Reentrant AC magnetic susceptibility in Josephson-junction arrays: An alternative explanation for the paramagnetic Meissner effect

摘要

The paramagnetic Meissner effect (PME) measured in high $T_{C}$ granularsuperconductors has been attributed to the presence of $\pi$-junctions betweenthe grains. Here we present measurements of complex AC magnetic susceptibilityfrom two-dimensional arrays of conventional (non $\pi$) Nb/Al/AlOx/Nb Josephsonjunctions. We measured the susceptibility as a function of the temperature $T$,the AC amplitude of the excitation field, $h_{AC}$, and the external magneticfield, $H_{DC}$. The experiments show a strong paramagnetic contribution fromthe multi-junction loops, which manifests itself as a reentrant screening atlow temperature, for values of $h_{AC}$ higher than 50 mOe. A highly simplifiedmodel, based on a single loop containing four junction, accounts for thisparamagnetic contribution and the range of parameters in which it appears. Thismodel offers an alternative explanation of PME which does not involve$\pi$-junctions.