Evidence of a Mixed State Transition Region Between the Vortex Solid and Liquid Phases in YBa_2Cu_3O_(7-δ) single crystals

摘要

Transport and a.c. susceptibility measurements have been performed on detwinned YBa_2Cu_3O_(7-δ) single crystals in the vicinity of the melting line. We have extracted threshold current (I_v) versus temperature dependences from a set of V(I) curves using three different voltage criteria. These dependences showed a sharp rise over a transition region in the vicinity of T_m, the width of which was independent of the particular voltage criterion. A similar sharp transition was observed in the out of phase component of the a.c. susceptibility, the width of which was independent of the applied a.c. field. We propose that within the transition region, the vortex solid and liquid phases coexist. From a comparison of the extracted I_v(T) curves with a low current ohmic R(T) dependence we estimate that the resistivity falls to zero when between 10 and 30 % of the vortices are in the solid phase. We explain this observation in terms of vortex liquid flowing in channels between solid domains.