Anisotropy in the upper and lower critical fields of MgB2 single crystals

MgB2 single crystals with a sharp transition width (DeltaT(c) similar to 0.3 K) were grown by the ambient-pressure synthesis. The upper and lower critical fields, H-c2 and H-c1 were evaluated from the magnetization for the applied field H-mu0 less than or equal to 5 T parallel to the three crystallographic directions. The coherence length xi and the penetration depth lambda for each direction were deduced from these critical fields with a little assumption for H-c1. The anisotropy ratio gamma(H) = xi(c)/xi(ab) was estimated to be similar to6.5 at 2.5 K and similar to2.3 near T-c (= 37.6 K), and gamma(lambda) = lambda(c)/lambda(ab) (= 3.0 +/- 0.2) to be almost constant over the range 2.5 K less than or equal to T < T-c. (C) 2004 Elsevier B.V. All rights reserved.