Evidence of a hidden-order pseudogap state in URu₂Si₂ using ultrafast optical spectroscopy

Ultrafast optical spectroscopy was utilized to investigate carrier dynamics in the heavy-fermion compound URu₂Si₂ from 5 to 300 K. The amplitude and decay time of the photoinduced reflectivity increase in the vicinity of the coherence temperature T^*∼57 K, consistent with the presence of a hybridization gap. At 25 K, a crossover regime manifests as a new feature in the carrier dynamics saturating below the hidden-order transition temperature of 17.5 K. This is indicative of a psuedogap region (17.5 K < T < 25 K) separating the normal Kondo-lattice state from the hidden-order phase. Rothwarf-Taylor modeling of the data yields values of ∼10 meV (5 meV) for the hybridization gap (hidden-order gap).