Out-of-plane and in-plane anisotropy of upper critical field in MgB2

The anisotropy of the upper critical field in MgB 2 has been investigated on single crystalline, dense polycrystalline and powder samples by both transport and magnetic measurements. On the single-crystal sample, H c c2 and H a b c2 have been measured directly with applied field parallel to the c axis and ab plane. The angular dependence of H c 2 shows deviation from the anisotropic Ginzburg-Landau model at lower temperatures. On polycrystalline and powder samples, H c c2 and H a b c2 have been determined by the method proposed by Bud'ko et al. The value of anisotropy parameter γ=H a b c2/H c c2 is temperature and sample dependent, and is about 3 and 4.5 for single and polycrystals, respectively, near T c . However, H c c2(T) are almost the same for all samples. These features could be an indication of anisotropic s-wave superconductivity with pancakelike energy gap or resulted from the different impurity levels in these samples. The anisotropy of H c 2 in the ab plane has also been measured on single crystal and we set an upper bound of 1% for the in-plane anisotropy.