Upper critical field and irreversibility line determined by transport measurement of the new superconductor MgB2

Resistive transitions of the new superconductor MgB2 are carried out under magnetic fields from 0 to 8 T. It is found that the irreversibility field H -irr(T) and the upper critical field H-c2(T) obtained are very close to tho se determined in the magnetic measurements. By using the Arrhenius activati on law p = p(0)exp(- U/k(B) T), the activation barrier U has been determine d by the low dissipation part of R(T) curves. It is found that U much great er than k(B)T manifesting a very weak thermal activation and thermal fluctu ation effect. This may further indicate that the flux dynamics near H-irr(T ) is induced by strong quantum fluctuation and tunnelling of vortices.