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-Tc superconductors, the transition is extremely difficult to observe, because it occurs in the immediate vicinity of the mean field upper critical field Hc2. In contrast, the melting transition is established in high-Tc cuprates at several tens of Kelvin, owing to large thermal fluctuations. Here we find that in ultraclean heavy-fermion superconductor URu2Si2 (Tc = 1.45 K) a distinct melting transition with outstanding characters occurs well below Hc2. 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.