Electrodynamics of the antiferromagnetic phase in URu$_2$Si$_2$

TL;DR

This study investigates the optical properties of URu$_2$Si$_2$ and its doped variants, revealing similar gap structures in hidden order and antiferromagnetic phases, supporting a nesting-induced density wave model.

Contribution

It provides detailed optical conductivity measurements showing identical gap features in both phases, suggesting a common underlying density wave mechanism.

Findings

Both phases exhibit a density wave-like gap with spectral weight transfer.

The antiferromagnetic phase shows increased transition temperature and gap energy.

Optical scattering rate follows a $ ext{omega}^2$ dependence above transitions.

Abstract

We present data on the optical conductivity of URu$_{2-x}$(Fe,Os)$_{x}$Si$_{2}$. While the parent material URu$_2$Si$_2$ enters the enigmatic hidden order phase below 17.5 K, an antiferromagnetic phase is induced by the substitution of Fe or Os onto the Ru sites. We find that both the HO and the AFM phases exhibit an identical gap structure that is characterized by a loss of conductivity below the gap energy with spectral weight transferred to a narrow frequency region just above the gap, the typical optical signature of a density wave. The AFM phase is marked by strong increases in both transition temperature and the energy of the gap associated with the transition. In the normal phase just above the transition the optical scattering rate varies as $\omega^2$. We find that in both the HO and the AFM phases, our data are consistent with elastic resonant scattering of a Fermi liquid. This indicates that the appearance of a coherent state is a necessary condition for either ordered phase to emerge. Our measurements favor models in which the HO and the AFM phases are driven by the common physics of a nesting-induced density-wave-gap.