Temperature and field dependence of the anisotropy of MgB2

The anisotropy gamma 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 gamma = 6 at 15 K to 2.8 at 35 K. Reversible torque data near Tc reveal a field dependent anisotropy, increasing nearly linearly from gamma = 2 in zero field to 3.7 in 10 kOe. The unusual temperature dependence is a true bulk property and can be explained by non-local effects of anisotropic pairing and/or the wave vector dependence of the effective mass tensor.