TRANSMISSION HIGH-ENERGY ELECTRON-ENERGY LOSS SPECTROMETRY (EELS) OF CUPRATE SUPERCONDUCTORS

The formation of charge carriers (holes) and charge transfer correlated with different types of dopants in cuprate superconductors has been studied by means of transmission high energy electron energy loss spectrometry (EELS). The O-K absorption edges of p-type doped systems exhibit a distinct pre-edge feature which is related to the hole formation on oxygen sites. The CuL3 edge of the doped compounds also exhibit a pronounced shoulder on the higher energy side of the CuL3 edge which represents the charge transfer between copper and oxygen. Er123 EELS data are similar to the Y123 where the hole density shows a strong dependency on the oxygen doping. Co-substitution in Y123 on Cu(I) sites does not seem to effect the hole density, but perhaps their mobility. The newly discovered Ga-based cuprate superconductors exhibit additional significant features. EELS studies on O-K edge appear to reveal two types of holes in this system, one induced by hole doping alone (Ca-doping) and the other by high pressure oxygenation treatment. Only the latter type of holes appears to be associated with occurrence of superconductivity. All the EELS absorption data are consistent with the doped charge-transfer insulator description of the electronic states.