Strong Quantum Fluctuation of Vortices in Bulk Samples of the New Superconductor MgB₂

By using transport and magnetic measurement, the upper critical field H_c2(T) and the irreversibility line H_irr(T) has been determined. A big separation between H_c2(0) and H_irr(0) has been found showing the existence of a quantum vortex liquid state induced by quantum fluctuation of vortices in the new superconductor MgB₂. Further investigation on the magnetic relaxation shows that both the quantum tunneling and the thermally activated flux creep depends weakly on temperature. However, when the melting field H_irr is approached, a drastic rise of the relaxation rate is observed. This may indicate that the melting of the vortex matter at a finite temperature is also induced by the quantum fluctuation of vortices.