Characterizing, modeling, and optimizing high-T c superconducting quantum interference devices

Abstract

We have measured the current-voltage, flux-voltage, and noise characteristics of 77 K superconducting quantum interference devices made with step edge junctions and multilevel edge junctions on SrTiO3 substrates and with step edge junctions on LaAlO3. We find in each case that the I-V curves can have excess currents of up to 50%–80%, that the responsivity, ∂V/∂Φ, can be substantially smaller than expected from the resistively shunted junction (RSJ) model, but that the white noise voltage spectral density is about as expected. We discuss the extent to which the reduction of the responsivity is correlated with the excess current. We note that the observed unchanged magnitude of the voltage noise and the reduced responsivity would both be expected if we based the RSJ predictions on a model in which the excess current is not considered part of the Josephson current. Finally and perhaps fundamentally, we find that simulations made assuming a nonsinusoidal current-phase relationship give rise to an excess current, a reduction of the responsivity, and little change in the voltage noise spectral density, at least qualitatively as is observed.