Anisotropies of the lower and upper critical fields in MgB2 single crystals -: art. no. 057001

The temperature dependence of the upper (H-c2) and lower (H-c1) critical fields has been deduced from Hall probe magnetization measurements of high quality MgB2 single crystals along the two main crystallographic directions. We show that Gamma(Hc2)=H-c2parallel toab/H-c2parallel toc and Gamma(Hc1)=H-c1parallel toc/H-c1parallel toab differ significantly at low temperature (being similar to5 and similar to1, respectively) and have opposite temperature dependencies. We suggest that MgB2 can be described by a single field dependent anisotropy parameter gamma(H) (=lambda(c)/lambda(ab)=xi(ab)/xi(c)) that increases from Gamma(Hc1) at low field to Gamma(Hc2) at high field.