isolates the so-called J_(eff) = 1/2 ground state. This peculiar ground state is strictly achieved only if the energies at play, most especially the cubic and tetragonal components of the crystal field splitting and the spin-orbit coupling, follow a precise hierarchy. The aim of this work is to determine the cubic crystal field splitting of the Ir 5d states in Sr2IrO4 and Sr3Ir2O7 by X-ray Raman scattering, a bulk sensitive and self-absorption free probe. Indeed, spin-orbit coupling strength and tetragonal crystal field splitting have already been experimentally determined by other authors. By focusing our attention on the O K edge and exploiting the orientation dependence of the spectra, we were able to assign features in the 528-535 eV energy loss range to specific transitions involving the Ir 5d orbitals. This has allowed us to extract values for the cubic crystal field splitting: 3.8 ± 0.82 eV in Sr2IrO4 and 3.55 ± 0.13 eV in Sr3Ir2O7. Furthermore, we found values for the tetragonal crystal field splitting acting on the e_g states: 1.6 ± 0.82 eV in Sr2IrO4 and 1.9 ± 0.13 in Sr3Ir2O7. This work is the first direct experimental determination of the cubic crystal field splitting in the two iridates. A complete electronic structure of the two compounds is finally achieved and the implicit theoretical assumptions, which are essential for the establishment of the J_(eff) = 1/2 ground state, are confirmed. Furthermore, this is one of the first X-ray Raman scattering studies at the O K edge of transition metal oxides in general: we demonstrate that this spectroscopic technique can be used to obtain a detailed picture of the electronic transitions in these materials and, more generally, our work paves the way for similar detailed studies of correlated electron systems such as the high temperature cuprate superconductors or nichelates. In particular, X-ray Raman spectroscopy is suitable for measurements in extreme environments, such as high pressure, and therefore it is a valid substitute of other techniques (e. g. soft X-ray absorption spectroscopy) which cannot be accomplished in such conditions.

Davide Pincini
Crystallographic investigation of gold nanoparticles embedded in a SrTiO3 thin film for plasmonics applications. (2014)

Metallic nanoparticles represent a widespread subject of study in several different fields of research, such as applied physics (nanotechnology), material science, chemistry and biology. In recent years they have been extensively studied by many authors (Kelly et al. [30], Link and El-Sayed [37], Jackson and Halas [27]) particularly with respect to their optical properties. These are characterized by the presence of the so called localized surface plasmon resonance, which earned the study of metal nanoparticles the name of plasmonics. Their properties have been probed with a wide selection of experimental techniques, ranging from microscopy (TEM, AFM, STM . . . ) to optical spectroscopy and X-ray diffraction, and a large variety of fabrication methods (Pelton et al. [44]) have been explored to obtain systems made up of nanoparticles of various metals. Furthermore, they have been proved to be promising for many different scientific and practical applications (Prasad [45] and Bell [3]), for which both the surrounding medium sensitivity of the plasmonic resonance and the associated local field enhancement play a major role. The morphological and structural features of the metallic nanoparticles are strictly related to their properties in most of the applications mentioned above. These features mainly depend on the particular preparation method exploited for the nanoparticles production: a fundamental understanding of the atomic-scale processes involved in the nanoparticles formation is thus of great importance. The samples object of the present work are composed by anisotropic monocrystalline gold (Au) nanoparticles embedded in a strontium titanate (SrTiO3) thin film. They were prepared through a novel two-steps deposition process, whose main parameters can be varied in order to obtain nanoparticles of different size and shape (Christke et al. [13] and Katzer et al. [29]). This process therefore allows to tune the optical properties of the nanoparticles and it constitutes a valid alternative to other traditionally used fabrication processes, particularly for the production of plasmonic active sensors in life sciences. Nanoparticles prepared with an analogous deposition process have been also exploited as flux pinning centers in high temperature superconducting YBCO thin films (Grosse et al. [23] and Katzer et al. [28]) or for the engineering of YBCO grain boundaries in Josephson junctions (Michalowski et al.[40]). The study here presented consists of a crystallographic characterization realized through synchrotron X-ray diffraction. The aim was to determine the preferred crystallographic orientations of the Au nanocrystals and their interaction with the surrounding SrTiO3 matrix. It represents the first step of a research project addressing the nanoparticles features (such as the shape and the dimension) with an impact on their optical properties and the modifications induced in the surrounding matrix. Samples with different amounts of deposited Au were probed with a hard X-ray beam and two different diffraction setups were used, exploiting both a two-dimensional and a zero-dimensional detector. The main vertical (normal to the substrate) growth direction of the nanoparticles was determined for all the samples, along with the crystalline quality of the SrTiO3 layer. For each vertical growth direction the in-plane orientation of the Au crystals was measured, in order to fully determine their crystallographic orientation with respect to the substrate. The correlation between the nanoparticles orientation and the amount of deposited Au was investigated, trying to understand the role of the SrTiO3 thin film in the nanoparticles formation process. Such a characterization is not only important in the light of the potential practical applications of the samples, but it is also valuable in itself. It indeed offers the possibility of a deeper understanding of the fundamental properties regarding the growth of Au (and transition metals in general) on ceramic substrates, which are still not well explored. The present work is organized as explained hereafter. After a general overview of metallic nanoparticles (Section 1.1), Chapter 1 describes the samples experimentally probed, with particular focus on the preparation methods used and their potential practical applications (Section 1.2). Chapter 2 provides the reader with the main theoretical concepts involved in the measurements presented in the following chapters: Sections 2.1 and 2.2 outline the basics of the X-ray diffraction technique and the properties of transition (face centered cubic) metals deposited on ceramic substrate respectively; then, Section 2.3 describes in detail all the mathematical framework necessary for the description of a diffraction experiment performed with a CCD area detector. Following the presentation of the experimental setup of Chapter 3, the main experimental results obtained are discussed. In particular Chapter 4 presents the preliminary textured analysis undertaken and the main vertical growth direction of the Au nanoparticles (along with the modification induced in the SrTiO3 thin film). Chapter 5 deals with the in-plane investigation of the Au crystals, addressing its relation with the presence of the SrTiO3 layer and the amount of Au deposited. Finally the appendices provide a brief overview of the properties of synchrotron radiation (Appendix A) and some details about the program used to performed the CCD diffraction data reduction (Appendix B). In Appendix C the most important Python scripts used for the data analysis are reported.

William Capra
The magnetic phase diagram of the magnetoelectric EuTiO3 studied by neutron powder diffraction (2014)

At room temperature (RT) EuTiO3 (ETO) is a prototypical perovskite with a long range magnetic order appearing below T_N = 5.5K. The magnetic moments are provided by the Eu ions which arrange in a G-type structure. Due to the strong absorption of natural Eu neutron diffraction experiments on ETO are difficult, data quality is limited and details of the magnetic structures (moment directions) have been refined only very recently, by resonant x-ray diffraction. Below T_N magnetoelectric (ME) properties appear in the ETO dielectric constant dependence versus applied magnetic field. Although no long range order of the electric polarisation is realized, in ETO the microscopic mechanisms source of the ME coupling are still matter of debate. In parallel, a structural phase transition just below RT has been detected and macroscopic magnetic measurements have questioned the magnetic structure deduced by x-rays, proposing a new magnetic phase diagram characterised by a spin-flop state at temperatures close to T_N. Clearly, in this panorama a systematic investigation of the magnetic structure of ETO by a microscopic technique with great thermal stability was necessary in order to suitably clarify the background for the ME properties development. This is the aim of this thesis. In order to achieve our goal we exploited neutron powder diffraction (NPD) technique as an investigation tool, by pushing the technique to its limits. For this reason specific NPD experiments were performed at the very high-resolution diffractometer D2B of the ILL in a special configuration and a new Python code was develop to finely correct the data. Solutions of ETO structures by Rietveld methods are presented together with the experimental methods employed and the correction strategies developed. An extensive investigation of the influence of absorption correction uncertainties due to experimental parameters and specific procedures implemented are discussed. As a result of our work we can state that ETO spin moments always lies in the a,b-plane over the temperature range investigated. We finally discuss the magnitude of the magnetic moment found higher than the values previously reported.

Luca Piovani
Mid-infrared tunable, carrier envelope phase stable, pulsed source (2014)

In the recent years growing attention has been paid in Mid-Infrared (Mid-IR) few cycle sources. These have been demonstrated to be capable of both driving and probing a vast multitude of low energy elementary excitations in molecules as well as in solids. Control over this processes is particularly fascinating in strongly correlated materials where even tiny changes in the crystallographic structure can lead to gigantic effects1. For this and many others applications2 shot to shot reproducibility of the pulses’ electric feld both in frequency and phase is crucial. The difference frequency generation process between Near-Infrared (NIR) beams coming from Optical Parametric Amplifers has been successfully exploited for long a time to generate intense Mid-IR feld with almost transform-limited duration. The relative temporal offset of the carrier of a Mid-IR pulse in respect to its envelope, called Carrier-Envelope Phase (CEP), is one of the most important parameter in the description of the electric feld of such pulses. Hence, good control over its stability is needed in all of the above mentioned scenarios. Passive stabilization methods can be used to generate Mid-IR pulses with intrinsically locked CEP. Indeed, difference frequency mixing of NIR pulses obtained by the same broadband super-continuum white light generates CEP stable Mid-IR felds. Often this is not enough to maintain the CEP constant over long periods of time, therefore active stabilization methods are also employed. In this text we present an experimental setup for generating Mid-IR pulses with the above characteristics. After some needed general theory and practical knowledge are given we discuss the possibility of a new time-domain stabilization method based on the well known free space Electro Optic Sampling (EOS) technique. Hence we compare it with the already working frequency-domain approach based on similar processes, which has been demonstrated by Manzoni et al. in their paper[1]. We then prove a proposed method to generate Mid-IR pulses with narrow-band spectra that can be used in particular experiments that call for an higher spectral resolution.


Mauro Fanciulli
Spin orbit and crystal field excitations in Cerium compounds probed by resonant inelastic X-ray scattering (2013)

In this Thesis a study of low energy excitations in three Cerium intermetallics by means of soft X-ray Absorption Spectroscopy (XAS) and Resonant Inelastic X-ray Scattering (RIXS) is presented. Measurements were performed at beamline ID08 of the ESRF. Rare Earths materials are peculiar because the partially filled 4f shell is well localized at the atomic site but lies close to the Fermi level. The 4f electronic structure of Rare Earth atoms is dominated by Spin-Orbit coupling (SO), and in Cerium, having one 4f electron, the split is into the two states 2F5/2 and 2F7/2. In addition, the interaction with the Crystal Field generated by the surrounding charges produces a further splitting of the (2j+1)-fold degeneracy because of a lowering of symmetry. Aim of this work is to analyze the excitations involving two f levels, so-called ff excitations. The levels can have a different total momentum J (SO excitations) or share the same J (CF excitations). From XAS we measured a different valence in the three samples Nd1.84Ce0.16CuO4, CeRu2P2 and CeCu2Si2: 4+, 3+ and mixed, respectively. Moreover, differences between Fluorescence Yield and Auger Electron Yield are discussed. X-ray Linear Dichroism is small, thus Crystal Field effects are small. Scattering measurements in resonance with M5 edge from Ce4+ sample show only the elastic line, as expected because no excitations shall take place in the empty 4f shell. In addition, a fluorescence feature in RIXS spectra is observed at higher energy. On the other hand, scattering from Ce3+ reveals an inelastic feature ascribable to SO excitation. CF excitations are not well resolved, nevertheless the broadening of inelastic features much larger than the spectrometer resolution suggests a CF substructure. Measurements on the M4 edge show in addition a high energy loss due to Coster-Kronig conversion. This work is a first measure by RIXS of the SO splitting in Ce, 300 meV, in agreement with ARPES measurements. Even if the resolution was not sufficient for CF excitations, there is a good fitting with calculations performed with parameters obtained by neutron scattering. This is promising in sight of the forthcoming costruction of a new high resolution spectrometer.

Andrea Amorese
Single photon acquisition in CCD detectors for high resolution resonant inelastic soft X-ray scattering (2013)

Resonant Inelastic X-ray Scattering (RIXS) is a synchrotron based spectroscopic technique that in the last 15 years has been growing to become one of the most innovative and powerful tools for the study of the magnetic and electronic structure of solids and molecules. One of the most profitable applications of RIXS is connected to the study of magnetic and electronic properties of transition elements and rare earth compounds. RIXS offers in fact the unique possibility of studying energy- and momentum-resolved neutral elementary excitations, with bulk sensitivity and chemical selectivity. Among others, one of the most original recent applications of RIXS has been the discovery of charge density modulations in high Tc superconducting cuprates, in particular underdoped YBCO. In this thesis I will present some recent results showing the presence of charge density waves (CDW) also in optimally doped Bi2212. This very important discovery can help to shade light on the nature of these charge orderings and of the pseudogap phase. The detection of this ordering was possible thanks to the momentum and energy resolution of RIXS, that allowed the study of a very small modulation of the elastic peak, by separating the elastic signal from the rest of the spectrum. Being this technique so promising, in the last years the synchrotron facilities have designed and built several dedicated instruments, with increasing energy resolution and sample position control. Among these, the group of Politecnico di Milano lead by Prof. Giacomo Ghiringhelli and Prof. Lucio Braicovich has been contributing to the design and construction of a new high resolution RIXS spectrometer for soft x-rays, called ERIXS, that will be mounted on the new ID32 beamline at ESRF. The group has a well-established experience in the field, having designed the spectrometers AXES, mounted at ID08 beamline at ESRF and SAXES, mounted at ADRESS beamline at SLS. Both these spectrometers have had a crucial influence on the success of RIXS, having provided almost all the important RIXS results of the last 10 years in the soft x-ray range. The ERIXS spectrometer at ID32 has been designed to improve the energy resolution by 5 times with respect to AXES at ID08, while increasing the overall count rate by a factor of 3. These are unprecedented performances that can be achieved only through an extreme optimization of all the components of the beam line and of the spectrometer. There comes the work described in this thesis, which deals with an innovative utilization of the CCD detector already in use on AXES. In this spectroscopy, the energy of scattered photons is determined by dispersing them with a diffraction grating and imaging their intensity distribution vs diffraction angle using a position sensitive detector. One of the crucial limiting factors is then the spatial resolution of the detector. The most used image sensors for x-rays applications, thanks to their high efficiency, low noise and versatility, are the charge coupled device (CCD) detectors. In these commercially available devices, the spatial resolution has been reported at ~25 micron FWHM independently of the pixel size, for soft x-rays. When a photon is absorbed in the active layer at the surface of the detector a large number of electorn-hole pairs are generated, and an electron cloud diffuses in the material before being captured by the potential wells of physical pixels fabricated in the wafer. Depending on the pixel size the charge cloud is spread over several neighboring pixels, thus reducing the spatial resolution of the detector. On the other hand, these spots can allow the use of centroiding techniques to determine the exact position of impact of photons, achieving a resolution which is better than the pixel size. This thesis describes the development and test of an algorithm that allows a significative enhancement in the detector resolution, using a single photon centroid reconstruction. The determination of the photon impact position is based on the calculation of the center of mass (COM) of a group of pixels containing the spot, i.e. it is performed an average of the pixels’ positions using as weight the signal accumulated inside each pixel. In order to evaluate the actual performances of our code (effective spatial resolution, speed and limitations due to overlapping events), a resolution test was carried out in the labs of Politecnico di Milano, using a traditional x-ray source. The experimental set-up allows a very direct measurement of the resolution, by analysing the sharpness of the shadow of a razor blade edge on acquired images. The detector used for these tests is a CCD of Princeton Instrument (PI-SX 1300, 20 micron pixel size), used at ESRF until some years ago for soft x-rays detection. In addition to the experimental test the software was used also on “virtual” images obtained by MonteCarlo generation of photon impact and of the relative charge cloud. In this way the ultimate performances of the code could be tested under controlled conditions and the effect of pixel size, cloud dimensions and readout noise on the final result could be simulated. The resolution achieved is excellent in both the tests, being around 7 micron on experimental data and 2 micron in the ideal case of simulations. The experimental value is excellent because it would fulfill the target of 10 micron resolution required for the optimal performances of ERIXS. The theoretical limit of 2 micron although obtained by neglecting the effects of read-out noise, demonstrate that a large margin of improvement is available for the future. The discrepancy between simulations and measurements can be attributed, at least partly, to geometrical limitations in the optical system used for the tests. Despite the encouraging results on resolution, some artifacts were observed on the images after the elaboration, due to possible errors related to the COM calculation. These can occur when some pixels of a spot are not considered for the reconstruction of a spot, or if their signal is overwhelmed by noise. Several methods to overcome these artifacts are under study while writing this thesis and they bode well for the elimination of the problem. Another problem related to the use of the software for synchrotron spectroscopies arises from the requirement of a low photon density on the images to be analyses with the developed software. As a matter of fact, the centroid reconstruction can work only if the spots generated by the photons do not overlap each other. A low local density of photons accumulated during each single exposure of the CCD to the incoming beam implies a much more frequent reading of the CCD in the single photon detection mode than in the traditional one. A quick test performed at ESRF to evaluate the exposure time under typical experimental conditions indicates that an exposure time lower than 40 seconds is necessary. The readout time of the CCD is around this same value if the whole detector area is used in the low noise / low speed read-out mode to acquire the image, but can be reduced to less than 10 seconds by selecting only a smaller region. However, this need of acquiring many images with short exposure time for every spectrum causes a significative loss in duty cycle and therefore a loss in efficiency, which is the most important limiting factor to the use of the centroid reconstruction methods on CCD detectors. Ultimately, the work presented in this thesis is a trailblazing study of the possibility of the single photon counting centroid reconstruction in the soft x-rays range. As a matter of fact, these resolution enhancement methods are already well established for photons of higher energy, which produce a bigger charge cloud that simplify the use of centroid reconstruction algorithms. Here, it is demonstrated the possibility of achieving an important resolution enhancement through a software elaboration of images acquired by an high quality traditional CCD detector for soft x-rays. The possibility to use a traditional detector instead of one specifically designed for this application is of fundamental importance. Apart from the lower cost of the device, the possibility to use a single detector to acquire images to be elaborated with the traditional or with the single photon counting algorithm can allow to shift between the two methods just by changing the CCD acquisition parameters. The only but important price to pay for using a traditional CCD detector instead of those designed for this purpose is a loss of duty cycle and thus a loss of overall efficiency of the RIXS apparatus: this is a crucial limitation for a technique hampered until now mainly by the counting rate. Anyway this problem can be reduced by modifying the parameters that influence the readout time and it will be probably further canceled during next years thanks to the increasing reading speed of these devices. The software is expected to be ready to be used for the application in synchrotron spectroscopies by the first RIXS experiment of ID32, foreseen for the last months of 2014.

Eric Vezzoli
Miniature plastic high pressure cell for x-ray spectroscopy in multi-extreme conditions (2013)

Since its first observation in 1947 at the General Electric Research Laboratory in New York, synchrotron radiation has been increasingly used by the scientific community thanks to the development of facilities able to exploit this sort of radiation as a tool to investigate the world down to the nanometer and angstrom scale in the universe of atoms and molecules. Third generation synchrotron facilities are dedicated structures to generate intense and directional x-rays and made it possible to exploit the power and brilliance of radiation with spatial, temporal and energy resolution. This thesis focuses on the development of energy dispersive absorption spectroscopy in multi-extreme conditions of temperature pressure and magnetic field. This technique studies material properties by observing X-ray absorption which is sensitive to its composition, local and electronic structure and magnetism. This gives to spectroscopy its peculiar characteristic of being element and orbital selective. The first two chapter of this thesis are dedicated to the properties of synchrotron radiation, the facilities for its generation and exploitation and to an overview of absorption spectroscopy techniques. Different techniques as XANES – X-ray Absorption Near Edge Structure and EXAFS – Extended X-ray Absorption Fine Structure are available to study the absorption of X-rays from materials of different composition. This technique provides information on the matter and electronic structure of materials. After this introduction, the beam line ID24 structure and the set-up for experiment in multi-extreme conditions will be described. A pulsed magnet coupled with a He flow sample cryostat and the particular set-up of ID24 beam line provide time resolved measurements on ms scale in fields up to 30T and temperatures down to 1.5K. The energy-dispersive concept and the high photon flux of this the beam line make it possible to acquire a full spectrum around the absorption edge in microseconds. After this technical introduction, chapter 3 gives an overview on strongly correlated electrons systems. In these solids the presence of unfilled d− or f− electron shells leads to the impossibility to explain the electronic behaviour with a non interacting electron-model. The appearance of high-T superconductivity, strange magnetic properties, Kondo effect and other exotic properties not described by classic theories, has raised interest in these compounds. In particular I will focus on heavy fermion compounds for their ability to have their properties tuned by pressure and magnetic field as well as temperature. The appearance of superconductivity driven by antiferromagnetic fluctuations and the presence of phase transitions determining Quantum Critical Points suggest more studies on this materials. The lacking of a system that can provide high pressure, low temperature and high magnetic field X-ray absorption studies lead to the first part of my work: the development of a miniature high pressure cell for pulsed magnetic field and cryogenic temperature. Detailed simulations on eddy current heating and forces arising from the interaction of the induced currents with the field lead to the conclusion that even a non-magnetic metallic cell could not fit our purpose. We therefore developed a cell of the turnbuckle type made entirely by engineering plastics to fit into the limited sample space inside the cryostat of the pulsed field set-up. In the following we proceed on the design and realization of this new cell and on the laboratory commissioning of it. To demonstrate the performance of the cell I observed the colour change at 3.5GP a of a sample of Crocoite P bCrO4 combined with a pressure measurement by the the ruby fluorescence technique. After that the same cell was tested in the multi-extreme condition set-up to demonstrate the operation of the cell for studies through the acquisition of spectra from a Cu foil during the field pulse. After this technical part we show a study of the valence as a function of temperature in the mixed valence compounds Y bCu2Si2 and Y bRh2Si2 with spectra taken on BM23. In this Heavy Fermion Compound(HFC) systems the 14th electron of the 4f− shell of the Y b2+ ion becomes a delocalized and goes into the Fermi sea as a function of the temperature. In these samples the competition of different phenomena leads to the presence of 2 valence states. Through the work carried out during this thesis, X-ray spectroscopy in multi-extreme conditions of field (30T), temperature (1.5K) can be combined with pressure up to 6GP a for the first time. This significantly increase the thermodynamical phase space accessible for the study of strongly correlated electron systems.


Greta Dellea
Charge transfer effects in Co-phthalocyanine single molecule magnets on graphene (2012)

In this thesis we investigated the electronic and magnetic coupling in hybrid graphene-single molecule magnets systems. This project is the result of a collaboration between the ESRF (European Synchrotron Radiation Facility) and the NANOsciences department of Istitut Néel, CNRS, Grenoble. Graphene samples have been produced using a chemical vapor deposition (CVD) technique that both allows to obtain large graphene areas and to transfer them on dielectric substrates (SiO2/Si++ and SrTiO3) in order to perform field effect measurements. The morphological and electrical characterization of our samples shows that they are formed by a single graphene layer with only a few percent of defected areas and intrinsic hole (graphene/SiO2/Si++) or electron (graphene/SrTiO3) doping. Cobalt phthalocyanine (CoPc) single molecule magnets have been deposited in-situ and at room temperature on graphene substrates and on a highly oriented pyrolytic graphite (HOPG) substrate used as a reference. The evaporation process has been calibrated in order to achieve a coverage of 10% of a monolayer of molecules on the surface. From X-ray absorption spectroscopy measurements (X-ray magnetic circular dichroism and X-ray linear dichroism) we found that CoPc molecules do not interact with HOPG or hole doped graphene, so that the free molecule electronic ground state is retained. Opposite to that, evidence of a charge transfer effect from the substrate is observed in CoPc deposited on electron doped graphene, resulting in an almost complete quenching of the magnetic moment associated to the central cobalt ion. These results are similar to experimental results and theoretical models for CoPc deposited on metals but in addition suggest the possibility of progressively controlling the magnetic moment of CoPc just by changing the density of carriers at the Fermi level in graphene.

Matteo Bianchini
Resonant X-ray spectroscopy of fractal TiO2 structures for photovoltaic applications (2012)

From its first observation in 1947 at the General Electric Research Laboratory in New York, synchrotron radiation has been increasingly used by the scientific community thanks to the development of facilities able to exploit this sort of radiation as a tool to investigate the world down to the scale of nanometers and angstroms, i.e. the world of atoms and molecules. Third generation synchrotron facilities, structures able to generate very intense and directional x-rays, made many different applications possible thanks to the chance to exploit power and brilliance of radiation, together with that one to choose spatial, temporal and energetic resolution. In this thesis an important application of synchrotron radiation is discussed, namely x-ray spectroscopy. Such a technique studies the properties of materials by observing x-ray absorption and emission, which depends on the material itself and on its electronic structure. This needs the ability to govern x-rays’ energy which can be achieved at synchrotrons. Moreover, this gives to spectroscopy its peculiar characteristic of being element selective, i.e. able to probe only the selected element and to observe how it behaves accordingly to its neighborhood. The first two chapter of this thesis are dedicated to the description of synchrotron radiation and its properties, together with the facilities that support its generation and exploitation, and to an overview of spectroscopic techniques which are currently used. In fact, spectroscopy developed and improved and many techniques such as XANES (x-ray absorption near edge structure), XES (x-ray emission spectroscopy) and RIXS (resonant inelastic x-ray scattering), became available to study the absorption and emission of x-rays from samples of every kind. XANES, in particular, probes the unoccupied levels above the Fermi energy and thus it is an interesting tool to study the electronic structure in this energy range. Somehow complementary is XES, which probes the occupied levels below the Fermi energy (valence levels); for this reason the two techniques can give a full understanding of the whole electronic structure in the neighborhood of the considered element. RIXS is an even more complete technique, because it combines the two processes in a second order photon-in photon-out technique which provides a bidimensional map rich of informations about the electronic structure. In this thesis the techniques above were exploited to study titanium dioxide (TiO2), a semiconductor material of great interest in the fields of catalysis and photovoltaic energy generation. Indeed, this material is employed as a photoanode in Dye Sensitized Solar Cells (DSSCs) which are an interesting and promising alternative to silicon based solar cells, since they are hybrid cells with very low production cost. Together with such a photoanode, indeed, there is a dye which provides the light absorption and an electrolyte (a redox couple) which closes the circuit between it and the dye, since it makes the electron recombination possible. Particularly, the physics of these cells (also known as Gr ̈atzel cells) is described in chapter four and special attention is given to the fact that they can be improved by the use of nanostructured TiO2 photoanodes that give the possibility to increase the light absorption of the cell because they maximize the surface-to-volume ratio and give the highest efficiency of electron transfer at the dye/semiconductor interface. In the attempt to find an even better efficiency, photoanodes were deposited at Politecnico di Milano using Pulsed Laser Deposition (PLD), a technique that allows to confer to TiO2 a peculiar morphology, resembling that one of a forest where every tree contributes to obtain an extremely high surface. These structures with extreme porosity were proved to be useful to hamper electron recombination and improve electron transport; thus their properties are studied for a possible use in industrial devices. For this reason, in the final part of this thesis a sample of nanostructured TiO2 and a similar sample sensitized with “black dye” were studied by means of XANES, XES and RIXS spectroscopies performed on the K edge of titanium in order to figure out the influence of such a dye on the semiconductor, since a better understanding of the physics that governs the electronic structure of these complex interfaces is necessary to obtain better efficiencies and therefore better photovoltaic devices. In particular, data analysis was addressed to understand whether there were differences in the energetic levels close to the Fermi energy between the sample with and without dye. Such modifications were observed, although this was not clearly predictable from the beginning because x-ray spectroscopy (differently than electron spectroscopies) is more a bulk sensitive technique than a surface sensitive one. Thus this means that the interaction between the dye molecules and the semiconductor atoms is deeper than expected and that surface atoms have a dominant contribution in the sample. Besides, chapter three of this thesis is a parenthesis that wants do describe X- count, a Matlab-based software that was developed at the European Synchrotron Radiation Facility to simulate the whole absorption and emission process, from the incident X-rays, via their interaction with the absorber atoms in the sample, to the fluorescent emission of another X-ray photon and its collection from a detector. This program mainly answers to the necessity to estimate the result of an experiment before performing it, since during beamtimes scientists have not much time and they must know whether an experiment is possible or not, i.e. if it gives an useful signal and which are the critical factors to improve it. Also, X-count gives the possibility to simulate and estimate many further effects, such as self absorption or emission of fluorescence from other atoms in the sample than the absorber atom. Thanks to these skills and to its utility, the program is currently used at the ESRF to support the planning of many experiments and this will hopefully bring to its further improvement.


Nicola Roberto Viganò
Design and development of a tomographic library with physical corrections for quantitative analysis (2011)

This thesis work is about the design and the implementation of a tomographic reconstruction library based on the Algebraic Reconstruction Techniques (ART), with the addition of a peculiar set of corrections. These corrections try to introduce some physical processes into the ART framework, in order to improve the reconstruction quality, and possibly deal quantitative results. The mathematical background, the implementation choices, and the code samples reported in this text, are the basis for the FreeART library, which is a tomographic library started from scratch and developed by the author, from the beginning of October 2010 until the end of March 2011 at the ESRF, Grenoble, France. This text can be considered both a handbook for the library, and a tutorial on how to write an ART code, with physical corrections. FreeART is an open source library and is distributed under a LGPLv2+ license, which makes it extremely useful for people interested in learning how to write a Simultaneous ART tool, or in using it freely. The sources can be downloaded from the website of the project: Where it is also possible to find information about the product and user guides.

Valerio Oliana
Role of oxygen vacancies in EuO thin films (2011)

Europium Oxide (EuO) is a ferromagnetic semiconductor with a Curie Temperature (Tc) of 69 K and a band gap of 1.2 eV at room temperature. Upon electron doping it shows spectacular metal-insulator transition (MIT) at TC and colossal magnetoresistance (CMR), whereas the change in resistivity can exceed 13 orders of magnitude. The exchange splitting of about 0.6 eV of the conduction band in the ferromagnetic phase leads in Eu-rich EuO thin lms to obtain 100% spin polarized conduction current, thus making of EuO a promising material for spintronics applications. The Curie temperature can be enhanced by doping with rare earths, e.g. Gd, and by exerting hy- drostatic pressure. The present thesis describes the work I carried out in the group of Prof. Liu Hao Tjeng at the II Institute of Physics of the University of Cologne in cooperation with the Max Planck Institute of Dresden (Germania). It deals with the growth of Europium Monoxide thin lms and in particular with the correlation of temperature dependent transport and magnetic properties with the oxygen de ciency concentration in electron-doped Eu-rich EuO thin lms. The lms were prepared on YSZ(100) substrates using the Molecular Beam Epitaxy (MBE) techinique under Ultra High Vacuum (UHV) condition, which allows to grow single-crystalline EuO films with fine tunable stoichiometry. A wide range Eu-rich EuO samples with di erent stoichiometry were grown, by systematically varying the oxygen pressure and substrate temperature. The quality of the films was monitored by means of surface sensitive electron di raction techniques, i. e. Refection High Energy Electron Diffraction (RHEED) and Low Energy Electron Diffraction (LEED). Subsequently the spectroscopic, transport and mangetic of each film were analyzed by means of X-Ray Photoemission Spectroscopy (XPS), resistance and Superconducting Quantum Interference Device (SQUID) measurements, respectively. Those properties were then related to the oxygen vacancy concentration, in order to find out the limiting concetration leading to MIT to occurr.