Voltage-flux-characteristics of asymmetric dc SQUIDs

Abstract

Abstract-We present a detailed analysis of voltage-flux V (Φ)-characteristics for asymmetric dc SQUIDs with various kinds of asymmetries. For finite asymmetry α I in the critical currents of the two Josephson junctions, the minima in the V (Φ)-characteristics for bias currents of opposite polarity are shifted along the flux axis by ∆Φ = α I β L relative to each other; β L is the screening parameter. This simple relation allows the determination of α I in our experiments on YBa 2 Cu 3 O 7−δ dc SQUIDs and comparison with theory. Extensive numerical simulations within a wide range of β L and noise parameter Γ reveal a systematic dependence of the transfer function V Φ on α I and α R (junction resistance asymmetry) . As for the symmetric dc SQUID,, where now f also depends on α I . For β L < ∼5 we find mostly a decrease of V Φ with increasing α I , which however can only partially account for the frequently observed discrepancy in V Φ between theory and experiment for high-Tc dc SQUIDs.

Spreads in Critical Current and Normal Conductance of High-$T_{\rm c}$ dc SQUID

“…The asymmetry typically leads to reduction in the transfer functions, i.e. and , of dc SQUID [13]. Nevertheless, the transfer functions can also be enhanced with the optimized asymmetry conditions [13], [15], [16].…”

“…The asymmetry is quantified by the parameter , where and are the critical currents of Josephson junctions [13]. For the geometrically symmetric SQUID, the bridge widths of junctions are identical, and the asymmetry due to the spread is the "intrinsic" asymmetry [13], i.e.…”

The voltage–current characteristic of high-TC DC SQUID: Theory, simulation, experiment

“…The practical importance of asymmetric DC SQUIDs is that they have a higher transfer function (the slope of the voltage-to-flux curve) as compared to the symmetric case 10,11,12,13,14 . This property is generally attributed to the distortion of the voltage-to-flux curve.…”

“…The second factor describes the critical current suppression by the thermal fluctuations, which is similar to the suppression effect in a single Josephson junction 9 . The factorization allows us to carry the first factor to the left hand side of expression (11), so that we consider below the reduced modulation ∆V R = exp(α/2)∆V /R, which depends on the critical and bias currents only.…”

“…However, it should be remembered that Eqs. (4) and (11) are the approximate expressions which account for the first order term in the perturbation expansion of the voltage over small parameter ε = exp(−α/2).…”