The response of a material to an external electromagnetic stimulus is encoded in the dynamical susceptibility χ(q, ω). In quantum materials, the full understanding of this property in the energy and momentum space is particularly relevant because of the sizeable level of correlation, which makes the spin and charge degrees of freedom highly intertwined. Although resonant inelastic x-ray scattering (RIXS) spectra are directly related to both the charge and the spin contributions, only the latter has been extensively studied with RIXS so far. In this work we carefully detail how RIXS measurements can be considered a unique tool to investigate the valence component of χ(q, ω), by extracting unique information as the superconducting gap and pseudogap openings in the non-ARPES friendly cuprate YBa2Cu3O7-δ.
Figure 1
Integrated RIXS intensity vs temperature cleaned from Bosonic excitations in two different momentum transfer points in YBCO UD. Q1 shows the superconducting and pseudogap transitions whereas Q2 does not, as predicted by our simulations.
Complementing the experimental activity with simulations of the charge dynamical structure factor to find the optimum q position to maximize the effects of the pseudogap, we carried out RIXS measurements on an underdoped YBa2Cu3O7-δ (YBCO UD) sample (p ≈ 0.15), where the putative T* is indirectly estimated to be far from Tc. We also measured an almost undoped sample (YBCO AF), which does not present any gap opening at the Fermi level in our range of temperatures. Performing a high resolution ( ≈40 meV) RIXS measurement as a function of temperature and at a fixed q, we have been able to observe two clear jumps in the integrated low energy spectral intensity, below 100 meV in energy loss, as shown in Fig.1. The first change of slope is at T ≈ 87 K, in good agreement with the Tc estimated from transport measurements, the second one at T ≈ 175 K, compatible with T*.
Our results establish RIXS as a valuable alternative to study the two gaps phenomenology in non-ARPES friendly materials. Moreover, we demonstrated the direct proportionality between the RIXS cross-section and the valence component of the dynamical charge susceptibility, thus offering a new way to track the energy and momentum dependence charge response function in a vast range of materials.
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