Articoli in riviste
|Oxygen-stoichiometry dependence of the electronic structure of YBa2Cu3O7- with (0<<0.7): Possibility of a highly correlated mixed-valent state|
|Gourieux T., Krill G., Maurer M., Ravet M.F., Menny A., Tolentino H., Fontaine A.|
|Physical Review B 37 (1988) 7516|
|DOI : 10.1103/PhysRevB.37.7516|
YBa2Cu3O7- is obtained over a wide range of stoichiometry (0<<0.7) by proper thermal treatment in O2. Samples are carefully characterized by x-ray powder diffraction and thermogravimetry. Their superconducting properties are investigated mainly by measuring the susceptibility (dc) in low fields (Meissner effect). The main part of this paper is devoted to an x-ray photoemission spectroscopy study of core levels (mainly O 1s and Cu 2p) with the aim of elucidating the change of the electronic structure on passing from the semiconducting (>0.5) to the superconducting metallic state (0). On the basis of our experimental results, we argue that the superconducting state is obtained from a mixture of configurations 3d9 and in the ground state of copper (L stands for a hole in the O-Cu ligand and k for a conduction electron). We find that the ratio of the configuration is directly related to the magnitude of Tc. The charge-neutrality rule is approximatively followed, provided that the Cu3+ state is identified as a configuration. We discuss also the presence of the 3d10 configuration (i.e., Cu+) in the ground state. A possible consequence of these Cu+ would be to increase the number of formal Cu3+ (here 3d9Lk).