Two-band Effect on the Superconducting Fluctuating Diamagnetism in MgB2

(Received 3 January 2005; published 22 June 2005) The field dependence of the magnetization above the transition temperature Tc in MgB2 is shown to evidence a diamagnetic contribution consistent with superconducting fluctuations reflecting both the  and  bands. In particular, the upturn field Hup in the magnetization curve, related to the incipient effect of the magnetic field in quenching the fluctuating pairs, displays a double structure, in correspondence with two correlation lengths. The experimental findings are satisfactorily described by the extension to the diamagnetism of a recent theory for paraconductivity, in the framework of a zero-dimensional model for the fluctuating superconducting droplets above Tc.