Upper critical field anisotropy in the Chevrel phase compound PbMo₆S₈

Abstract

The anisotropy of the upper critical field,H _c2, was measured on deoxygenated single crystals of the Chevrel phase compound PbMo₆S₈. The superconducting transition temperature,T _c, was determined with the applied field pointing in various directions in the hexagonal$(2\overline 1 0)$ plane. At a fixed value of the external field,T _c is an oscillating function of direction. Since theH _c2 versusT curve is linear in the field region we probed, the shape of the anisotropy inT _c at constantH is the same as the anisotropy ofH _c2 at constantT. The data reflect the low-temperature symmetry of the crystal structure. Two-fold symmetry was observed in the$(2\overline 1 0)$ plane. The data were fitted to the effective mass model with uniaxial symmetry. The anisotropy ofH _c2 is more complex than that previously observed by us in Cu_1.8Mo₆S₈. We relate this observation to the structural differences of these compounds.