Theoretical study of anharmonicity in La2-xSrxCuO4.

Abstract

We propose an anharmonic model for the lattice dynamics of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ based on the perturbation theory for weakly anharmonic crystals. It is found that only small anharmonic force constants are required to describe correctly the thermal expansion and equilibrium properties related to the tetragonal-to-orthorhombic phase transition and the strong T dependence of the soft-mode frequency. As a consequence, these properties are related to anharmonicity, which does not enhance the lattice polarizability of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ significantly and should therefore play a minor role for the pairing mechanism leading to superconductivity.