Time Dependent Transport Properties of Superconducting MgB$_{2}$

The time dependent transport properties of polycrystalline superconducting MgB$_{2}$ sample was investigated in details by means of current-voltage (I -- V curves) measurements with different current sweep rates (dI/dt) and transport relaxation (V -- t curves) measurements. The corresponding measurements were carried out as functions of temperature (T), transport current (I) and external magnetic field (H). Upon cycling of the transport current, it was observed that the hysteresis effects in I -- V curves are negligible. Further, for slow or fast sweep rates (dI/dt) of the transport current, the measured voltage dissipation at a given transport current is approximately stable and does not change much with time. Analysis of the I -- V curves shows that the pinning potential is also practically independent of dI/dt. The time evolution of sample voltage reveals that the increase in sample voltage is quite a sharp and non-linear. The V -- t curves were interpreted in terms of re-organization of flux lines and suppression of the effective superconducting order parameter along the sample as the time progresses. The quenched state created in V -- t curves was correlated to the supercooling of flux lines. Finally, the experimental results of polycrystalline MgB$_{2}$ sample were compared to previous similar studies on superconducting Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-x}$ and Bi$_{1.7}$Pb$_{0.3}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{x}$ polycrystalline samples.