Two-phonon bound states in La2-xSrxCuO4: Evidence of hard-phonon anharmonicity.

Abstract

Phonon anharmonicity is the most likely origin of anomalous isotope effects. In the resonant Raman spectrum of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$, the two-phonon peak energy of the breathing mode is higher than twice the single-phonon peak energy by 13 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at x=0. This indicates that a two-phonon bound state (biphonon) is formed by the large phonon anharmonicity. The energy difference and the two-phonon scattering intensity decrease rapidly with increasing x. These results indicate that the anharmonicity is affected strongly by the carrier concentration, and induces the anomalous isotope effects.