Temperature and Field Dependence of the Anisotropy ofMgB2

Abstract
The anisotropy γ of the superconducting state of high quality single crystals of MgB2 was determined, using torque magnetometry with two different methods. The anisotropy of the upper critical field was found to be temperature dependent, decreasing from γ6 at 15 K to 2.8 at 35 K. Reversible torque data near Tc reveal a field dependent anisotropy, increasing nearly linearly from γ2 in zero field to 3.7 in 10 kOe. The unusual temperature dependence is a true bulk property and can be explained by nonlocal effects of anisotropic pairing and/or the k dependence of the effective mass tensor.