By using transport and magnetic measurements, the upper critical field HC2(T) and the irreversibility line Hirr(T) were determined. A large separation between HC2(0) and Hirr(0) was found showing the existence of a quantum vortex liquid state induced by quantum fluctuation of vortices in the new superconductor MgB2. Further investigation of the magnetic relaxation shows that both the quantum tunnelling and the thermally activated flux creep depended weakly upon temperature. However, when the melting field Hirr was approached, a drastic rise of the relaxation rate was observed. This was suggested to indicate that the melting of the vortex matter at a finite temperature was also induced by the quantum fluctuation of vortices.

Strong Quantum Fluctuation of Vortices in Bulk Samples of the New Superconductor MgB2. - H.H.Wen, S.L.Li, Z.W.Zhao, Y.M.Ni, Z.A.Ren, G.C.Che, H.P.Yang, Z.Y.Liu, Z.X.Zhao: Chinese Physics Letters, 2001, 18, 816-9