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Applied Physics Letters

Digital logic with YBa2Cu3O7−x dc SQUIDs

S. Schwarzbek, Roger A. Davidheiser, G.R. Fischer, Jerome A. Luine, N.J. Schneier

Published Aug 12, 1991• 14 citations
Publisher

Abstract

We report the operation of and, exor, exnor, and inverting gates in a circuit architecture suitable for the set of all Boolean gates. Series arrays of YBa2Cu3O7−x dc superconducting quantum interference devices with integrated coils exhibit sufficient modulation to allow low-speed tests demonstrating gate logic, and lack of oscillation or latching. The best inverter operated (without load) at 36 μV, with 2 nW of gate dissipation.

DOI
10.1063/1.105262

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We report the operation of and, exor, exnor, and inverting gates in a circuit architecture suitable for the set of all Boolean gates. Series arrays of YBa2Cu3O7−x dc superconducting quantum interference devices with integrated coils exhibit sufficient modulation to allow low-speed tests demonstrating gate logic, and lack of oscillation or latching. The best inverter operated (without load) at 36 μV, with 2 nW of gate dissipation.
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Arrays of dc-superconducting quantum interference devices (dc-SQUIDs) were investigated, in which each dc-SQUID element has a multiturn input coil. The dependence of flux noise on the number of turns of the input coil was studied for current amplifiers. SQUID arrays were fabricated by a Nb/AlO x /Nb Josephson tunnel junction fabrication process. Results show that the flux noise of the SQUID array amplifier does not depend on the turn number of the input coil. A SQUID array amplifier consisting of 128 dc-SQUID elements was fabricated, in which each element had a 7-turn input coil and a mutual inductance between the input coil and the SQUID loop of 460 pH. This amplifier showed an ultrahigh current resolution of 1 pA/Hz0.5 and a maximum input current of 11 µA for signals of 50 kHz, which is sufficient bandwidth for X-ray detection applications. These values correspond to a large dynamic range of 87 dB. The flux-voltage characteristics of this SQUID array, however, showed asymmetric behavior, which originated from electronic resonance in stray capacitance and the mutual inductance between the SQUID loop and the multiturn input coil.
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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.
One can use a direct current (dc) superconducting quantum interference device (SQUID) as a current amplifier by injecting a signal current Is into part of the superconducting loop and detecting the resultant change in critical current Ic. The current gain max ‖dIc/dIs‖ can be increased by making the inductances of the two arms of the SQUID asymmetric, thereby skewing the transfer function Ic vs Is. Detailed simulations of the device performance include an analysis of the impact of inductance and junction asymmetry on the transfer function. At 77 K a maximum current gain of 5 should be achievable albeit for frequencies below 1 GHz. An increase in the operating speed to 2 GHz can be achieved by reducing the gain to 2. We have fabricated asymmetric YBa2Cu3O7−x dc SQUIDs on SrTiO3 bicrystal substrates and operated them at 77 K. Using a second, readout SQUID to monitor changes in the critical current, we have achieved a low-frequency current gain up to 2.3. By coupling two SQUIDs together, we have increased the gain to 8.5.