Reconciling strange metal transport in CeCoIn$_5$ through the difference of optical and cyclotron effective masses

Abstract

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 (TDTS). 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 scales roughly like 1/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.

Figures (5)

• Figure 1: THz conductivity of the CeCoIn$_5$ thin film. Real and imaginary part of the THz conductivity ($\sigma_1$ and $\sigma_2$, respectively) in the circular basis at the temperatures and in the magnetic fields indicated, showing systematic electron-like cyclotron resonance. a) b) at low temperature in the JHU setup. c) d) higher temperature data including the pulsed field data taken at LANL. The dashed lines are fits using two Drude terms. Superconducting phase data (1.6K, below 2T) are included for completeness.
• Figure 2: Cyclotron frequency $\omega_c$ and cyclotron mass $m_c$. a) Field dependence of $\omega_c$ at different temperatures. $m_c$ is field independent in the low field region as $\omega_c$ is linear in $B$. In high fields $m_c$ increases as indicated by the decreased slope. The dashed line is a guide to the eye. b) Temperature dependence of the optical effective mass $m^*$ extracted from low field EDM analysis and $m_c$ from a linear fit to $\omega_c$ below 2T.
• Figure 3: Zero-frequency scattering rates and effective masses extracted from EDM analysis. a) Temperature dependence of zero-frequency value for scattering rate $1/\tau_0$ and normalized scattering rate $1/\tau^*_0$. $1/\tau_0$ is linear in $T$ while $1/\tau^*_0$ follows $T^2$, as indicated by the dashed lines. Larger symbols denote pulse field data. b), c) Field dependence of the effective mass and the scattering rate $1/\tau_0$ at zero frequency.
• Figure 4: Frequency dependent effective masses and scattering rates extracted from EDM analysis a) Effective masses, frequency dependent mass is an evidence for strong electronic correlation. No polarization or field dependence are observed. b) Scattering rate, favored (left circularly polarized) polarization has a lower scattering rate. The overall magnitude of the scattering rate decreases in field, consistent with the field suppressed DC resistivity.
• Figure S1: DC Hall coefficient obtained by electrical transport measurement on a similarly prepared film.