Exotic superconducting state embedded in the hidden order state of URu2Si2

The heavy-fermion compound URu2Si2 has mystified researchers since the superconducting state (T-c = 1.45 K) is embedded within the enigmatic "hidden order" phase (T-h = 17.5 K). Here, we report charge and thermal transport measurements on ultraclean single crystals of URu2Si2 with very large residual-resistivity-ratio down to 30 m K (similar to T-c/50), which reveal a number of unprecedented superconducting properties. The results provide strong evidence for a new type of unconventional superconductivity with two distinct gaps having different nodal topology. We propose a gap function with chiral d-wave form Delta(k) = Delta(0)k(z)(k(x) + ik(y)). We also demonstrate that a distinct flux line lattice melting transition with outstanding characters occurs well below the upper critical fields even at sub-Kelvin temperature. The intriguing superconducting state of URu2Si2 adds a unique and exciting example to the list of unconventional superconductors. (C) 2010 Elsevier B.V. All rights reserved.