Reconciling strange metal transport in CeCoIn 5 through the difference of optical and cyclotron effective masses

The strange metal behavior in cuprate superconductors—characterized by linear in temperature resistivity and anomalous Hall transport—stands in stark contrast to the expectation of conventional Fermi liquid (FL) theory. Remarkably, the similar transport behavior has also been observed in the heavy fermion metal CeCoIn 5 , whose d-wave superconducting ground state and strong antiferromagnetic fluctuations draw parallels to the cuprates. Here we have investigated the optical conductivity of the strange metal state of CeCoIn 5 over a wide magnetic field range using time-domain THz spectroscopy. Using unique high-field THz spectroscopy we have shown that the current relaxation rate scales approximately as T 2 , giving evidence for a hidden Fermi liquid state over a large field range. This result can be reconciled with linear in T resistivity with the realization that heavy quasiparticles have an optical mass that becomes very enhanced at low T . This optical mass contrasts with the mass that characterizes cyclotron motion, which does not suffer the same large temperature-dependent renormalization. Although by itself anomalous, this allows one to understand a number of other phenomena in CeCoIn 5 that have been taken to be signatures of strange metals, including the coexistence of a conventional T 2 dependence of the cotangent of the Hall angle with the linear in T resistivity, which with our observation also reflects FL-like physics.