Figure 2 (left panel) shows Cu L 3 -XAS data from optimally doped YBCO, measured across the metal-superconducting transition (MST). The XAS and RIXS measurements were performed at the Advanced Resonant Spectroscopies (ADRESS) beamline17 at the Swiss Light Source (SLS), Paul Scherrer Institut, Switzerland, using the Super-Advanced X-ray Emission Spectrometer (SAXES)18. The XAS spectra have been measured in the bulk-sensitive total fluorescence yield (TFY) mode using polarization in-plane (E||ab) or out of plane (E||c), respectively. At the Cu 2p edge, the excitation process creates a 2p core hole at the metal site. While the main peak at 930.7 eV can be assigned to the 3d9→2p53d10 transition, the satellites around 932.4 and 933.4 eV are due to metal-to-ligand charge transfer (MLCT) transitions on the Cu(2)- and Cu(1)-sites of the planes and the chains, respectively19,20.

Figure 2 Left panel: experimental Cu 2p XAS-TFY spectra of optimally doped YBCO measured at 300 K (red lines), respectively 15 K (blue lines) and the difference spectra (15 K–300 K).In the upper (lower) part of the panel, in-plane, E||ab (out-of-plane, E||c), polarized x-rays have been used for excitation. The vertical ticks indicate the excitation energies for the RIXS measurements. Right panel: The corresponding simulated spectra are shown in the top part of the panel. They are obtained using fitted linear combinations of model calculations of spectra from core-hole excited 3d10, 3d10L−14s1, 3d9L+1, 3d104s1 and 3d10L−1 final states, which are shown in the lower part of the panel. Full size image

We find a strong temperature dependence of the spectral weight around the main peak but the most striking observation is that, regardless of the x-ray polarization direction, there are opposing intensity trends in the “chains satellite” and the “planes satellite” upon cooling. This suggests the intriguing possibility that there is a temperature driven redistribution of charge between the planes and the chains of YBCO. The spectral difference in percentages displayed in the figure are derived from the hatched areas relative to the total area (taking into account the atomic step between L 3 and L 2 ) under the room temperature spectra. The difference highlights the opposing behavior of the satellites in the MST. Formally, Cu ions in YBa 2 Cu 3 O 6.9 have a charge state of +2.3. For an intermediate mixed valence system, such as YBCO, one can expect that the ground-state wave function configurations of YBCO can fluctuate4 between different Cu-site configurations and give rise to the transitions listed in Table I, where L+1(L−1) denotes an additional (a missing) electron in the oxygen ligand band and such configurations are the effect of MLCT (see Supplementary Materials).

Table 1 Cu transitions in different configurations Full size table

At the top of the right panel of Fig. 2, we show simulated Cu L 3 -spectra created by using linear combinations of the pure-configuration spectra at the bottom of the right panel. Since a full theoretical description of the Cu L-XAS spectrum of YBCO has proven to be too complex for state-of-the-art ab initio methods we employ an ionic model approach to project the charge-transfer contributions from essential configurations. Our calculations show that the MLCT transitions 3d9→2p53d9L+1 deriving from formally divalent copper sites must heavily contribute to the spectral weight of the two satellites (reproduced by the double-peaked blue traces in Fig. 2). However, the opposing temperature trends in the satellites suggest the existence of additional contributions besides those from divalent copper sites. Thus the theoretical spectrum belonging to 3d9L+1→2p53d10L+1 and 3d9L−1→2p53d10L−1 (dashed blacked traces in Fig. 2) peaking only at the first satellite, indicates that monovalent and/or trivalent copper sites may contribute to the charge redistribution between the chains and planes. In order to distinguish between contributions from different configurations and to learn more about the nature of the states involved, we have performed incident-energy and -polarization dependent RIXS, in particular to study the transitions belonging to the satellite peaks.

Figure 3 (left panel) shows an overview of the polarization dependent Cu L 3 RIXS spectra of optimally doped YBCO measured in the normal state (300 K) and in the superconducting state (15 K), respectively. The RIXS spectra were recorded at a 90°-scattering geometry with a grazing incidence angle of 20° with respect to the sample surface, i.e. for the geometry denoted by E||c there is an angle of 20° between the polarization of the incident x-rays and the crystal c-axis. The RIXS spectra have been normalized to the incident x-ray flux and plotted relative to the incident x-ray energy, i.e. plotted on an energy loss scale. For convenience when comparing spectral shapes, we have applied scaling factors as indicated to spectral pairs of identical geometry.

Figure 3 Left panel: Energy and polarization dependent Cu L 3 RIXS spectra (in-plane, E||ab and out of plane, E||c) of optimally doped YBCO measured in the normal state (300 K) and in the superconducting state (15 K), respectively. Spectral pairs are normalized to the incident photon flux. The arrow indicates the spin excitation peak (paramagnons) with −0.21 eV energy loss in the L 3 spectrum at 930.7 eV incident energy. Right panel: RIXS spectra excited at 933.4 eV (“chain satellite”) (top) and at 932.4 eV (ZRS satellite) (bottom). The red (blue) lines are spectra recorded at 300 K (at 15 K). The grey lines are the respective experimental difference spectra (300 K–15 K). The black lines are calculated spectra with enhanced configuration 3d9L−1 for excitation at 932.4 eV and 3d8L+1 for excitation at 933.4 eV (see Supplementary Materials for details). Full size image

Excitation close to the L 3 resonance (929.8 eV and 930.7 eV) yields spectra dominated by dd-excitations around −1.5 eV that are localized on divalent Cu orbitals. Moreover, magnetic and lattice excitations (here para-magnons and phonons, respectively) at energy losses within a few hundred meV are prominent at this excitation energy10,21. Particularly, out-of-plane polarized components are found to be temperature sensitive (see also Supplemental Materials). Intriguingly, already early on, specific phonon modes have been suggested to be of crucial importance for HTSC of cuprates, e.g. by facilitating interplane pairing. Indeed, the A g Cu(1)-O(4) stretching vibration has been found to have anomalous strength in infrared reflectivity22 and could be coupled to charge transfer23. Together with structural lattice instabilities in YBCO, the observation of transverse modes sensitive to the MST of YBCO is possible evidence for unconventional, e.g. polaron-mediated, superconductivity. Below, we show that YBCO indeed undergoes a massive out-of-plane electronic reconstruction in the MST as well.