Phonon Raman scattering in (La1−xSrx)2CuO4 single crystals

Abstract

Anomalous Raman spectra were observed in (${\mathrm{La}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${)}_2$ ${\mathrm{CuO}}_4$ single crystals for the light polarized parallel to the layer. The number of peaks are much larger than expected from group-theoretical analysis, supposely due to the strong phonon-magnon and phonon-free carrier interactions. With increasing x, two-phonon peaks decrease simultaneously with the decrease of the two-magnon peaks; this in turn is related with the decrease of the correlation length of the antiferromagnetic spin order. Two-phonon scattering is composed of a combination of two layer-breathing modes and two layer-quadratic modes, modes that have strong electron-phonon interaction. A soft mode for the orthorhombic-tetragonal phase transition was found for the incident light polarized perpendicular to the layer, its energy is much higher than that observed by neutron scattering. The present Raman scattering indicates that the symmetry of the crystal with x>0.01 is lower than orthorhombic.