Low-temperature specific heat studies on superconductors and mott insulators in κ-(BEDT-TTF)2X system

The low-temperature specific heat of the insulating phase and the superconducting phase in κ-(BEDT-TTF)2X [X=Cu[N(CN)2]Br, Cu(NCS)2, and Cu[N(CN)2]Cl] is studied to characterize the ground state of each phase. The insulating salts do not have any finite γ term, which ensures the existence of finite charge-gap around the Fermi level. The superconducting salts, on the other hand, show a quadratic temperature dependence at low-temperatures. This temperature dependence and magnetic field dependence below 1T are consistently understood with the existence of line-nodes in the superconductive gap.