Positional disorder in Josephson-junction arrays: Experiments and simulations

We present the results of measurements on proximity-effect arrays with deliberate positional disorder, characterized by a parameter ${\Delta }^{\mathrm{*}}$. This system provides an experimental realization of XY magnet with random Dzyaloshinskii-Moriya interactions. In measurements of resistance versus perpendicular magnetic field, R($f_0$), we find strong evidence for a critical field $f_c$∝1/${\Delta }^{\mathrm{*}}$ beyond which phase coherence is destroyed. Our Monte Carlo simulations show evidence for reentrant behavior in the helicity modulus Y, but with Y always finite at the lowest temperatures. Such a critical field and reentrance were predicted by Granato and Kosterlitz.