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

Abstract

Magnetic-field-angle-resolved heat capacity of dimer-Mott organic superconductors κ-(BEDT-TTF)2X is reported. 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 x 2 −y 2 +s± for X=Cu[N(CN)2]Br and dxy for X=Ag(CN)2H2O. We suggest that the symmetry difference comes from the competition of dxy and d x 2 −y 2 antiferromagnetic fluctuations depending on lattice geometry.