Detailed optical spectroscopy of hybridization gap and hidden-order transition in high-qualityURu2Si2single crystals

We present a detailed temperature and frequency dependence of the optical conductivity measured on clean high-quality single crystals of ${\mathrm{URu}}₂{\mathrm{Si}}₂$ of $ac\text{\ensuremath{-}}$ and $ab\text{-plane}$ surfaces. Our data demonstrate the itinerant character of the narrow $5f$ bands, becoming progressively coherent as the temperature is lowered below a crossover temperature ${T}^*\ensuremath{\sim}75$ 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 $a\text{\ensuremath{-}}$ and $c\text{-axis}$ polarization. Our observation of a suppression of optical conductivity below 50 meV along both the $a$ and $c$ axes, 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 ${T}_{\mathrm{HO}}=17.5$ K. In the hidden order phase the low-frequency optical conductivity shows a single gap at $\ensuremath{\sim}6.5$ meV, which closes at ${T}_{\mathrm{HO}}$.