Superconducting state of the two-dimensional superconductor α-(ET)₂NH₄Hg(SCN)₄ under the magnetic field parallel to the conducting plane

The resistive transition to the superconducting (SC) state in α-(ET)₂NH₄Hg(SCN)₄ was studied under magnetic fields applied parallel to the conducting plane. The SC state is divided into three regions: (1) temperature-dependent resistive region, (2) zero-resistive region, and (3) temperature-independent resistive region. We observed twofold symmetry in the resistance under an in-plane magnetic field near the midpoint of the SC transition, which indicates that the upper critical field μ₀H_c2 has the same twofold symmetry. This in-plane anisotropy can be ascribed to the crystal structure.