Hybridization gap versus hidden order gap in URu$_2$Si$_2$ as revealed by optical spectroscopy

TL;DR

This study uses optical spectroscopy to distinguish between the hybridization gap and hidden order gap in URu2Si2, revealing their different energy scales and spectral features.

Contribution

It provides the first detailed optical spectroscopy comparison showing the distinct nature of hybridization and hidden order gaps in URu2Si2.

Findings

Hybridization gap appears below the coherence temperature with spectral weight transfer near 400 cm$^{-1}$.

Hidden order gap emerges below the hidden order temperature, significantly reducing the Drude component.

Hybridization and hidden order gaps occur at different energy scales and have different spectral signatures.

Abstract

We present the in-plane optical reflectance measurement on single crystals of URu$_2$As$_2$. 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 life time 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 4000 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 orger gap are distinctly different: they occur at different energy scales and exhibit completely different spectral characteristics.