Upper Critical Field and Irreversibility Line Determined byTransport Measurement of the New Superconductor MgB2

Abstract

Resistive transitions of the new superconductor MgB2 are carried outunder magnetic fields from 0 to 8 T. It is found that the irreversibilityfield Hirr(T) and the upper critical field Hc2(T) obtained are very close to those determined in themagnetic measurements. By using the Arrhenius activation law ρ=ρ0exp(-U/kBT), the activationbarrier U has been determined by the low dissipation part of R(T) curves. It is found that U>>kBTmanifesting a very weak thermal activation and thermal fluctuation effect.This may further indicate that the flux dynamics near Hirr(T) is induced by strong quantum fluctuation and tunnelling of vortices.