Detailed optical spectroscopy of hybridization gap and hidden-order transition in high-quality URu₂Si₂ single crystals

We present a detailed temperature and frequency dependence of the optical conductivity measured on clean high-quality single crystals of URu₂Si₂ of ac− and ab-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^∗∼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 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_HO=17.5 K. In the hidden order phase the low-frequency optical conductivity shows a single gap at ∼6.5 meV, which closes at T_HO.