In-plane anisotropy of the upper critical field of λ-(BETS)2GaCl4

The upper critical field Ho of a two-dimensional organic superconductor lambda-(BETS)(2)GaCl4 with T-c similar to 5 K was studied in a low temperature limit T much less thanT(c) (at 0.3 K) under a magnetic field applied parallel to the superconducting plane. The H(c)2 dependence on the angle within the plane showed a notable distorted four-fold symmetry, with the maximum anisotropy of similar to 5 %. This is in contrast to a two-fold symmetry determined by the effective mass anisotropy, as found in an angular dependence of the magnetoresistance in the normal state. The four-fold like term is in agreement with the theoretical prediction for a d-wave superconducting state.