Raman studies of lanthanum cuprate superconductors

Abstract

Raman-scattering studies of the high-Tc superconductor La2−x(Sr, Ba)xCuO4 are briefly reviewed. A detailed analysis of the phonon-mode symmetries is given along with a discussion of the effects expected from the orthorhombic-to-tetragonal phase transition, which is known to occur in these materials. Survey spectra are given for powders, ceramics, and single crystals, and an oxide of Cu is identified as the primary impurity phase. Extensive spectra are given from oriented single crystals of La2CuO4 and La1.85Sr0.15CuO4, and several mode assignments are made. Spectra for the high-temperature tetragonal phase and the low-temperature orthorhombic phase are given for each material. The soft phonon associated with the phase transition is seen in La2CuO4 as is a broad peak arising from two-magnon scattering. Neither of these features is seen in the Sr-doped sample.