Two-band effect on the superconducting fluctuating diamagnetism in MgB2

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 sigma and pi 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.