REENTRANT AC MAGNETIC-SUSCEPTIBILITY IN JOSEPHSON-JUNCTION ARRAYS

We have measured the complex ac magnetic susceptibility of unshunted J osephson-junction arrays as a function of temperature T, amplitude of the excitation field h(ac), and external magnetic field H-dc. For smal l h(ac) Meissner screening occurs. For larger h(ac), however, the scre ening is reentrant in T. This reentrance is not thermodynamic but dyna mic and arises from the paramagnetic contribution of multijunction loo ps. This result gives an alternative explanation of the paramagnetic M eissner effect observed in granular superconductors. Experimental resu lts are in agreement with a simplified model based on a single loop co ntaining four junctions.