Determination of irreversibility fields in low temperature superconductors

The existence of the irreversibility line is one of the most intrigue features of high temperature superconductors. The irreversibility line is resulted from the short coherence length, large anisotropy and high working temperature of high-Tc oxides. For low temperature superconductors, however, there has been experimental evidence of the existence of the irreversibility line even though it is previously thought that the irreversibility line is too close to the upper critical field line to be observed. In this work, systematic measurements of dc magnetization, ac susceptibility and magnetoresistivity as a function of applied field and temperature have been performed on Nb alloy samples in order to investigate the irreversibility line in low temperature superconductors. The results show that there exits an observable region below the mean-field critical field, Hc2, where the magnetization is reversible during a cycle of increasing and decreasing field. Furthermore, results obtained on other low temperature superconductors such as the Chevrel phase superconductor PbMo6S8, Ba0.25Pb0.75BiO3, the newly discovered intermetallic compound MgB2 and overdoped cuprate Tl2Ba2CuO6+δ (Tc∼20 K) are also discussed