Optical conductivity of URu$_2$Si$_2$ in the Kondo Liquid and Hidden-Order Phases

TL;DR

This study investigates the optical conductivity of URu$_2$Si$_2$ across different electronic phases, revealing anisotropic behaviors, hybridization gap formation, and a density-wave gap associated with the hidden-order phase.

Contribution

It provides detailed polarization-dependent optical conductivity measurements revealing the evolution of electronic structure through Kondo and hidden-order phases in URu$_2$Si$_2$.

Findings

Strong hybridization gap at 12 meV in ab plane

Drude peak persists at 300 K along c axis

Density-wave like gap appears at 6.5 meV in hidden order

Abstract

We measured the polarized optical conductivity of URu$_2$Si$_2$ from room temperature down to 5 K, covering the Kondo state, the coherent Kondo liquid regime, and the hidden-order phase. The normal state is characterized by an anisotropic behavior between the ab plane and c axis responses. The ab plane optical conductivity is strongly influenced by the formation of the coherent Kondo liquid: a sharp Drude peak develops and a hybridization gap at 12 meV leads to a spectral weight transfer to mid-infrared energies. The c axis conductivity has a different behavior: the Drude peak already exists at 300 K and no particular anomaly or gap signature appears in the coherent Kondo liquid regime. When entering the hidden-order state, both polarizations see a dramatic decrease in the Drude spectral weight and scattering rate, compatible with a loss of about 50 % of the carriers at the Fermi level. At the same time a density-wave like gap appears along both polarizations at about 6.5 meV at 5 K. This gap closes respecting a mean field thermal evolution in the ab plane. Along the c axis it remains roughly constant and it "fills up" rather than closing.