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Japanese Journal of Applied Physics

Logic Operation Using a High Tc Coupled Superconducting Quantum Interference Device

Kazuo Saitoh, Hiroyuki Fuke, Tadashi Utagawa, Youichi Enomoto

Published Nov 1, 1996• 4 citations
Publisher

Abstract

Operation principle of AND/OR logic has been realized in a coupled high-T c Superconducting QUantum Interference Device (SQUID). The device consists of rf- and dc-SQUID's. The logic operation is based on the nonlinear characteristics of the internal and external magnetic flux in the rf-SQUID. Results of the logic operation can be read out by the dc-SQUID. The device is made of NdBa2Cu3O7-δ superconducting thin film with superconducting weak links fabricated using a narrow-focused Ga-ion beam technology.

DOI
10.1143/jjap.35.l1415

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Operation principle of AND/OR logic has been realized in a coupled high-T c Superconducting QUantum Interference Device (SQUID). The device consists of rf- and dc-SQUID's. The logic operation is based on the nonlinear characteristics of the internal and external magnetic flux in the rf-SQUID. Results of the logic operation can be read out by the dc-SQUID. The device is made of NdBa2Cu3O7-δ superconducting thin film with superconducting weak links fabricated using a narrow-focused Ga-ion beam technology.
The characteristics of a simple logic gate which consists of an rf-Superconducting QUantum Interference Device (SQUID) and dc-SQUID has been investigated in connection with the Single-Flux-Quantum (SFQ) logic operation and Josephson junction characteristics. The Josephson junctions were made of Neodymium Barium Copper Oxide thin film employing Focused Ion Beam technology. We confirmed the generating SFQ in the rf-SQUID and simultaneous read-out by the dc-SQUID up to 50 K. It has been found that the temperature dependence of output voltage level decreased more rapid than that of critical current normal resistance product. The origin of the discrepancy was studied using a phenomenological model of the Josephson junction and numerical simulation.
A thin-film dc superconducting quantum interference device (SQUID) consisting of quasiparticle-injected superconducting weak links was fabricated and successfully operated. This SQUID was made on a silicon substrate and it consists of a ∼100-nm-thick lead alloy with two injector junctions. The characteristics of this dc SQUID are controllable by varying the injecting rate of quasiparticles; therefore, it can be operated at arbitrary temperatures below the transition temperature of the superconducting material.
Conventional rf SQUIDs (superconducting quantum interference devices) consist of a superconducting loop interrupted by one weak link, and when read out via a tank circuit they produce output signals in the order of a few tens of microvolts. When the single weak link is replaced by a dc SQUID (a superconducting loop with two junctions) a so-called double quantization loop SQUID is formed. In this paper it is shown theoretically, and supported experimentally with thin-film devices of three different designs, that the general behaviour can be explained by a simplified picture of the fixed critical current of the single junction of the rf SQUID being replaced by an effective critical current of a dc SQUID loop that is varied by the applied magnetic flux (i.e., we have a rf SQUID with a variable β parameter). A more detailed analysis of the experimental data and the theory of this novel device reveals its interesting nonlinear character and shows the deviations from this simple picture. It is demonstrated experimentally that output signals in the order of ≥1 mV can be obtained with a suitable read-out mode for the tank circuit, a clear advantage when compared to conventional rf SQUIDs. With respect to dc SQUIDs, the coupling to integrated planar input coils will be unproblematic since no microwave resonances should occur in the structures.
For a superconductive quantum interference device (SQUID) with rather large loop inductance for a single flux quantum, store–restore operation of flux quanta has been examined by external current pulses. To sense the stored flux quanta, the device includes another directly coupled SQUID as a magnetometer. Both of the coplanar type dc SQUIDs have been composed of Josephson junctions made by the narrow-focused ion beam irradiation technique in single-layer NdBa2Cu3O7−δ superconducting films. The SQUID has a critical current of 375 μA and an inductive parameter (βL) of 32. The number of stored flux quanta is controlled by the pulse signal height and the input terminal, but not by the number of pulses.
We have fabricated superconducting quantum interference devices (dc SQUID’s) from thin films of the superconducting oxide YBa2Cu3Oy. The devices were made by first lithographically patterning an ion implant mask containing a 40 by 40 μm loop and two 17-μm-wide weak links over a ∼1-μm-thick oxide film. Ion implantation was then used to destroy the superconductivity in the film surrounding the device without actually removing material, resulting in a completely planar structure for the SQUID’s. The SQUID’s were operated in the temperature range from 4.2 to 68 K. The superconducting flux quantum was measured to be h/2e in these materials.
We describe a new dc-superconducting quantum interference device (SQUID) circuit of the FM type, which has a large coefficient for converting an applied magnetic flux into a frequency of MHz band or a lower band. The circuit consists of a SQUID amplifier composed of two stages, formed by a superconducting weak link and two dc-SQUID elements with nonhysteretic characteristics. In the first stage, the weak link and dc-SQUID elements are connected in parallel and operated by a common bias voltage. Since the respective elements oscillate with high frequencies by the ac Josephson effect, the two frequencies interact with each other, so that, the first stage generates a beat signal of a lower frequency. In the second stage, the other dc-SQUID element with a intrinsic flux gain enhances the beat signal generated during the first stage. A theoretical analysis of the circuit with computer simulation shows that the beat frequency is a periodical function of the magnetic flux, and the SQUID amplifier with two stages can improve the flux sensitivity.