The upper critical field HC2 for fields perpendicular and parallel to the Mg and B planes of magnesium diboride was studied from 5.4K up to TC by comparing high-field magnetotransport, ac-susceptibility and specific heat measurements. The onset of the finite resistivity yielded HC2, while the end of the resistive transition to the normal state was due to surface effects. The HC2 perpendicular to the planes revealed a conventional temperature dependence with μ0HC2ab(0) = 3.5T but the parallel critical field with μ0HC2||ab(0) = 17T had a positive curvature at temperatures above 20K. Consequently, the anisotropy factor Γ = HC2||ab/HC2ab was temperature dependent with Γ equal to 4.8 at low temperatures and about 2 near TC. The angular dependence of the upper critical field of an elliptic form was obtained at several temperatures.

Upper Critical Magnetic Fields in Single Crystal MgB2. L.Lyard, P.Samuely, P.Szabó, C.Marcenat, T.Klein, K.H.P.Kim, C.U.Jung, H.S.Lee, B.Kang, S.Choi, S.I.Lee, L.Paulius, J.Marcus, S.Blanchard, A.G.M.Jansen, U.Welp, G.Karapetrov, W.K.Kwok: Superconductor Science and Technology, 2003, 16, 193-8