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IEEE Transactions on Magnetics

Microwave properties of thin film NbN SQUIDs with granular weak lines

R. Steiner, Allen Paige Flora, Bascom S. Deaver, Edward J. Cukauskas

Published Jan 1, 1981• 1 citation
Publisher

Abstract

A series of NbN thin film SQUIDs with transition temperatures ranging from 5 K to ∼14 K are being characterized by microwave measurements at 9.6 GHz. The data are being used to determine the critical current, resistance, and energy sensitivity of each device, to identify features that may be related particularly to the granular nature of the weak links, and to study systematically the variations of the SQUID response with temperature. The devices are nominally 1 μm long, 10 nm thick and from 1 μm to ∼50 μm wide and are fabricated on 1 mm-diameter quartz rods. All have previously been well characterized as SQUIDs at 23 MHz. Detailed results are presented for the first five devices studied.

DOI
10.1109/tmag.1981.1061058

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A series of NbN thin film SQUIDs with transition temperatures ranging from 5 K to ∼14 K are being characterized by microwave measurements at 9.6 GHz. The data are being used to determine the critical current, resistance, and energy sensitivity of each device, to identify features that may be related particularly to the granular nature of the weak links, and to study systematically the variations of the SQUID response with temperature. The devices are nominally 1 μm long, 10 nm thick and from 1 μm to ∼50 μm wide and are fabricated on 1 mm-diameter quartz rods. All have previously been well characterized as SQUIDs at 23 MHz. Detailed results are presented for the first five devices studied.
Thin-film SQUID’s fabricated from the high Tc refractory material NbN have been studied at 9.4 GHz. Devices with granular weak links display a Josephson critical current dependence over a wide range of temperatures below Tc. We have observed open loop signal-to-noise ratios as high as 20 000 corresponding to a minimum detectable energy of 1×10−30 J in a 1 Hz bandwidth over a temperature range of several °K. In two samples, the response time was found to be limited by the intrinsic gap response time, τg∼h//2Δ (T). These devices are rugged, stable, cycle well, and have good SQUID properties at both 9.4 GHz and 20 MHz.
Thin-film NbN SQUID’s with granular weak links have been fabricated and their rf I-V characteristics measured at 23 MHz. Three different temperature variations of the critical currents were observed. A Josephson-like temperature dependence for those devices with strongly coupled grains and a bulk-mean-field-like dependence for those with the weakest coupled grains were found. An intermediate case where the critical current follows the bulk-mean-field theory at low critical currents and switches over to Josephson-like for larger currents was also observed. The noise properties of these devices were deduced from the measurement of the step rise parameter a. The data were in good agreement with the theory of Kurkijarvi over a wide range of temperature and critical currents for those devices with a bulk-mean-field-like critical current dependence and a low critical current density (∼104 A/cm2). Other devices were in agreement over a much smaller region of temperature and critical current. These devices are rugged, stable, and show very little change in their characteristics after extended storage at room temperature.
A study of the magnetic flux noise in thin film NbN SQUIDS with variable thickness weak links has been initiated using the fractional step rise parameter, a , to characterize the noise. A series of devices have been fabricated and measured which exhibit Kurkijarvi magnetic flux noise, each over a restricted temperature range below their T . The T ' s for the devices ranged from below 5K to as (high as 15.7K. The variable thickness weak links are nominally 1 micrometer in length, between 50 microaeters to submicrometer in width and less than 10 MI. in thickness. For these thicknesses, the weak link region is granular ("inhomogeneous") in nature and the temperature dependence of the critical current obeys either the Josephson or the mean field theories depending on the strength of the intergranular coupling. The deduced magnetic flux noise obeys Kurki-jarvi theory for critical current values greater than 2@ /L but lessthanapproximately 61p /L for most deGices tested. The dependence of the 'measured magnetic flux noise on the degree of granularity of the weak link and the temperature dependence of the critical current is discussed.
A substrate step technique has been used to fabricate superconducting NbN weak links of length ∼750 Å, width ∼1.5 μm. The film in the weak-link region is less than 100 Å thick, and shows evidence of being granular. These devices have very high resistance (as much as 30 Ω), are quite rugged, and show almost ideal Josephson response to microwave radiation at 70 GHz. A vortex flow model is expected to apply because of the small coherence length in the weak-link material. However, the well-known resistively shunted junction model is found to be a significantly better predictor of device behavior.
SQUIDs employing short (\sim 1 \mu) ultra thin weak link sections of "granular" niobium and niobium nitride have been prepared on cylindrical quartz substrates by rf and rf reactive sputtering, photolithography and anodization. These SQUIDs have demonstrated near ideal intrinsic noise characteristics when biased at 20 MHz. Niobium nitride devices have operated with this nearly intrinsic noise at temperatures up to 12.5 K. Current-phase relation and flux entry versus applied flux were measured on one niobium SQUID. Results indicate a nearly [if not exactly] sinusoidal current-phase relation from Tcto almost a degree below Tcand flux entry noise in good agreement with the intrinsic flux noise theory. Biasing these devices at 9.2 GHz gives a degraded response which can be related to relaxation effects in the weak links. Thermal noise switching between the superconducting and dissipative states was also observed at 9.2 GHz and could be fit by a simple model.