Hybridization gap versus hidden-order gap in URu2Si2 as revealed by optical spectroscopy

We present the in-plane optical reflectance measurement on single crystals of URu2Si2. The study revealed a strong temperature-dependent spectral evolution. Above 50 K, the low frequency optical conductivity is rather flat without a clear Drude-like response, indicating a very short transport lifetime of the free carriers. Well below the coherence temperature, there appears an abrupt spectral weight suppression below 400 cm−1, yielding evidence for the formation of a hybridization energy gap arising from the mixing of the conduction electron and narrow f-electron bands. A small part of the suppressed spectral weight was transferred to the low frequency side, leading to a narrow Drude component, while the majority of the suppressed spectral weight was transferred to the high frequency side centered near 3500 cm−1. Below the hidden order temperature, another very prominent energy gap structure was observed, which leads to the removal of a large part of the Drude component and a sharp reduction of the carrier scattering rate. The study revealed that the hybridization gap and the hidden order gap are distinctly different: they occur at different energy scales and exhibit completely different spectral characteristics.