Raman study of local lattice distortions and charge redistribution in cuprates

D.Palles¹, D. Lampakis¹, E. Liarokapis¹, J.Karpinski², and C. Panagopoulos³

¹Department of Physics, National Technical Univ. of Athens, Athens 157 80, Greece

²Laboratory for Neutron Scattering ETH and PSI, Villigen PSI, Switzerland

³Cavendish Laboratory and IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge, CB3 0HE United Kingdom.

Abstract

Small changes in doping or the application of hydrostatic pressure can induce non-linear lattice distortions, lattice instabilities, and phase separation in the cuprates. We report evidence from Raman measurements for the presence of small lattice distortions and soft modes in the superconductors YBa₂Cu₃O_6.5, YBa₂Cu₃O_over, YBa₂Cu₄O₈, and Bi₂Sr₂CaCu₂O₈. The non-linear effects have been induced by hydrostatic pressure and involve modifications in the spectral characteristics of A_g–symmetry phonons. The soft modes are related with the tilting of the oxygen cages involving vibrations of the out-of-plane oxygens parallel to the planes. In all compounds studied a coexistence of phases seems to appear at a critical pressure where changes in the superconducting transition temperature have been observed. We have observed similar correlations between the transition temperature and spectral modifications by chemical doping in the Raman spectra of La_2-xSr_xCuO₄. In all cases, changes in the relative intensity of certain phonons indicate a charge redistribution, which connects the Raman data with the variation of the transition temperature induced by doping or by the application of external or internal (substitutional) pressure.