Raman and infrared spectroscopy in high T_c superconducting oxides

Abstract

Magnon, phonon, and luminescence in a visible region of high T_c superconducting oxides (La_1−x Sr _x )₂CuO₄, YBa₂Cu₃O_7−y , and Bi₂Sr₂CaCu₂O _y are investigated by Raman spectroscopy using single crystals. The peak energies and intensities of two-magnon scattering in insulating (La_1−x Sr _x )₂CuO₄ and YBa₂Cu₃O_7−y decrease with the increase of Sr or O concentration, but still scattering ranging from low energy to over 4000 cm⁻¹ remains even in the superconducting phase as overdamped two-magnon scattering. Phonon modes show strong dependence on the Sr or O concentration. The phonon and magnon scattering is in a resonant Raman condition with localized electronic levels of 19520, 18770, and 18030 cm⁻¹, which are observed as very weak luminescence. A soft mode for the tetragonal-orthorhombic phase transition is observed in (La_1−x Sr _x )₂CuO₄.