Resonant Inelastic X-ray Scattering
One of the main activites of the POLIMIX group is the investigation of Strongly Correlated Systems and other Quantum Materials using Resonant Inelastic X-ray Scattering and other synchrotron-based tehcniques.
Our main tool is Resonant Inelastic X-ray Scattering, an innovative X-ray tehcnique whose technical development and scientific exploration in the last 15 years carry the name of our group.
The first modern RIXS spectrometers were built here at the Physics Department, and then mounted at ESRF and Swiss Light Source. After that, our group took an active role in the design of the ERIXS spectrometer at the ESRF, which started operation in 2015, and hRIXS, the new hRIXS spectrometer of the European-XFEL, the first soft x-ray RIXS intruments for pump-probe experiments.
In the last 5 years, many more RIXS spectrometers were built in state-of-the-art synchrotron in the United States, England, Sweden, and Germany.
What is RIXS?
RIXS is a photon-in photon-out technique which inherits the characteristics of two different families of x-ray tehcniques: spectroscopy and diffraction. The process is similar to the more-common (non-resonant) IXS and Inelastic Neutron Scattering (INS): we shine our materials with monochromatic x-rays, and by analysing the energy and direction of the scattered photons we can retrieve the energy and momentum of the excitations created in the material during the scattering event.
However, differently from IXS and INS, we tune the photon energy to a core-absorption edge, such as the L-edge of transition metals, the K-edge of light elements, or the M-edge of rare earths. This resonant character brings tremendous advantages:
- element and even site-sensitivity inside the unit cell, meaning that we analyse the excitations of just the atomic species we have selected
- orders-of-magnitued increase of the cross section which allows us to study not only crystals, but also thin films and even monolayers, overcoming the limitations of INS
- access to purely-magnetic excitations.
We currently perform experiments using many RIXS spectrometers in synchrotrons all around the world.
What do we study?
Our interest lies in Strongly Correlated Materials, were the interaction between electrons produces extremely complex and fascinating physics.
Among them, our main focus is on High Temperature Superconductors. In the last years, we have obtained a number of important results in the field:
- We have discovered different forms of charge order in YBa2Cu3O7-x and other HTS families
- We have used RIXS to measure the crystal field energies in all HTS families.
- We have extensively studied their magnetic excitations, establishing the presence of short-range AF correlations even at high doping levels.
- We have for the first time used RIXS to measure the electron-phonon coupling in cuprate superconductors.
Our group has also worked extensively in the field of iridium compounds and other candidates for the observation of Kitaev physics: in these systems, the reduced value of the spin and the strong geometric frustration are predicted to generate complex physics, with the appearance of Majorana fermions.
Design of a lab-XAS spectrometer
X-Ray Absorption spectroscopy (XAS) is a powerful technique used to characterize materials, which can give information on the present atomic species, their oxidation state and their electronic configuration.
However, performing XAS experiments outside synchrotron facilities is extremely challenging due to a lower photon flux, larger spot size and much lower brilliance.
The NeXt-GAME project aims to overcome this limitation. We are currently desiging a new table-top XAS spectrometer to be installed here at PoliMi, to be used jointly with the Chemistry Department for studies on new materials, next-generation batteries, catalysis and biochemistry.
The mechanical and optical components will be designed in collaboration with Cinel, a top-level company in the field of x-ray and vacuum technology.
What are we doing?
The correct design of the spectrometer requires extensive ray tracing simulations to optimize the energy resolution, the flux and the spot size on the detector.
In the near future, we will characterize the performance of the source and the dector.
XNext is an Italian company specialized in the development of advanced X-ray inspection systems. It is owner of XSpectra patent, a disrupting inspection system based on the new concept of multispectral X-Ray analysis. The goal is to identify the nature of contaminants and non-conformities in the inspected products through an X-ray transmission spectroscopy.
Our work mainly deals with the optimization of the detector performances.
It is the same system that is mounted on Xnext X-ray inspection machines. It consists of a detection unit, made of a linear array of CdTe pixels, characterized by an integrated circuit (ASIC) for the data acquisition and management. The X-ray sources at our disposal are different type of X-ray tubes and an Americium radioactive source. The measurements are done in appropriate leaded chamber.
Pockels effect experimental setup
We will arrange an experimental setup to electro-optically measure the electric field distribution inside Xnext detectors.
IR transmission microscope
We are currently buying a IR/visible microscope to study the CdTe quality, spotting Te inclusions inside the crystals
Unfortunately, CdTe Schottky diodes suffers a time instability known as bias induced polarization due to crystal defects (especially Cd vacancies). These defects produce deep trap states in the semiconductor gap that can be thermally filled, the build up of charge over time downgrades the spectroscopic performances of the detector deforming the electric field profile. We are currently working to solve these stability problems.
In the near future we will focus our efforts in set up a Pockels system to map the electric field in CdTe crystals. Parallelly, we will map Te inclusions in CdTe crystals to correlate these microscopic defects with pixel response.