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 MgB2 was achieved for the bulk nucleation field H-c2, in samples of aligned crystallites. A ratio H-c2(ab)/H(c2)(c)approximate to1.7 was found between the critical field parallel to the ab plane and parallel to the c axis direction. Further, detailed study of the H-c2 angular dependence confirmed its bulk origin, in contrast with what would be expected for the surface nucleation field H-c3. A Fermi velocity anisotropy was evaluated to be V(F)(ab)approximate to1.6 V-F(c), assuming an isotropic order parameter. For an anisotropic s-wave pairing symmetry it has been estimated that V(F)(ab)greater than or equal to2.5 V-F(c). Different H-c2 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 axe the field penetration depth lambda, 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.