Two-band effect on the temperature and the angle dependences of the ratio of the surface to the bulk superconductivity in MgB2

We investigated the ratio of the surface superconducting field (Hc3) to the bulk superconducting field (Hc2) in MgB2 single crystals at different temperatures and at different angles between the field and the c-axis of the crystal by using electrical transport measurements. The temperature dependence of the ratio Hc3/Hc2 at different angles was qualitatively well described by a recent theory based on the two-gap nature of MgB2. The angular dependence of the ratio shows that it can be both enhanced and reduced, which can be explained by the two-band theory. If slight deviations of Hc3/Hc2 from the value for the halfinfinite geometry used in the theory are to be accommodated, effects due to the finite geometry and to the orientation of the applied field relative to the crystal plane need to be considered. 4 Author to whom any correspondence should be addressed. New Journal of Physics 11 (2009) 073028 1367-2630/09/073028+08$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft