Critical field anisotropy in (TMTSF)₂ CLO₄ in the basal plane

We have performed detailed measurements of the critical field of (TMTSF)₂ ClO₄ in the plane perpendicular to the highly conducting a axis. We find a large anisotropy with the field in the c direction having a value which is more than an order of magnitude less than along the b direction. The b axis critical field is only slightly (50%) less than the a axis (the highly conducting direction). The angular and temperature dependences suggest that the anisotropy is a band structure effect along b and c, but that the a direction critical field is Pauli limited. These results imply that critical field anisotropy in the directions parallel and perpendicular to the highly conducting a axis cannot be used go obtain information about the band structure anisotropy and that the observed superconductivity is singlet rather than triplet.