Gap Symmetry of the Organic Superconductor λ-(BETS)₂GaCl₄ Determined by Magnetic-Field-Angle-Resolved Heat Capacity

Abstract

Angle-resolved heat capacity (ARHC) measurements with multi-directional magnetic fields for an organic superconductor λ-(BETS)2GaCl4 are performed to investigate the out-of-plane (polar angle) and in-plane (azimuthal angle) anisotropic features in the superconducting electronic heat capacity Cele. The polar angle (θ) dependences of Cele have a simple twofold periodicity caused by the anisotropy of the critical field Hc2. In contrast, the azimuthal angle (ϕ) dependences show asymmetric anisotropies including twofold and fourfold-like terms originating from the anisotropy of the superconducting gap function. From the results, we discuss the gap function from the perspective of the mixed symmetry caused by the antiferromagnetic spin fluctuation on the low-symmetry λ-type lattice.