Detailed optical spectroscopy of the hybridization gap and the hidden order transition in high quality URu$_{2}$Si$_{2}$ single crystals

TL;DR

This study uses optical spectroscopy on high-quality URu$_{2}$Si$_{2}$ crystals to explore the hybridization gap and hidden order transition, revealing coherent charge carriers and a gap opening below the hidden order temperature.

Contribution

It provides detailed temperature and frequency-dependent optical data on URu$_{2}$Si$_{2}$, highlighting the emergence of coherence and a hybridization gap associated with the hidden order phase.

Findings

Coherence develops below T* ~75 K, higher than previously reported.

A hybridization gap of ~6.5 meV opens below T_{HO} = 17.5 K.

Heavy fermion plasmons observed at 18 meV along different axes.

Abstract

We present a detailed temperature and frequency dependence of the optical conductivity measured on clean high quality single crystals of URu$_{2}$Si$_{2}$ of $ac$- and $ab$-plane surfaces. Our data demonstrate the itinerant character of the narrow 5f bands, becoming progressively coherent as temperature is lowered below a cross-over temperature $T^*{\sim}75~K$. $T^*$ is higher than in previous reports as a result of a different sample preparation, which minimizes residual strain. We furthermore present the density-response (energy-loss) function of this compound, and determine the energies of the heavy fermion plasmons with $a$-and $c$-axis polarization. Our observation of a suppression of optical conductivity below 50~meV both along $a$ and $c$-axis, along with a heavy fermion plasmon at 18~meV, points toward the emergence of a band of coherent charge carriers crossing the Fermi energy and the emergence of a hybridization gap on part of the Fermi surface. The evolution towards coherent itinerant states is accelerated below the hidden order temperature $T_{HO}=17.5$~K. In the hidden order phase the low frequency optical conductivity shows a single gap at $\sim 6.5$meV, which closes at $T_{HO}$.