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Superconductor Science and Technology

Tunable HTS rf Josephson-effect oscillator based on YBCO-Au-YBCO resistive SQUID

L Hao, D.A. Peden, John C. Gallop, J.C. Macfarlane

Published Nov 21, 2001• 5 citations
Publisher

Abstract

We report the successful realization of a resistively shunted two-junction high temperature superconducting (HTS) SQUID in which the heterodyne signal generated between the two YBCO grain-boundary junctions is accurately controlled in the range of 1–50 MHz by adjustment of the bias current in the milliampere range through a thin-film on-chip YBCO-Au-YBCO shunt resistor (∼25 μΩ). Oscillations in the temperature range 20–60 K and signal powers up to 2 nW in a 20 kHz bandwidth have been recorded.

DOI
10.1088/0953-2048/14/12/329

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Electronics Letters•Jun 22, 2011•16 citations
We report the successful realization of a resistively shunted two-junction high temperature superconducting (HTS) SQUID in which the heterodyne signal generated between the two YBCO grain-boundary junctions is accurately controlled in the range of 1–50 MHz by adjustment of the bias current in the milliampere range through a thin-film on-chip YBCO-Au-YBCO shunt resistor (∼25 μΩ). Oscillations in the temperature range 20–60 K and signal powers up to 2 nW in a 20 kHz bandwidth have been recorded.
Double Josephson junction HTS resistive SQUIDs have been fabricated using an on-chip shunt Au resistor across a gap in an YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// loop, providing a low shunt resistance of R/sub s/ /spl sim/26 /spl mu//spl Omega/ at T/spl les/77 K. The dependence of the heterodyne oscillation linewidth of the R-SQUID on the thermal noise across the shunt resistor is under investigation with a view to the development of an absolute noise thermometer. Signal powers /spl sim/nW have been measured and the device has been shown to be tunable from 5 to 50 MHz. The measured oscillation linewidth (/spl sim/30 kHz at 17 K) is believed to be the narrowest reported for such an HTS device, however non-thermal broadening of the linewidth is also observed, as has been reported for single junctions at GHz frequencies. We describe measurements on such devices over a range of temperatures and bias conditions.
High-temperature-superconducting (HTS) resistively shunted superconducting quantum interference devices, consisting of two yttrium–barium–copper–oxide (YBCO) grain boundary junctions shunted by gold-film resistors of ∼20 μΩ, have been prepared and characterized at temperatures from 17 to 74 K. The small value and stability of the resistor allows narrow-band heterodyne oscillations to be generated between the Josephson oscillations in the individual junctions. The frequency can be precisely controlled in accordance with the ac Josephson effect by adjustment of an external current applied to the YBCO–Au–YBCO shunt resistor, and the linewidth is determined in principle only by Johnson noise in the resistor. Off-chip detection and spectral analysis of the heterodyne signal generated in this way has not been previously reported. Measurements of the signal power (∼nW), the tunability of the center frequency from 5 to 50 MHz, the frequency stability and the linewidth of the heterodyne Josephson oscillation are described. Although linewidth broadening due to nonthermal fluctuations is observed, the linewidth of ∼30 kHz at T=17 K, at a center frequency of 24 MHz, is believed to be the narrowest reported for Josephson oscillations in an HTS device of this type.
Experimental evaluation of a high-temperature superconducting Josephson heterodyne mixer based on a 'resistive-SQUID' configuration is reported. The device consists of two YBa 2 Cu 3 O 7− x step-edge Josephson junctions connected via a small resistor in an otherwise superconducting loop. It has been previously shown to generate a heterodyne oscillation, which is frequency-tunable by a control current through the resistor. Under certain conditions, this device can operate as a frequency-tunable heterodyne mixer (down-converter) in the presence of an RF signal. In this paper, we describe the operation of the autonomous Josephson mixer–local oscillator device and present the experimental results on the mixer performances in terms of the junction current–voltage characteristics, the frequency tunability, linearity, and dynamic range as well as their temperature dependence for signal frequencies from 1 to 5 GHz.
We report continuing investigations into practical applications of the AC Josephson effect as the basis for a voltage-tunable radio-frequency oscillator. We have previously demonstrated experimentally that useful power levels (10 s of nW) and linewidths of a few kHz can be achieved in the heterodyne output from a high-temperature-superconducting resistive SQUID (HTS-RSQUID) operating in the frequency range 1-50 MHz. Those results were achieved with 2-junction R-SQUIDs incorporating current-biased shunt resistors of a few micro-ohms. We have now modified the fabrication procedures, and adjusted the shunt resistors and bias current values so that higher frequencies can be achieved. The Josephson junctions are of step-edge type, rather than the bi-crystal type used in our earlier work. The step-edge technique permits much more flexibility in the geometrical lay-out and utilizes the more cost-effective single-crystal MgO substrates. In the present paper, we report numerical simulations and experimental measurements on these devices in the frequency range up to 2 GHz.
An all thin-film resistive SQUID with two Josephson tunnel junctions has been designed, fabricated and studied. As well as the two junctions, this device consists of a low ohmic (1.9*10 -6 Omega ) resistive part connected by superconducting leads to both junctions in a loop with very low ( approximately 10 -11 H) inductance. A direct current flowing through the resistive part fixes the voltage difference between the two Josephson junctions that are additionally fed by a bias current. The voltage difference is converted into a signal frequency that can be picked up from the SQUID with a conversion factor of ( phi o ) -1 =483.6 MHz mu V -1 , defined only by the electron charge and Planck's constant. An output signal level (peak-to-peak) of about 50 mu V has been obtained. The low frequency range (below 100 kHz) of the output signal frequency has been investigated. The possibilities of considerably extending this frequency range are also discussed.
A pair of high-temperature-superconducting (HTS) step-edge Josephson junctions connected in the ‘resistive-SQUID’ (R-SQUID) configuration has been previously shown to generate a heterodyne oscillation which is frequency-tunable by a control current across the resistor. A method is described which enables the R-SQUID to act both as the local oscillator (LO) and simultaneously as a radio- frequency down-converter element. For the first time the autonomous Josephson mixer–local oscillator device for signal frequencies between 1–5 GHz, over a temperature range of 20–70 K, is demonstrated.