Studies on the anisotropic properties of MgB2

This paper presents a review on reported anisotropic properties of MgB2. The first direct measurement of an anisotropic superconducting property in MgB2was achieved for the bulk nucleation field Hc2, in samples of aligned crystallites. A ratio / ~ 1:7 was found between the critical field parallel to the ab plane and parallel to the c axis direction. Further, detailed study of the Hc2 angular dependence confirmed its bulk origin, in contrast with what would be expected for the surface nucleation field Hc3. A Fermi velocity anisotropy was evaluated to be ~1:6 , assuming an isotropic order parameter. For an anisotropic s-wave pairing symmetry it has been estimated that > or = 2:5 . Different Hc2 anisotropy has been found by different authors, using different samples, measured in varied temperature ranges. Other reported anisotropic properties of MgB2 in the superconducting state are the field penetration depth , coherence length xi, and energy gap delta; in the normal state are the magnetoresistance, compressibility, and thermal expansion. So far, most of the reported results have been obtained using aligned crystallites, c-axis oriented thin films and sub-millimeter crystals