Quantum Matter Semianr

Dyanmics and energy relaxation in correlated metals 

For almost four decades the condensed matter community has struggled to understand the physics of various "strange" metals.   These systems — often interesting superconductors at low temperatures — show unconventional power laws in their transport.  They are conventionally typified by a linear in T dependent resistivity that persists to high temperature, which is in apparent conflict with the cotangent of the Hall angle that goes as a more conventional T^2, and often a strong violation of Kohler's rule.  In this talk will a discuss a number of low energy THz range optical experiments that give insight on the physics of strange metals.  I will emphasize our recent measurements of correlated metals using the new nonlinear technique of THz 2D coherent spectroscopy.  The cuprate superconductor La2−xSrxCuO4 shows an extremely large normal state nonlinearity.  Irrespective of its origin, the large normal state nonlinearity provides an opportunity to measure the energy relaxation rate at temperatures where the momentum relaxation rate is linear in T and close to its ``Planckian" form.  The energy relaxation rate is a fundamental response time that is difficult to access by other means.  We find the energy relaxation rate to be 10−40 times smaller than the momentum relaxation rate. This shows that the scattering that causes momentum loss (and T-linear) resistivity do not remove appreciable energy from the electrons. Although the T-dependence of the momentum relaxation is consistent with quasi-elastic scattering off bosonic collective modes at temperatures above their characteristic energy (the Bloch-Gruneisen temperature for acoustic phonons) it is inconsistent with the energy relaxation's temperature dependence. It is an increasing function of T, which is indicative of inelastic scattering to the phonon bath.