Optical investigation of the heavy-fermion normal state in superconducting UTe2

The recently discovered superconductor, UTe2, has attracted immense scientific interest due to observations that suggest odd-parity superconductivity. It is believed that the material is a heavy-fermion metal at low temperatures although details of the normal state are unclear. Using Fourier transform infrared spectroscopy, we investigated the normal state electronic structure of UTe2 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 gives spectra in close correspondence to the experiment. Via this comparison we attribute the prominent MIR peak to interband transitions involving a narrow U 5 f feature that develops near the Fermi level. In this regard, this comparison along with data that shows the scale of the low-temperature mass renormalization 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.