Electronic Structure of La_2-xSr_xCuO₄ and YBa₂Cu₃O_7-y

Abstract

Since the discovery of the new superconducting cuprates, numerous models for the mechanism for the high superconducting transition temperatures have been developed.^1–3 At present there is no clear evidence for one or another model. Therefore, it is extremely important to elaborate the electronic structure of this new class of materials and to determine the relevant parameters by experiments in order to guide theory in the choice of a model. In particular, there is a strong discussion whether band-structure calculations using the local density approximation (LDA) yield reasonable information on the electronic structure or whether the electron-electron interaction of the 3d electrons on the Cu sites is important for the high-T_c superconductors similar to the oxides, chalcogenides and halides of the late 3d transition metals. Having accepted the strong e-e correlation, a further question is, what is the influence of the correlation effects on the electronic structure and what is the nature of the charge carriers in the new superconductors. This contribution reviews recent high-energy spectroscopic investigations on the electronic structure of the superconducting cuprates.^4–6 Recent results on single crystals have been included. X-ray induced photoemission (XPS), Auger-electron spectroscopy (AES), bremsstrahlung-isochromat spectroscopy (BIS) and electron energy-loss spectroscopy (EELS) were used.