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Chinese Journal of Physics

Noise Properties of YBa2Cu3O7-δ Josephson Devices

J. Chen, Hiroyuki Nakamura, Takuya Yamashita

Published Apr 1, 1996• 0 citations
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Abstract

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.

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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.
We have studied white (high-frequency)-noise properties of YBa2Cu3O7-δ grain-boundary dc superconducting quantum interference devices (SQUIDs). Bias voltage (V) dependence of the white-noise voltage spectral density (S v) was measured for dc-SQUIDs and junctions. As a result, S v at low bias voltage can be explained by the thermal noise theory of tunnel junctions. Furthermore, the energy resolution of 1.96×10-32 J/Hz is obtained at 43 K in the white-noise region. The measured energy resolution of the dc-SQUIDs is in good agreement with the theoretical results. According to the above results, YBa2Cu3O7-δ dc-SQUIDs with a quantum noise limit might be realized at 4.2 K.
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.
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.
Autonomous single-layer dc superconducting quantum interference devices (SQUIDs) have been prepared from epitaxial, laser-deposited YBa2Cu3Ox films on step edge SrTiO3 and LaAlO3 substrates. For device patterning, a SiOx inhibit technique as well as conventional ethylenediaminetetraacetic wet etch was used. The Josephson junctions are of grain boundary type. Their widths are 5 μm. The SQUID hole is a square of about 5 μm; the SQUID inductance is estimated to be about 20 pH. We obtained a very regular, nonhysteretic flux to voltage modulation over more than 100 flux quanta (Φ0). The maximum voltage signal is of the order of 15 μV peak to peak and the maximum transfer function dV/dΦ at the appropriate flux bias is 50 μV/Φ0 at 77 K. The best value of the equivalent flux noise as measured in the flux locked loop mode is 1.4×10−5 Φ0/Hz1/2 at 1 Hz and 1×10−5 Φ0/Hz1/2 in the white noise region for f≳5 Hz. This results in an energy resolution εn,w(f≳5 Hz)=1×10−29 J/Hz in the white noise region and 2×10−29 J/Hz at 1 Hz.
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.
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).