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 alpha_I 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 Delta_Phi = (alpha_I)*(beta_L) relative to each other; beta_L is the screening parameter. This simple relation allows the determination of alpha_I in our experiments on YBa_2Cu_3O_(7-x} dc SQUIDs and comparison with theory. Extensive numerical simulations within a wide range of beta_L and noise parameter Gamma reveal a systematic dependence of the transfer function V_Phi on alpha_I and alpha_R (junction resistance asymmetry). As for the symmetric dc SQUID, V_Phi factorizes into g(Gamma*beta_L)*f(alpha_I,beta_L), where now f also depends on alpha_I. For β_L below five we find mostly a decrease of V_Phi with increasing alpha_I, which however can only partially account for the frequently observed discrepancy in V_Phi between theory and experiment for high-T_c dc SQUIDs.