Dynamic Instabilities and Current-Voltage Curves of Polycrystalline Superconducting Y1Ba2Cu3O7−x /Ag

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We performed systematic current-voltage measurements (I–V curves) in Ag—added the polycrystalline Y1Ba2Cu3O7−x sample (YBCO/Ag) as a function of the transport current (I), temperature (T), external magnetic field (H), and sweeping rates (dI/dt). Standard and reverse procedures were used in the measurements of I–V curves. The obtained results were compared to I–V curves of the YBCO and polycrystalline sample of MgB2. Upon cycling transport current, the I–V curves of the YBCO/Ag sample exhibit hysteresis effects for both procedures and are sensitive to the variation of dI/dt. The experimental data reveal that the irreversibilities in the I–V curves of YBCO are more prominent than those of YBCO/Ag. Furthermore, there are no considerable hysteresis effects in the I–V curve of MgB2 and this behavior is attributed to the absence of the weak-link structure in MgB2. We suggest that the physical origin of the rather small hysteresis effects in the I–V curves of the YBCO/Ag sample can be related to the destruction of weak-link structure due to the addition of Ag into the superconducting matrix. Ag destroys partly the intergranular pinning properties of the YBCO ceramic by increasing grain coupling and, therefore, the irreversibilities in the I–V curves of the YBCO/Ag decrease considerably. The instabilities and short- and long-lived plateau regions observed in I–V curves were explained in terms of plastic flow of flux lines along easy motion channels, which are considered mainly as metallic silver paths in the YBCO/Ag sample. At moderate dissipation levels, we could not observe any difference between the data obtained by using the standard and reverse procedures. This suggests that the surface effects are also weakened by the addition of Ag into the superconducting structure.