Voltage-flux-characteristics of asymmetric dc SQUIDs
Abstract
We present a detailed analysis of voltage-flux V(Phi)-characteristics for asymmetric dc SQUIDs with various kinds of asymmetries. For finite asymmetry alphaI in the critical currents of the two Josephson junctions, the minima in the V(Phi)-characteristics for bias currents of opposite polarity are shifted along the flux axis by DeltaPhi = (alphaI)*(betaL) relative to each other; betaL is the screening parameter. This simple relation allows the determination of alphaI in our experiments on YBa2Cu3O(7-x dc SQUIDs and comparison with theory. Extensive numerical simulations within a wide range of betaL and noise parameter Gamma reveal a systematic dependence of the transfer function VPhi on alphaI and alphaR (junction resistance asymmetry). As for the symmetric dc SQUID, VPhi factorizes into g(Gamma*betaL)*f(alphaI,betaL), where now f also depends on alphaI. For βL below five we find mostly a decrease of VPhi with increasing alphaI, which however can only partially account for the frequently observed discrepancy in VPhi between theory and experiment for high-Tc dc SQUIDs.
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