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Applied Physics Letters

Direct-injection high-Tc dc-SQUID with an upper YBa2Cu3O7−x ground plane

Hirotaka Terai, Mutsuo Hidaka, T. Satoh, Shuichi Tahara

Published May 19, 1997• 27 citations
Publisher

Abstract

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/□.

DOI
10.1063/1.118995

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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/□.
A dc superconducting quantum interference device (SQUID) with coplanar control lines has been fabricated from a single layer of YBa2Cu3O7 thin film. Step-edge junctions were formed using resist/Si double-mask layers. Current–voltage characteristics of this SQUID at 77 K show Shapiro steps under microwave irradiation and are qualitatively consistent with the resistively shunted junction (RSJ) model. The spacing between the closed loop and the coplanar control lines in the tested SQUIDs ranged from 0.2 to 1.5 μm. The measured output voltage-control current characteristics showed that the experimentally derived mutual inductances were a function of the spacing, in agreement with the calculated results. The typical mutual inductance for a spacing of 1.0 μm was ∼0.5 pH, which is adequate for detecting signals of flux-transfer logic gates in which the typical logic amplitude is ∼0.1 mA. A coplanar structure is thus useful for logic gate applications.
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.
In this work, we present nanoscale superconducting quantum interference devices (SQUIDs) with dimensions as small as 10 nm from the high-transition-temperature superconductor YBa2Cu3O7−δ (YBCO). The SQUID features and Josephson junctions are directly written into a 35-nm thick YBCO film with a focused helium ion beam. We integrate these nano-SQUIDs with directly written nano-isolated inductively coupled control lines to demonstrate a low power superconducting output driver capable of transimpedance conversion over a very wide temperature range of 4–50 K.
All high-Tc, multilevel edge junction superconducting quantum interference devices (SQUIDs) which now operate at 77 K have been fabricated using laser-ablated YBaCuO electrodes and in situ laser-ablated SrTiO3 for the barrier material. Devices with a SQUID inductance of about 70 pH have a peak to peak voltage swing, VΦ, up to 8 μV (dV/dΦ≂25 μV) and a flux noise (SΦ) of about 3×10−10 Φ20/Hz in the white noise region. IcRn, products for these devices are about 35 μV at 77 K with Ic in the range of 3–100 μA and operation to temperatures as high as 84 K observed. Scaling of the junction conductance and critical current with junction size has been measured on many devices and we observe an IcRn product that varies approximately as J0.89c.
We have fabricated a YBa2Cu3O7 (YBCO) thin-film dc superconducting quantum interference device (SQUID) with lithographically defined deep-submicron weak links. At 77 K the voltage response to dc flux is sinusoidal and nonhysteretic, with maximum peak-to-peak amplitude of 5 μV. The maximum response is 8 μV at 70 K, where 2LI0/Φ0≊1. At lower temperatures, the maximum response oscillates in the range 4–6 μV. Random telegraph noise was observed near 30–40 K. Well-behaved Shapiro steps were observable at all measured temperatures below Tc.
We have investigated the electrical properties of high-T c dc-SQUID's formed in a coplanar waveguide, and discussed their applicability to single flux quantum (SFQ) circuits. The planar type Josephson junctions were fabricated in NdBa2Cu3O y (NBCO) superconducting thin films by a narrow-focused ion beam (FIB) irradiation technique. The critical current and normal resistance values of the Josephson junction at 4.2 K were 115 µA and 8 Ω, respectively. The inductance was estimated by applying a direct injection of the control current into the dc-SQUID loop and measuring the periodic voltage response. A loop inductance of 47 pH was obtained for a 25 µm-wide and 20 µm-long dc-SQUID at 4.2 K. It has been clarified that for a stable operation of SFQ logic circuits, a 10 µm-square dc-SQUID's are required.