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Applied Physics Letters

High-Tc interferometers using all YBa2Cu3O7−δ trilayer junctions

Harumichi Sato, Eiji Fujimoto, Tomoaki Yamada, Hiroshi Akoh, T. Nakajima, K. Hohkawa, Hiroshi Nakagawa, Masahiro Aoyagi

Published Sep 18, 2003• 0 citations
Publisher

Abstract

We have demonstrated high-Tc interferometers using YBaCuO trilayer junctions, which are two-junction-type interferometers with a direct current injection. In order to realize a small loop inductance value without a ground plane, the interferometers were designed to have loop inductance with a vertical stacked structure by overlapping junction bridges on base electrodes. The interferometers showed clear threshold characteristics in the control current dependence of the gate current at temperatures below 70 K. The temperature dependence of the loop inductance showed good agreement with a calculation result based on a strip-line model. The sheet inductance was estimated to be 0.85 pH/□ at 4.2 K from the threshold characteristics of the interferometers by changing the bridge length between the two junctions, which was in good agreement with the design value.

DOI
10.1063/1.1613037

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We have fabricated direct-injection high-Tc superconducting quantum interference devices (SQUIDs) with integrated YBa2Cu3O7−x (YBCO) ground planes in the upper part of structure. The YBCO/PrBa2Cu3O7−x(PBCO)/YBCO edge junctions with 10-nm-thick PBCO barriers showed RSJ-like current-voltage (I–V) characteristics before and after the ground plane formation. We measured the voltage modulations of these SQUIDs with two different hole sizes to evaluate the line inductance. At a temperature of 30 K, the estimated line inductance under the ground plane was about 1.0 pH/□, while the estimated line inductance without the ground plane was 2.8 pH/□.
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Abstract Superconducting quantum interference devices (SQUIDs) with 3D nano-bridge junctions can be miniaturized into nano-SQUIDs that are able to sense a few spins in a large magnetic field. Among all device parameters, the inductance is key to the performance of SQUIDs with 3D nano-bridge junctions. Here, we measured the critical-current magnetic flux modulation curves of 12 devices with three design types using a current strip-line directly coupled to the SQUID loop. A best flux modulation depth of 71% was achieved for our 3D Nb SQUID. From the modulation curves, we extracted the inductance values of the current stripe-line in each design and compared them with the corresponding simulation results of InductEX. In this way, London penetration depths of 110 and 420 nm were determined for our Nb (niobium) and NbN (niobium nitride) films, respectively. Furthermore, we showed that inductances of 11 and 119 pH for Nb and NbN 3D nano-bridge junctions, respectively, dominated the total inductance of our SQUID loops which are 23 pH for Nb and 255 pH for NbN. A screening parameter being equal to one suggests optimal critical currents of 89.6 and 8.1 μ A for Nb and NbN SQUIDs, respectively. Additionally, intrinsic flux noise of 110 ± 40 n Φ 0 /(Hz) 1/2 is calculated for the Nb SQUIDs with 3D nano-bridge junctions by Langevin simulation.
Threshold curves for multijunction superconducting interferometers have been calculated previously, showing general agreement with observed features, especially in symmetric cases. We here add some more details to the analysis, paying particular attention to the effects of asymmetries in coupling, inductance, or critical currents. Feed-loop inductance and flux quantization in the feed loop can be important. A changing lobe pattern over many periods, asymmetries within a period, shifting patterns between runs spanning a warm-up, and sudden changes in pattern because of noise in the environment are all quantitatively explainable on the basis of this model. By use of a single ’’calibration curve’’, the inductance for symmetric two- or three-junction interferometers can be obtained immediately.
It has long been recognized that a control current Ia injected into the section of a two- junction superconducting quantum interference device is able to produce a change of its critical current Ic, so that a current gain g = |dIc/dIa| can be identified. We investigate the circumstances under which large gains can be achieved by using vertical Josephson interferometers, which are characterized by small loop inductances. We discuss the theory of operation of such a device, its performances, and its advantages with respect to planar interferometers used in the previous works. Two potential applications are addressed.
The performance of Josephson based devices strongly depend on the inductance properties associated with the material and circuit design. Here we compare the inductance of YBa2Cu3O7-δfilms with and without superconducting ground planes grown by reactive coevaporation. Specifically, we fabricated several superconducting quantum interference devices from single and multi-layer films with different geometries using a focused helium ion beam. Measurements of device electrical transport properties were analyzed to experimentally determine the sheet inductance. Additionally, measurements of the temperature dependence of the inductance was used to separate the contributions from geometric and kinetic inductance. We find that the presence of the ground plane in the multi-layer structure reduces the contribution of geometric inductance with no detectable change in the kinetic inductance.