THE VORTEX LIQUID AND ITS TRANSITION TO A SOLID FOR YBA2CU3O7-X

Evidence for the plastic vortex liquid and its transition to a plastic solid for YBa2Cu3O7-x (YBCO) is given based on measurements of the ef fects of the ac field amplitude, frequency and de field Ed On the ac s usceptibility. The time scale to and effective activation energies U(B -d,j) of vortex motion are extracted, where j is the current density. to is smaller by three orders of magnitude at temperatures T>T-m than at T<T-m, where T-m is a field-dependent characteristic temperature de termined experimentally. This implies that the vortex velocity is larg er at T>T-m than at T<T-m by the same orders of magnitude and the tran sition from vortex liquid to solid has occurred. The energy barriers, U-1(B-d,j) proportional to B-d(-0.66) depending weakly on j at T>T-m a nd U-2(B-d,j) proportional to B-d(-0.43)[1-(j/j(c))(0.15)] at T<T-m, d emonstrate the characteristics of the plastic motion of the liquid and vortex solid, respectively. Thus Tm(Bd) is considered to be the liqui d-solid transition temperature and obeys the power law B-d proportiona l to (1 - T-m/Tc)(1.47), which is modestly in agreement with the measu red value. A method is proposed for constructing the vortex liquid-sol id transition line by ac susceptibility measurements.