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IEEE Transactions on Applied Superconductivity

Noise properties of SQUIDs with step-edge Josephson junctions

V. N. Glyantsev, M. Siegel, Jürgen Schubert, W. Zander, U. Poppe, Helmut Soltner, Alex I. Braginski, Christoph Heiden

Published Mar 1, 1993• 20 citations
Publisher

Abstract

The authors investigated low-frequency noise in RF and DC superconducting quantum interference devices (SQUIDs) with step-edge Josephson junctions (SEJs). The noise properties have been measured as a function of frequency, temperature, bias current, and magnetic field. The low-frequency noise has a telegraph-like character in certain conditions and depends quasi-periodically on flux and temperature. This telegraph noise was also observed in single step-edge junctions. The experimental results were explained by a model of such a junction in which an array of parallel and series connected weak links acts like a two-level fluctuator. This fluctuator arises from the quantum states of a multijunction SQUID with spatially inhomogeneous SEJs resembling a multijunction array.>

DOI
10.1109/77.233957

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Journal of Applied Physics•Jan 15, 1989•5 citations
The authors investigated low-frequency noise in RF and DC superconducting quantum interference devices (SQUIDs) with step-edge Josephson junctions (SEJs). The noise properties have been measured as a function of frequency, temperature, bias current, and magnetic field. The low-frequency noise has a telegraph-like character in certain conditions and depends quasi-periodically on flux and temperature. This telegraph noise was also observed in single step-edge junctions. The experimental results were explained by a model of such a junction in which an array of parallel and series connected weak links acts like a two-level fluctuator. This fluctuator arises from the quantum states of a multijunction SQUID with spatially inhomogeneous SEJs resembling a multijunction array.>
We have fabricated RF and DC SQUIDS with step-edge Josephson junctions (SEJ) and YBa 2 Cu 3 O 7 thin films. The low-frequency noise of these SQUIDS has been investigated as a function of temperature, bias current and magnetic field. Typically, the energy resolution in the white noise region of the DC SQUIDS with an inductance of 45 pH was 3*10 -30 J Hz -1 at 77.5 K. Depending on the bias current, the low-frequency spectrum could be of either 1/f type or Lorentzian type. The Lorentzian spectrum was generated by random telegraph signals (RTSS). An Arrhenius plot was obtained from the temperature dependence of the switching time of these RTSS. We have determined two barrier lengths of 0.23 eV and 1 eV of the two-level fluctuator (TLF) generating this RTS noise. From the magnetic-field dependence of the low-frequency noise we have found that the noise is periodic with the flux corresponding to the area of the SEJ. The results were discussed in terms of a model treating the SEJ as a multi-junction interferometer which forms a two-level fluctuator when the junction is placed inside a SQUID loop. The noise is generated by thermally activated switching of the SQUID between the two different states of the SQUID.
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 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.
A seven-channel RF SQUID (superconducting quantum interference device) with anodized short weak links has been fabricated. The devices exhibit a sensitivity of 23.6 fT/ square root Hz and a 1/f noise onset of 0.05 Hz at a white noise level of 1.8*10/sup -4/ phi /sub 0// square root Hz. The intrinsic flux noise is estimated from an observed step pattern to be 4.9*10/sup -5/ phi /sub 0// square root Hz. It is shown that Arnold theory explains very well the behavior of Josephson junctions with a critical current density of around 10/sup 4/ A/cm/sup 2/.>
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.
The noise characteristics at low frequencies are studied for various planar dc SQUIDs made of junctions with different barrier materials of a-Si, aluminum oxide, and niobium oxide. It is found that fluctuations of the parameters of individual junctions contribute to the flux noise of the SQUIDs in a magnitude of the same order with the apparent flux noise. The magnitude of the flux noise due to the parameter fluctuations depends on the barrier material and on the inductance parameter βL . We have observed a telegraph noise in the voltage time trace and a Lorentzian in the voltage power spectrum for a current-biased SQUID. As a source of the parameter fluctuations, capture and emission of electrons at the trapping sites in the barrier are indicated.