Comparative study on the anisotropic properties of MgB2

The anisotropy of upper critical field of MgB2 has been studied on single crystals and poly-crystals by both transport and magnetic measurements. The angular dependence of Hc2 shows deviation from the anisotropic Ginzburg–Landau model at lower temperatures. The value of anisotropy parameter is temperature and sample dependent, and is about 3 and 4.5 for single and poly-crystals, respectively, at temperatures near Tc. However, Hc2c(T) are almost the same for all samples. These features could be an indication of anisotropic s-wave superconductivity with pancake-like energy gap or resulted from the different impurity levels in these samples. The anisotropy of Hc2 in ab-plane has also been measured and we set an upper bound of 1% for the in-plane anisotropy.