Vortex lattice melting in the ultraclean heavy-fermion superconductor URu₂Si₂

The vortex states and the quasiparticle dynamics in the ultraclean heavy-fermion superconductor URu₂Si₂ (T_c0=1.45 K) are studied by the electronic and thermal transport measurements. We find that a distinct vortex lattice melting transition, which is similar to that in high-T_c cuprates, occurs well below the mean-field upper critical field. The thermal fluctuations are exceptionally enhanced by very low carrier number with heavy mass even at sub-Kelvin temperatures. Additionally, we find a thermal conductivity anomaly below the melting transition, indicating enhancement of the quasiparticle mean free path possibly due to the formation of a novel quasiparticle Bloch state.