"Renormalized masses and their effects on transport in correlated metals.”
This talk will present our recent results using optical spectroscopy on two different correlated metals, the cuprate high temperature superconductors and the heavy fermion CeCoIn5. The recent observation of cyclotron resonance in optimally-doped La2−xSrxCuO4 using time-domain THz spectroscopy in high magnetic field has given new possibilities for the study of cuprate superconductors. I will present the measurement of the cyclotron mass of the hole doped cuprate La2−xSrxCuO4 across a range of dopings spanning the slightly underdoped (x=0.13) to highly overdoped (x=0.26), near to the termination of the superconducting dome. These results reveal a systematic increase of mc with doping, up to values greater than thirteen times the bare electron mass. This is in contrast with those masses extracted from the heat capacity, which show a peak near the pseudogap critical point p∗ and/or Lifshitz transition. Among other aspects, these results are surprising as photoemission reveals a Lifshitz transition in the middle of our doping range and the sign of the cyclotron mass determined from a finite frequency resonance is -- in conventional theories — a topological quantity only sensitive to whether or not the Fermi surface is closed around holes or electrons. We see no sign of a divergence of the mass near p∗ nor near the Lifshitz transition, showing that any singularity -- if it exists -- is not strong enough to affect the cyclotron mass. I will also discuss our recent work on thin films of the heavy-fermion superconductor CeCoIn_5. The complex optical conductivity is analyzed through a Drude model and extended Drude model analysis. Below the 40 K Kondo coherence temperature, a narrow Drude-like peak forms, as the result of the $f$ orbital-conduction electron hybridization and the formation of the heavy-fermion state. Its width shows a T^2 dependence giving evidence for a hidden Fermi state.