The anisotropy, γ, of the superconducting state of high quality single crystals was determined, using torque magnetometry with 2 different methods. The anisotropy of the upper critical field was found to be temperature dependent, decreasing from about 6 at 15K to 2.8 at 35K. Reversible torque data near TC revealed a field-dependent anisotropy, increasing nearly linearly from about 2 in zero field to 3.7 in 10kOe. The unusual temperature dependence was a true bulk property and could be explained by non-local effects of anisotropic pairing and/or k--> dependence of the effective mass tensor.

Temperature and Field Dependence of the Anisotropy of MgB2. M.Angst, R.Puzniak, A.Wisniewski, J.Jun, S.M.Kazakov, J.Karpinski, J.Roos, H.Keller: Physical Review Letters, 2002, 88[16], 167004 (3pp)