Decoupling momentum and energy relaxation rates in cuprate strange metals via giant THz nonlinearities

Abstract

Understanding the $T$-linear normal-state resistivity of cuprates remains a central physics challenge. The associated momentum relaxation rate, $\hat{I}{“}_M$, saturates near the conjectured ``Planckian" bound $\hat{I}{“}_M\sim kT/\hslash$, but the mechanism underlying the anomalous scattering remains unresolved. Here we employ nonlinear terahertz spectroscopy to systematically study La${}_{2-x}$Sr${}_x$CuO${}_4$ across a broad temperature and doping range. We measure the normal-state third-order susceptibility, $|\ddot{I}{‡}^{(3)}|\approx 6\times 10^{-9}$ m${}^2$/V${}^2$, among the largest in the THz regime, enabling direct access to the rarely measured electronic energy relaxation rate, $\hat{I}{“}_E$. Strikingly, $\hat{I}{“}_E$ is 10-40 times smaller than $\hat{I}{“}_M$, revealing that the scatterings responsible for momentum loss and $T$-linear resistivity do not remove appreciable energy from the electrons. While $\hat{I}{“}_M(T)$ is consistent with quasi-elastic scattering from bosonic modes above their characteristic energy scale, this is incompatible with the increasing temperature dependence of $\hat{I}{“}_E(T)$. Our results exclude phonons as the source of $T$-linear resistivity and impose strong constraints on possible mechanisms.

15 pages, 11 figures,