Anomalous shifts of oxygen-mode frequencies inLa2−xSrxCuO4,YBa2Cu3O7−δandTl2Ba2Ca1−xGdxCu2O8studied by photoinduced infrared absorption and Raman spectroscopy

Abstract

Comparison of photoinduced local modes (PILM's) of insulating ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.3}$ and ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Gd}}_{\mathit{x}}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8}$ with Raman spectra of their metallic counterparts suggests that carrier injection anomalously increases the frequencies of phonons involving z-axis apex-oxygen motion in both the dilute (insulator) and concentrated (metallic) limit. In ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$, we observe PILM's whose frequencies suggest a correspondence with the ``ghost modes'' observed in neutron scattering [Rietschel et al., Physica C 162--164, 1705 (1989)]. These data, together with Raman data on apex-oxygen anharmonicity, enable us to discuss the vibronic mode couplings involved in polaron formation in the three materials.