Soft X-Ray Spectroscopies of High-Temperature Superconductors.

High-temperature superconductors (HTSC) such as the cuprate superconductors and the iron-based superconductors belong to the class of "correlated materials", in which strong electron correlations play essential roles. Collective mode excitation X-ray absorption generally becomes the strongest when the X-ray energy is tuned to just above the absorption threshold, where the excited electron does not have enough energy to propagate in the conduction band and to excite an electron-hole pair, as shown in Figure 3. Electron-hole pair excitation In path (1) of the RIXS process, the energy of the electron excited into the conduction band changes with the energy of the incident X-ray while the energy of the emitted X-ray basically does not change with the incident X-ray energy. The collective mode excitation appears as Raman signals, for which the energy loss I i I SB i sb i - I SB f sb i remains unchanged when the incident X-ray energy is varied near the absorption peak. [Extracted from the article]