Probing the Lattice Anharmonicity of Superconducting YBa${}_2$Cu${}_3$O${}_{7-\delta }$ Via Phonon Harmonics

Abstract

We examine coherent phonons in a strongly driven sample of optimally-doped high temperature superconductor YBa${}_2$Cu${}_3$O${}_{7-\delta }$. We observe a non-linear lattice response of the 4.5\,THz copper-oxygen vibrational mode at high excitation densities, evidenced by the observation of the phonon third harmonic and indicating the mode is strongly anharmonic. In addition, we observe how high-amplitude phonon vibrations modify the position of the electronic charge transfer resonance. Both of these results have important implications for possible phonon-driven non-equilibrium superconductivity.

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