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Applied Physics Letters

Asymmetric YBa2Cu3O7−x dc SQUID: A three terminal device with current gain at 77 K

D. Koelle, R. Kleiner, Frank Ludwig, A. H. Miklich, E. Dantsker, John L. Clarke

Published Jan 30, 1995• 19 citations
Publisher

Abstract

We have fabricated asymmetric, two-junction YBa2Cu3O7−x dc superconducting quantum interference devices (SQUIDs) on SrTiO3 bicrystal substrates and used them as three-terminal amplifiers by injecting signal currents directly into part of the superconducting loop. Using a second, readout SQUID to monitor changes in the critical current, we have achieved current gains up to 2.3 at 77 K. By coupling two SQUIDs together, we have increased the gain to 8.5 at 77 K. The experimental results are compared with numerical simulations, which are used to discuss potential increases in the gain.

DOI
10.1063/1.114145

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We have fabricated asymmetric, two-junction YBa2Cu3O7−x dc superconducting quantum interference devices (SQUIDs) on SrTiO3 bicrystal substrates and used them as three-terminal amplifiers by injecting signal currents directly into part of the superconducting loop. Using a second, readout SQUID to monitor changes in the critical current, we have achieved current gains up to 2.3 at 77 K. By coupling two SQUIDs together, we have increased the gain to 8.5 at 77 K. The experimental results are compared with numerical simulations, which are used to discuss potential increases in the gain.
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.
We have studied low frequency (1/f) noise of YBa2Cu3O7−δ dc superconducting quantum interference devices (SQUIDs) on SrTiO3 bicrystal substrates. 1/f flux noise, either measured at different temperatures for optimized bias current or measured at 77 K for different bias currents, is almost constant. These facts imply that 1/f noise mainly comes from fluctuations of the critical current of the Josephson junction that form the SQUID. Also, we explain the critical current fluctuations in the junction by an equilibrium temperature fluctuation model.
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 high T c dc superconducting quantum interference device (SQUID) was fabricated using a bicrystal junction with a misorientation angle of θ=30°. The junction with a width of 2 µ m exhibited high resistance R s=10 Ω and critical current I 0=25 µ A at T=77 K. This gives a I 0 R s as high as 250 µ V. These values indicate that the bicrystal junction has great potential for the development of high performance SQUIDs. The SQUID fabricated with a 30° junction exhibited voltage modulation depths as high as ΔV=85 µ V, 40 µ V and 20 µ V at T=77 K for the inductance values of L s=60 pH, 100 pH and 160 pH, respectively. These are the highest values reported so far. Preliminary noise measurement showed that the flux noise of the SQUID is less than 8 µΦ0/ Hz1/2 for the white noise region, and less than 70 µΦ0/ Hz1/2 at f=1 Hz when the 60 pH-SQUID is dc biased and operated with a conventional Flux Locked Loop (FLL) circuit.
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.
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.