Transport Relaxation Measurements in Ag-Doped Superconducting Y-Ba-Cu-O

Systematic transport relaxation measurements were studied in an Ag doped ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7-{\rm x}}$ sample (YBCO/Ag) as functions of transport current $(I)$, temperature $(T)$ and external magnetic field $(H)$ . Similar measurements were repeated for YBCO sample without Ag for a comparison. The time effects were examined by the evolution of sample voltage ($V-t$ curves) as a function of time. Comparison between the $V-t$ curves YBCO-Ag and that of YBCO clearly reveals the effect of adding of Ag. At early times of relaxation process, it was observed that the sample voltage rises up rapidly in the $V-t$ curves of YBCO-Ag and levels off within a very short time. Furthermore, the time evolution of quenched state is quite different from that of YBCO as the transport current was reduced to a finite value. This difference in $V-t$ curves was interpreted in terms of presence of Ag which destroys the order parameter along the whole sample. In order to show the effect of Ag, we also studied current-voltage curves $(I-V)$ with different sweeping rates $({\rm d}I/{\rm d}t)$ of transport current. During the measurements, the transport current was cycled in up and down direction. In addition, in measuring of $I-V$ curves, two different procedures were applied, standard and reverse procedures. However, at moderate dissipation levels, we could not observe any difference between these two procedures. This experimental observation suggests that the surface effects are also weakened by adding of Ag into the superconducting matrix.