Vortex lattice melting in the heavy-fermion superconductor URu₂Si₂

The most fascinating phenomenon of vortex matter is the first-order melting transition from flux line lattice to liquid. In low-T_c superconductors, the transition is extremely difficult to observe, because it occurs in the immediate vicinity of the mean field upper critical field H_c2. In contrast, the melting transition is established in high-T_c cuprates at several tens of Kelvin, owing to large thermal fluctuations. Here we find that in ultraclean heavy-fermion superconductor URu₂Si₂ (T_c = 1.45 K) a distinct melting transition with outstanding characters occurs well below H_c2. We show that very low carrier number with heavy mass results in exceptionally large fluctuations even at subkelvin temperatures. We also find a thermal conductivity anomaly below the melting transition, indicating enhancement of the quasiparticle mean free path possibly due to the formation of a quasiparticle Bloch state.