An optical investigation of the heavy fermion normal state in superconducting UTe$_2$

TL;DR

This study uses optical spectroscopy and theoretical modeling to explore the normal state of UTe$_2$, revealing a low-energy Kondo resonance and heavy fermion behavior that are likely linked to its superconductivity.

Contribution

It provides the first detailed optical investigation of UTe$_2$'s normal state, combining experimental data with DFT+DMFT calculations to identify a Kondo resonance near the Fermi level.

Findings

Detection of a MIR peak around 4000 cm$^{-1}$ in optical conductivity.

Observation of a narrow Drude peak developing below 40 K.

Spectroscopic evidence for a low-energy Kondo resonance above superconducting transition.

Abstract

The recently discovered superconductor, UTe$_2$, has attracted immense scientific interest due to the experimental observations that suggest odd-parity superconductivity. It is believed that the material becomes a heavy-fermion metal at low temperatures although details of the normal state are unclear. Using Fourier transform infrared spectroscopy (FTIR), the normal state electronic structure of UTe$_2$ was investigated at zero applied magnetic field. Combining the measured reflectivity with the dc resistivity, the complex optical conductivity was obtained over a large frequency range. The frequency dependence of the real part of the optical conductivity exhibits a MIR peak around 4000 cm$^{-1}$ and a narrow Drude peak that develops below 40 K. A combination of density functional and dynamic mean field theory (DFT + DMFT) gives spectra in close correspondence to the experiment. Via this comparison we attribute the prominent MIR peak to inter-band transitions involving a narrow U 5$f$ feature that develops near the Fermi level. In this regard, our data gives spectroscopic evidence for the existence of a low energy Kondo resonance at temperatures just above the onset of superconductivity and implicates heavy electrons in the formation of the superconducting state. We find that the coherent Kondo resonance is primarily associated with a collapse of scattering and less with a transfer of spectral weight.