Dynamical reentrance and geometry imposed quantization effects in Nb-AlOx-Nb Josephson junction arrays

摘要

In this paper, we report on different phenomena related to the magnetic properties of artificially prepared highly ordered ( periodic) two- dimensional Josephson junction arrays ( 2D- JJAs) of both shunted and unshunted Nb - AlOx - Nb tunnel junctions. By employing mutual- inductance measurements and using a high- sensitivity home- made bridge, we have thoroughly investigated ( both experimentally and theoretically) the temperature and magnetic field dependence of complex AC susceptibility of 2D- JJAs. After a brief description of the measurement technique and the numerical simulations method, we proceed to demonstrate that the observed dynamic reentrance ( DR) phenomenon is directly linked to the value of the Stewart - McCumber parameter beta(C). By simultaneously varying the inductance related parameter beta(L), we obtain a phase diagram beta(C) - beta L ( which demarcates the border between the reentrant and non- reentrant behavior) and show that only arrays with sufficiently large value of beta(C) will exhibit DR behavior. The second topic of this paper is related to the step- like structure ( with the number of steps n = 4 corresponding to the number of flux quanta that can be screened by the maximum critical current of the junctions) which has been observed in the temperature dependence of AC susceptibility in our unshunted 2D- JJA with beta(L) ( 4.2 K) = 30 and attributed to the geometric properties of the array. The steps are predicted to manifest themselves in arrays with beta(L)( T) matching a 'quantization' condition beta(L)( 0) = 2 pi( n + 1). In conclusion, we demonstrate the use of the scanning SQUID microscope for imaging the local flux distribution within our unshunted arrays.