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IEEE Transactions on Applied Superconductivity

Thin-film resistive SQUIDS

G.S. Krivoy, H. Koch

Published Jun 1, 1995• 4 citations
Publisher

Abstract

Different all thin-film partly resistive superconducting quantum interference devices (RSQUIDs) for noise thermometry in the liquid helium temperature range were designed and investigated. Three different types of RSQUID were realised: (i) dc RSQUID, (ii) rf RSQUID, and (iii) RSQUID with two quantization loops (double RSQUID). Shunted tunnel junctions were used as weak links. The resistive parts have been made as a Cu thin film combined with Nb thin films either as a sandwich or a planar element. The devices using a rf pump have integrated rf coupling coils. Resistances of the resistive parts in the range from 2.69 /spl mu//spl Omega/ to 246 /spl mu//spl Omega/ were obtained. The output signal was /spl ap/50 /spl mu/V for the dc RSQUIDs, up to 80 /spl mu/V for the rf RSQUIDs, and up to 1.2 mV for the double RSQUIDs. The linear dependences of the output signal frequency versus a dc current passed through the resistive part were observed up to 40 MHz. The double RSQUIDS described have a clear advantage in the output signal compared to the other RSQUIDs.>

DOI
10.1109/77.403283

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Different all thin-film partly resistive superconducting quantum interference devices (RSQUIDs) for noise thermometry in the liquid helium temperature range were designed and investigated. Three different types of RSQUID were realised: (i) dc RSQUID, (ii) rf RSQUID, and (iii) RSQUID with two quantization loops (double RSQUID). Shunted tunnel junctions were used as weak links. The resistive parts have been made as a Cu thin film combined with Nb thin films either as a sandwich or a planar element. The devices using a rf pump have integrated rf coupling coils. Resistances of the resistive parts in the range from 2.69 /spl mu//spl Omega/ to 246 /spl mu//spl Omega/ were obtained. The output signal was /spl ap/50 /spl mu/V for the dc RSQUIDs, up to 80 /spl mu/V for the rf RSQUIDs, and up to 1.2 mV for the double RSQUIDs. The linear dependences of the output signal frequency versus a dc current passed through the resistive part were observed up to 40 MHz. The double RSQUIDS described have a clear advantage in the output signal compared to the other RSQUIDs.>
Partly resistive superconducting quantum interference devices (RSQUIDs) are used in noise thermometry in the liquid helium temperature range. Here we analyze one type of RSQUID that has not been thoroughly investigated until now: the direct current biased with two Josephson junctions and a resistive part connected in series with the junctions and closed by a superconducting ring. The resistive shunted model of the Josephson junctions is used for the analysis. Two cases of the RSQUID ring inductance were analyzed: (i) negligibly small and (ii) small but finite inductance. The expressions for the low frequency output signal, the current–voltage (I–V) characteristics, and the current flowing through the resistive part are obtained. In the dependence of the output frequency versus the dc current passed through the resistive part often an offset is observed experimentally. This phenomenon can be explained as an influence of an asymmetry in the critical currents of the junctions.
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.
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.
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.
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.
We have fabricated dc-superconducting quantum interference devices (SQUIDs) with all-thin-film point contacts (Nb–Nb nanoconstriction-NbN). The size of the nanoconstriction may be smaller than the superconducting coherence length of bulk Nb. The quasiparticle characteristic of the all-thin-film point contact can be explained using the Blonder, Tinkham and Klapwijk (BTK) quasiparticle current which is applicable to the superconducting quantum point contact. The flux noise of SQUID with 10 pH operated in a flux locked loop is 8 µΦ0/ Hz1/2 at 1 Hz, and 4 µΦ0/ Hz1/2 at frequencies above 10 Hz is limited by the preamplifier.