Symmetry change of d-wave superconductivity in κ-type organic superconductors

Magnetic-field-angle-resolved heat capacity of dimer-Mott organic superconductors kappa-(BEDT-TTF)(2)X is reported (BEDT-TTF and X represent bis(ethylenedithio)tetrathiafulvalene and a monovalent anion, respectively). Temperature and field dependence of heat capacity indicates that the superconductivity has line nodes on the Fermi surface, which is a typical feature of d-wave superconductivity. In-plane field-angle dependence exhibits fourfold oscillations as well as twofold oscillations due to anisotropy of superconducting gap functions. From analyses of the fourfold term, the gap symmetry is determined as d(x2-y2) + s(+/-) for X = Cu[N(CN)(2)]Br and d(xy) for X = Ag(CN)(2)H2O. We suggest that the symmetry difference comes from the competition of d(xy) and d(x2-y2) antiferromagnetic fluctuations depending on lattice geometry.