Vortex Lattice Melting Transition

Abstract

The most fascinating phenomenon of “vortex matter”, which is a highly correlated system composed of the magnetic flux lines in superconductors, 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. In this chapter, through the electro- and thermal-transport transport study, we show that in ultraclean heavy-fermion superconductor $$\mathrm{{URu}}_2\mathrm{{Si}}_2$$ ( $$T_c =1.4$$ K) a distinct melting transition with outstanding characters occurs well below $$H_{c2}$$ . In $$\mathrm{{URu}}_2\mathrm{{Si}}_2$$ , very low carrier number with heavy mass results in exceptionally large fluctuations even below 1 K. The uniqueness is further highlighted by 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.