THz response of CeCoIn₅ and evolution of effective mass in the non-Fermi liquid regime

CeCoIn₅ is a heavy-fermion superconductor that exhibits non-Fermi-liquid properties in the metallic state below 30 K that are interpreted in terms of quantum criticality. The strong evolution of the quasiparticle effective mass with temperature or energy, which is characteristic for heavy fermions, takes place in this normal-state regime, but is typically difficult to access experimentally. Optical spectroscopy on metals can reveal the temperature- and frequency-dependent effective mass and transport scattering rate in and beyond the quantum-critical regime. Here we perform THz spectroscopy on CeCoIn₅ thin films in wide temperature (3 K to 150 K) and frequency (0.2 THz to 1.1 THz) regions that match in energy, and we employ an extended Drude analysis. We find strong frequency and temperature dependence of the scattering rate and the effective mass, and we discuss the implications for understanding the evolution of the heavy-fermion, non-Fermi-liquid state of CeCoIn₅.