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Journal of Applied Physics

White-noise measurements and Joule heating in superconducting point contacts

Kenneth R. Carroll, Ho Jung Paik

Published Jun 1, 1989• 4 citations
Publisher

Abstract

We report measurements of the white noise in superconducting niobium point contacts. These measurements, the junction parameters, and the method of fabrication of the point contacts are compared with those of other experiments. The power spectral density was found to have an approximately parabolic dependence on the average voltage V rather than the linear dependence expected in the shot-noise limit. We also found that the magnitude of the excess noise was larger than has been previously reported. Because of these differences, the noise data presented add to the existing body of data. The V2 dependence of the power spectral density suggests that Joule heating may have been responsible for the excess noise. Joule heating was determined to be present in the junctions from the relatively small critical currents I0 ≲2 μA and the effect that a resonant cavity had on the Josephson oscillation. However, we show that Joule heating cannot account for the level of the excess noise observed in present or previous data.

DOI
10.1063/1.343293

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Experimental measurements on critical current noise in underdamped niobium based Josephson devices by a technique based on the switching current measurements is reported. By sweeping the junction with a current ramp we measure the critical current switching as a function of the time using the standard time of flight technique. In such a way it is possible to obtain the critical current fluctuations ΔIc=Ic(t)-<Ic(t)> and the relative standard deviations which corresponds to the root square of the current fluctuation power. Pointing at the white noise fluctuations (above few Hz) and taking into account the physical frequency of the device, it is possible to evaluate the power spectral density of the critical current. The analysis has involved high quality underdamped Josephson junctions having an area ranging from (4x4) μm2 to (40x40) μm2 in the temperature range from 4.2 K to few tenth of mK. These measurement provide very useful information about the intrinsic noise of Josephson devices involving SQUIDs and qubits.

Physics Procedia•Jan 1, 2012•2 citations
We report measurements of the white noise in superconducting niobium point contacts. These measurements, the junction parameters, and the method of fabrication of the point contacts are compared with those of other experiments. The power spectral density was found to have an approximately parabolic dependence on the average voltage V rather than the linear dependence expected in the shot-noise limit. We also found that the magnitude of the excess noise was larger than has been previously reported. Because of these differences, the noise data presented add to the existing body of data. The V2 dependence of the power spectral density suggests that Joule heating may have been responsible for the excess noise. Joule heating was determined to be present in the junctions from the relatively small critical currents I0 ≲2 μA and the effect that a resonant cavity had on the Josephson oscillation. However, we show that Joule heating cannot account for the level of the excess noise observed in present or previous data.
Thermodynamic fluctuations limit the ultimate performance of superconducting devices of practical interest, devices such as the quantum interference magnetometer and the Josephson junction radiation detector. We report direct observations of thermodynamically driven quantum phase fluctuations near the superconducting critical temperature. The experiments used point-contact quantum interference techniques with niobium points on a single-crystal tin substrate. Near the superconducting transition of tin, TC=3.72°K, the niobium points served as probes to measure fluctuations in the phase of the superconducting order parameter within the tin crystal. The temperature was slowly increased, beginning well below TC for tin, where the quantum phase was stable. Quantum phase fluctuations were first observed approximately 5 mdeg below TC. As the temperature continued to increase through this 5 mdeg range near TC, the characteristic frequency of the phase fluctuations increased 6 orders of magnitude. At TC a discontinuous transition to a resistive state gave rise to a finite voltage difference in the tin with the associated driven quantum phase precession. Thermodynamic Johnson noise voltages developed across this resistance of order 10−10Ω observably broadened the precession line. We will present the detailed temperature dependence of the frequency power spectrum of quantum phase fluctuations below and above TC.
The subject of electronic noise in high- superconducting Josephson devices and in their applications is considered. Several types of grain boundary junctions, prepared in different ways by four separate international laboratories, are fully characterized in terms of their electrical and noise properties at a range of temperatures, frequencies, magnetic fields and in the presence of microwaves. Similar characterization is carried out for multijunction Josephson flux-flow arrays, and bi-epitaxial SQUIDs. The theory of Likharev and Semenov for thermal noise in low- junctions is adopted as a reference against which excess noise can be identified in high- junctions. It is combined with theoretical models of excess noise based on fluctuations of critical current and resistance to provide accurate fitting of the observed noise curves. It is shown that the normalized levels of critical current and normal resistance fluctuations for all of the junction types examined are remarkably close (typically around and at 100 Hz) and are nearly independent of temperature in the range T = 30 - 80 K. The frequency dependence is close to the universal 1/f law with some deviations due to trapping of charge carriers at discrete states in the tunnelling barrier. Measurements of voltage noise levels and critical current fluctuations in multijunction flux-flow amplifiers, reported for the first time, are consistent with levels in single junctions of the same type. Excess noise is studied as a function of external magnetic field, in a way that has not been previously described. A new interpretation of the magnetic field-dependent noise data indicates that absolute levels of critical current fluctuations are nearly independent of applied magnetic field. This observation does not appear to have been reported elsewhere. It has important implications for the operation of grain boundary Josephson devices in real applications. Voltage noise in the presence of microwave irradiation, at operating points between Shapiro steps, is consistent with a modified form of the Likharev - Semenov equation which takes into account the known level of critical current fluctuations as measured in field-free conditions. The implications of these results for further improvements in device performance are discussed, and directions for further work are suggested.
Spectral density of current fluctuations in a short ballistic superconducting quantum point contact is calculated for arbitrary bias voltages VVV. Contrary to a commonly held view that the supercurrent flow in Josephson junctions is a coherent process without fluctuations, we find extremely large current noise that is caused by the supercurrent coherence. At finite voltages, this noise can be qualitatively understood as the shot noise of the large charge quanta of magnitude $2\ensuremath{\Delta}/V$ equal to the charge transferred through the contact during one period of the Josephson oscillations.
A fabrication technique for Nb-Al2O3-Nb Josephson tunnel junctions is described that is an alternative to the trilayer method generally used. At 4.2 K the magnitude of the low-frequency noise in the critical current I0 of four junctions with areas 𝒜 ranging from 7.8 to 115 μm2 was characterized by S1/2I0𝒜1/2/I0 =(14±6)pA μm/μA Hz1/2, where SI0 is the spectral density of the excess noise at 1 Hz. The noise power spectrum of one 3×3 μm2 junction exhibited a Lorentzian feature associated with the emptying and filling of a single trap in the barrier. The low level of noise makes these junctions attractive for use in superconducting quantum interference devices.
For a current-driven Josephson junction shunted by an Ohmic resistance, the dc voltage response and impedance to external high-frequency currents is calculated with a second-order perturbation method based on the unperturbed solution for the time evolution of the voltage as given by Aslamazov and Larkin. The response is proportional to signal power and has three characteristic-frequency regions depending on whether the internal self-generated Josephson frequency, ω0, is larger than, equal to, or smaller than the signal frequency, ω. For ω0< or >ω the response could be expressed by easily observable parameters of the dc voltage-current characteristics. This permits comparison of the predictions with experimental results obtained on point-contact junctions whose dc characteristics were only approximately represented by the simple model chosen for analysis. For ω0≪ω the response is found to be proportional to (i) the slope of the dc characteristic, (ii) the inverse square of the applied frequency, (iii) the inverse battery current in terms of the critical current, and (iv) the voltage amplitude of the inherent Josephson oscillations. For ω0≫ω ``classical'' detection proportional to curvature is obtained. For ω0∼ω resonance detection occurs, which depends to a considerable degree on fluctuations of the average contact voltage. These predictions are in reasonable agreement with experiments in which point-contact junctions were illuminated with klystron radiation at 10 and 90 GHz. Evidence is presented that the voltage fluctuations limiting the resonance response are the result of noise generated in the junction. Comparison of the noise-equivalent power expected from the simple model with measured results suggests that parasitic elements, such as shunt capacity, are not negligible at 90 GHz.
Experimental measurements on critical current noise in underdamped niobium based Josephson devices by a technique based on the switching current measurements is reported. By sweeping the junction with a current ramp we measure the critical current switching as a function of the time using the standard time of flight technique. In such a way it is possible to obtain the critical current fluctuations ΔIc=Ic(t)-<Ic(t)> and the relative standard deviations which corresponds to the root square of the current fluctuation power. Pointing at the white noise fluctuations (above few Hz) and taking into account the physical frequency of the device, it is possible to evaluate the power spectral density of the critical current. The analysis has involved high quality underdamped Josephson junctions having an area ranging from (4x4) μm2 to (40x40) μm2 in the temperature range from 4.2 K to few tenth of mK. These measurement provide very useful information about the intrinsic noise of Josephson devices involving SQUIDs and qubits.