Magnetic relaxation rate, critical current density and transport properties were investigated in bulk MgB2 from 1.6K to TC at magnetic fields up to 8T. A vortex phase diagram was depicted based upon these measurements. A large separation between the bulk irreversibility field Hirr(T) and the upper critical field HC2(T) was found. It was thus proposed that there was a quantum vortex liquid due to strong quantum fluctuation of vortices at 0K. It was also found that the magnetic relaxation rate was weakly dependent upon temperature but strongly dependent upon field, indicating a trivial influence of thermal fluctuation on the vortex de-pinning process. Therefore, the phase line Hirr(T) was attributed to quantum vortex melting in the rather clean system at a finite temperature.

Magnetic Relaxation and Critical Current Density of the New Superconductor MgB2. H.H.Wen, S.L.Li, Z.W.Zhao, H.Jin, Y.M.Ni, Z.A.Ren, G.C.Che, Z.X.Zhao: Superconductor Science and Technology, 2002, 15, 315-9