Optical conductivity of URu2Si2 in the Kondo liquid and hidden-order phases

We measured the polarized optical conductivity of URu2Si2 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.