Out-of-plane and in-plane anisotropy of upper critical field in MgB2 -: art. no. 104513

The anisotropy of the upper critical field in MgB2 has been investigated on single crystalline, dense polycrystalline and powder samples by both transport and magnetic measurements. On the single-crystal sample, H-c2(c) and H-c2(ab) have been measured directly with applied field parallel to the c axis and ab plane. The angular dependence of H-c2 shows deviation from the anisotropic Ginzburg-Landau model at lower temperatures. On polycrystalline and powder samples, H-c2(c) and H-c2(ab) have been determined by the method proposed by Bud'ko The value of anisotropy parameter gamma=H-c2(ab)/H-c2(c) is temperature and sample dependent, and is about 3 and 4.5 for single and polycrystals, respectively, near T-c. However, H-c2(c)(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-c2 in the ab plane has also been measured on single crystal and we set an upper bound of 1% for the in-plane anisotropy.