Reconciling strange metal transport in CeCoIn₅ 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 CeCoIn₅, 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 CeCoIn₅ 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 T², 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 [Formula: see text]. 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 CeCoIn₅ that have been taken to be signatures of strange metals, including the coexistence of a conventional T² dependence of the cotangent of the Hall angle with the linear in T resistivity, which with our observation also reflects FL-like physics.