Upper Critical Field and Irreversibility Line Determined byTransport 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 those determined in the magnetic measurement. By using the Arrhenius activation law ρ = ρ₀exp(-U/k_BT), the activation barrier U has been determined by the low dissipation part of R(T) curves. It is found that U k_BT manifests a very weak thermal activation and thermal fluctuation effect. This may further indicate that the flux dynamics near H_irr(T) is induced by strong quantum fluctuation and tunneling of vortices.