Anomolous Charge Transfer in High-Tc Superconducting Oxides Probed by Raman Scattering

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The superconductivity in copper-based high-Tc superconducting oxides is intimately linked with the insulator-to-metal transitions, which are induced by carrier doping into the parent insulator. In this study, Raman scattering has been used as a tool to monitor the charge transfer and charge redistribution near the insulator-to-metal transitions induced by atomic substitutions (doping). The system chosen for the study includes Bi2Sr2Ca1-xYxCu2O8+d (x=0-1), and thier Raman spectra have been investigated in a number of well-defined single crystal and polycrystalline samples. From the polarization and Y-doping dependence ; and from a comparison with previous reports on Bi-based cuprates, we have for the first time established clearly mode assignments of the 7 modes that are Raman allowed within the ideal body-centered tetragonal unit-cell. For instance, confusions of the assignments for the two most prominent Raman bands around 630 cm-1 and 460 cm-1 have been solved without ambiguity in th … More is study by the single-crystal polarized Raman measurements including the difficult c-axis polarized geometry. A more exciting result in this study was that the direct evidence for a decreased number of hole carriers onto the CuO2 planes was extracted from the change in intensities of phonon modes associated with CuO2 plane vibrations : i.e.with increasing Y doping we find that the vibrational modes involving atoms in the CuO2 planes rapidly increase in intensity as a result of the reduced metallic screening in the hole-depleted Y doped samples. Similar intensity variations have been found in almost all the known high-Tc superconducting oxides with different hole concentration. We have proposed that the hole concentration of a given superconducting sample (thereby superconducting transition temperature Tc) can be estimated by measuring the Raman intensity of the CuO2 plane. During the course of this study, we have also discovered in oxygen depleted Y-based high-Tc superconductors unusual phase transitions at room temperature induced by visible light irradiations (photoinduced phase transition), which may open up new opportunities for future industrial applications. Less