Reentrant ac Magnetic Susceptibility in Josephson-Junction Arrays

Abstract

We have measured the complex ac magnetic susceptibility of unshunted Josephson-junction arrays as a function of temperature $T$, amplitude of the excitation field ${h}_{\mathrm{ac}}$, and external magnetic field ${H}_{\mathrm{dc}}$. For small ${h}_{\mathrm{ac}}$ Meissner screening occurs. For larger ${h}_{\mathrm{ac}}$, however, the screening is reentrant in $T$. This reentrance is not thermodynamic but dynamic and arises from the paramagnetic contribution of multijunction loops. This result gives an alternative explanation of the paramagnetic Meissner effect observed in granular superconductors. Experimental results are in agreement with a simplified model based on a single loop containing four junctions.