Probing the lattice anharmonicity of superconducting YBa2Cu3O7−δ via phonon harmonics

We examine coherent phonons in a strongly driven sample of optimally-doped high temperature superconductor ${\mathrm{YBa}}₂{\mathrm{Cu}}₃{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$. We observe a nonlinear 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 nonequilibrium superconductivity.