Energy and Momentum Relaxation in the Normal State of LSCO Thin Films

The strange metal regime of the cuprate superconductors characterized by T-linear resistivity above T_c has been found to be adjacent to the pseudogap and Fermi liquid phases, at the intersection of bad metallic and normal Fermi-liquid like metallic behavior. In our work, we explore the nonlinear electrodynamic response of LSCO thin films from the mildly underdoped to extremely overdoped region using THz-pump, THz-probe spectroscopy. While there is a large nonlinear response in the superconducting state due to injection of quasiparticles via breaking Cooper pairs, we find that the nonlinear response persists deep into the normal state up to 120 K over the whole range of superconducting samples. This large nonlinearity is quite unlike normal metals. We use this nonlinearity as a tool to measure the energy relaxation rate of these materials via all THz range pump-probe spectroscopy. The energy relaxation time is a fundamental time-scale that cannot be measured by conventional linear response spectroscopic methods. A comparison of the energy relaxation rate to the current relaxation rate (measured with conventional THz spectroscopy) is particularly illuminating. We find across the doping range studied that the energy relaxation rate is much smaller than the momentum relaxation rate by a factor of 10-20x. This observation deviates strongly from expectations for Fermi-liquid behavior.