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Chinese Journal of Physics

Flicker (1/f) Noise in 45^± Bi-Epitaxial Grain Boundary Junctions and dc SQUID of YBa2Cu3Oy

Baodian Yao, S.-H. Tsai, C. C., M. K. Wu

Published Apr 1, 2000• 0 citations
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Abstract

Wehavefabricated dc superconductingquantuminterferencedevices (SQUIDs) incorpo-rating 45 ± bi-epitaxial grain boundary junction, i.e. YBCO/YSZ and YBCO/YBCO/YSZ, in washer geometry. The1=f noise power of single junction comes fromresistance fluctuation for high current bias andcritical current fluctuation for low current bias. Thenoisespectrum of thedcSQUIDswasmeasuredwithdcandacbiasschemesandshowedvalueof » 1 £ 10 ¡ 5 © 0/Hz1=2 at 100 Hz. The possible noise sources, critical current fluctuations, flux motion noise and resistance fluctuation, are investigated. The results are compared with previous published data. PACS. 85.25.Cp – Josephson devices. PACS. 85.25.Dq – Superconducting quantuminterferencedevices (SQUIDs).

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Wehavefabricated dc superconductingquantuminterferencedevices (SQUIDs) incorpo-rating 45 ± bi-epitaxial grain boundary junction, i.e. YBCO/YSZ and YBCO/YBCO/YSZ, in washer geometry. The1=f noise power of single junction comes fromresistance fluctuation for high current bias andcritical current fluctuation for low current bias. Thenoisespectrum of thedcSQUIDswasmeasuredwithdcandacbiasschemesandshowedvalueof » 1 £ 10 ¡ 5 © 0/Hz1=2 at 100 Hz. The possible noise sources, critical current fluctuations, flux motion noise and resistance fluctuation, are investigated. The results are compared with previous published data. PACS. 85.25.Cp – Josephson devices. PACS. 85.25.Dq – Superconducting quantuminterferencedevices (SQUIDs).
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 Y1Ba2Cu3O7−x step-edge junction dc superconducting quantum interference devices (SQUIDs) and characterized their noise performance. The current-voltage characteristics of our SQUIDs are of resistively shunted junction type with critical current densities jc of about 104 A/cm2 and maximum flux to voltage transfer functions δV/δΦ of 20 μV/Φ0 at 77 K. We compare model predictions for 1/f noise in dc SQUIDs with experimental data and show that the frequency dependent noise in our devices results from critical current fluctuations of the Josephson junctions. A bias current reversing technique is effective in suppressing this noise contribution. A flux noise Φn of 80 μΦ0/Hz1/2 at 1 Hz and 77 K was measured on a dc SQUID with an inductance Ls of 60 pH.
We have measured the low-frequency noise in the critical current Ic of six dc superconducting quantum interference devices (SQUIDs) with resistively shunted Nb–NbOx–PbIn Josephson junctions in the temperature range T=0.09–4.2K. Each device is voltage biased, the applied flux is an integer number of flux quanta, and the current fluctuations are measured with a second dc SQUID. At low frequencies f, there is a component of the power spectrum of the critical current fluctuations given approximately by SIc(f)=CIc2T2∕Af, where A is the area of both junctions, and C≈(3.9±0.4)×10−23m2∕K2. For quantum bits based on Josephson junctions, the scaling of SIc(f) with T2 implies that the dephasing time limited by critical current l∕f noise should scale as 1∕T for temperatures down to at least 0.09K.
We have fabricated and measured the properties of micron-size single grain boundary junctions and dc superconducting quantum interference devices (SQUIDs). The width of the junctions was varied between 0.25 and 2 μm. The junctions were formed in c-axis oriented epitaxial films with a single 24° tilt boundary. They were fabricated using electron beam lithography and ion milling. All of the junctions operate at 77 K. The noise properties of SQUIDs made of these junctions have been characterized. They show a significant 1/f component which is comparable to published values. However, the white noise level, 3×10−31 J/Hz measured at 77 K, is the lowest ever reported.
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.
White noise properties of YBa2Cu3O7-δ (YBCO) grain boundary Josephson devices have been studied. Bias voltage (V) dependence of the white voltage noise spectral density (SV) were measured in detail for YBCO dc-SQUIDS and single Josephson junctions on different kinds of substrates with various misorientation angles. It was found that Sv was in good agreement with the thermal noise theory hypothesizing that the device noise was only due to Johnson noise. This implies that the quantum noise can be achieved with optimized YBCO dc-SQUIDS. We have designed a YBCO dc-SQUID with a cross section of junctions as small as 0.1 μm^2. It was predicted that the minimum intrinsic white energy resolution (E””'‘) should be about Planck's constant (h) at 4.2 K. The actual minimum eW, value of 1.76 x 10^-33 J/Hz = 2.7 h was obtained. This result demonstrates that E” '”of YBCO dc-SQUID s can be minimized to the order of h.