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Applied Physics Letters

Fundamental operation of single-flux-quantum circuits using coplanar-type high-T c SQUIDs

Hiroyuki Fuke, Kazuo Saitoh, Tadashi Utagawa, Youichi Enomoto

Published Nov 4, 1996• 11 citations
Publisher

Abstract

We have fabricated coplanar type dc SQUIDs using NdBa2Cu3Oy superconducting thin films and operated fundamental single-flux-quantum (SFQ) circuits. The Josephson junctions were made by the narrow-focused ion beam irradiation technique. For a 145 μm wide and 10 μm long logic SQUID having a critical current of 105 μA and an inductive parameter (βL) of 28, a store and a restore of the flux quantum have been demonstrated at temperatures of 4.2–30 K. These operations were performed with an input pulsewidth of 5 ns (5 ns was the shortest input pulse width available from our function generating equipment). These results show experimentally the possibility of high speed operation in all high-Tc superconducting digital circuits.

DOI
10.1063/1.117324

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We have fabricated coplanar type dc SQUIDs using NdBa2Cu3Oy superconducting thin films and operated fundamental single-flux-quantum (SFQ) circuits. The Josephson junctions were made by the narrow-focused ion beam irradiation technique. For a 145 μm wide and 10 μm long logic SQUID having a critical current of 105 μA and an inductive parameter (βL) of 28, a store and a restore of the flux quantum have been demonstrated at temperatures of 4.2–30 K. These operations were performed with an input pulsewidth of 5 ns (5 ns was the shortest input pulse width available from our function generating equipment). These results show experimentally the possibility of high speed operation in all high-Tc superconducting digital circuits.
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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.
Basic characteristics of a simple single-flux-quantum (SFQ) logic gate using high-Tc material and Josephson junction (NdBa2Cu3O7−δ and focused ion beam junction) have been investigated. The logic gate consists of an rf-superconducting quantum interference device (rf-SQUID) and a dc-SQUID. In the logic gate, elementary SFQ logic operations, such as generating SFQ (dc/SFQ) and providing simultaneous readout (SFQ/dc), have been confirmed up to 50 K. The temperature dependencies of the output voltage level and the critical current-normal resistance (IcRn) product were compared, and the decreasing tendency of the output voltage level for increasing temperature was found to be more rapid than that of the IcRn product.
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.
YBa2_{2}2​Cu3_{3}3​O7−δ_{7-\delta }7−δ​Single Flux Quantum Flip Flop Directly Written With a Focused Helium Ion Beam
Superconducting logic circuits based on low transition temperature (Tc) are attractive due to their intrinsic low power consumption and high operating frequency. They have a wide variety of applications (particularly in future computing), but are difficult to use because they require cooling with liquid helium to operate. The current high Tclogic circuit based on YBa2Cu3O7-δ(YBCO) already has the ability to operate at a temperature above 4 K. However, this material is difficult to integrate into large-scale circuits by the traditional YBCO junction fabrication process. The limits of fabrication methods are the bottleneck stifling this material's potential for integration into logic circuits. As such, in this paper we aim to advance the state of the art in junction fabrication technology. Here we report the demonstration of a high Tcsuperconducting single flux quantum (SFQ) circuit made by a focused helium ion beam that is easily fabricated and can readily be incorporated into a large-scale integration with nano-scale junctions for future applications. An inductively coupled readout superconducting quantum interference device (SQUID) is employed to reduce the circuit complexity. The circuit parameters and performance are discussed.
We have implemented a simple circuit of the rapid single-flux-quantum (RSFQ) logic family using a single-layer YBa2Cu3O7−x thin-film structure with 14 in-plane Josephson junctions formed by direct electron beam writing. The circuit includes two dc/SFQ converters, two Josephson transmission lines, a complete RS SFQ flip-flop, and an SFQ/dc converter (readout SQUID). Low-frequency testing has shown that the dc-current-biased circuit operates correctly and reliably at T∼30 K, a few degrees below the effective critical temperature of the junctions. Prospects for a further increase of the operation temperature and implementation of more complex RSFQ circuits are discussed in brief.