Performance of YBa2Cu3O7−δ direct current SQUIDS with high-resistance step-edge junctions

Abstract

We measured the performance of dc superconducting quantum interference devices (SQUIDs) fabricated from YBa2Cu3O7−δ films as a function of the SQUID inductance, L, and normal resistance, Rn, of step-edge Josephson junctions and compared it to theoretical expectations. A process for adjusting critical currents and obtaining high normal resistances of junctions has been developed. All measurements were performed in liquid nitrogen, i.e., at 77 K. We could obtain controllably normal resistances of 5–10 Ω per junction at critical currents of 20–50 μA. A spectral density of white flux noise of SΦ1/2=2.5μΦ0/√Hz was obtained with a dc SQUID having L=50 pH, Rn=7 Ω per junction, βL=0.5 and ∂V/∂Φ=110 μV/Φ0. The best magnetic field resolution at 1 kHz was 20 fT/√Hz, with L=150 pH.

Optimization and passivation of HTS step-edge Josephson junction rf SQUIDs

“…The optimum ratio is lower than most reported values in the range of 0.5-1.0 [1]- [3], [5], [7], [8]. It is also significantly different from our rf SQUIDs of the same structure but fabricated using sputtered films for which is often 0.7.…”

“…Environmental degradation of HTS YBCO devices has been observed by many groups including ourselves [2], [8], [14]. In particular, junction trimming has been observed to increase the instability of the junction parameters.…”

“…A method of tuning device parameters after fabrication is desirable. There are a limited number of reports in the literature on junction trimming by heat annealing in Ar [8] or ion beam etching (IBE) [11]- [13], which showed improved performance for dc SQUIDs. In this work, junction-trimming techniques were applied to optimize our rf SQUIDs.…”

“…However, there is a lower limit on the film thickness before film discontinuity occurs over the step. The most commonly Manuscript [3], [5], [7], [8]. The low value of 0.5 was used by Ma et al [7] to optimize their rf SQUIDs.…”

High-frequency properties of high-T c Josephson junctions and mixing experiments in the W-band

“…The bow-tie antenna (Fig. la) Vc=IcRn, where Ic is the critical current and R~ is the normal resistance of junction, in the range between 1 mV and 1.8 mV at 4.2 K. A reduced oxygen concentration in the barrier region of the junction generally results in reduced critical temperature Tc, higher Rn and lower Ic, in accordance with known scaling laws [4]. To achieve optimal junction parameters, e.g., to adjust Vc, we performed a microwave oxygen plasma treatment.…”