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IEEE Transactions on Magnetics

Detailed measurements of the response of an rf SQUID in the regime LIc< Φ0/2π

Robert Rifkin, Daniel A. Vincent, Paul K. Hansma, Bascom S. Deaver

Published Mar 1, 1975• 5 citations
Publisher

Abstract

Using a superconducting ring containing a Nb point contact inductively coupled to a resonant circuit fed with an rf current source, (typical rf SQUID configuration) we have measured the tank voltage as a function of frequency in the regimeLI_{c}< \Phi_{0}/2\pi. ForLI_{c} \ll \Phi_{0}/2\pi, the curves of tank voltage versus drive current for various values of external flux\Phi_{x}through the ring agree with the calculations of Hansma. For higher critical currents, there is an asymmetry in these curves for frequencies above and below ω0, the resonant frequency of the circuit. Calculations using an expanded theory including the effects of a conductanceG_{o}(1+\alpha \cos \phi)and of screening flux give good agreement with the data. In principle α can be determined from these measurements, however the\cos \phiterm and the screening flux produce qualitatively similar effects. A quantitative determination of α requires a precise knowledge of Go/Ic. Using values of Goand Icfrom independent impedance measurements, the data yield a negative value of α in agreement with another experiment.

DOI
10.1109/tmag.1975.1058755

Related papers

Detailed measurements of the response of an rf SQUID in the regime LI/sub c/ < PHI/sub 0//2. pi

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A superconducting ring containing a Nb point contact inductively coupled to a resonant circuit fed with an rf current source (typical rf SQUID configuration) was used to measure the tank voltage as a function of frequency in the regime LI/sub c/ less than PHOI/sub 0//2..pi... For LI/sub c/ much less than PHI/sub 0//2..pi.., the curves of tank voltage versus drive current for various values of external flux PHI/sub x/ through the ring agree with the calculations of Hansma. For higher critical currents, there is an asymmetry in these curves for frequencies above and below ..omega../sub 0/, the resonant frequency of the circuit. Calculations using an expanded theory including the effects of a conductance G/sub 0/(1 + ..cap alpha.. cos phi) and of screening flux give good agreement with the data. In principle, ..cap alpha.. can be determined from these measurements, but the cos phi term and the screening flux produce qualitatively similar effects. A quantitative determination of ..cap alpha.. requires a precise knowledge of G/sub 0//I/sub c/. Using values of G/sub 0/ and I/sub c/ from independent impedance measurements, the data yield a negative value of ..cap alpha.., in agreement with another experiment. (auth)

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The performance of an rf SQUID is analyzed when the pumping frequency ..omega.. is comparable to the resonant frequency R/L of the interferometer. The SQUID input impedance is measured for different values of the parameter q = ..omega..L/R< or =..pi../4, input signal frequency, and pump frequency mismatch relative to the resonant frequency of the oscillating SQUID circuit. It is shown that the input resistance and input inductance of the squId may be either positive or negative.

•Oct 1, 1982•0 citations
Using a superconducting ring containing a Nb point contact inductively coupled to a resonant circuit fed with an rf current source, (typical rf SQUID configuration) we have measured the tank voltage as a function of frequency in the regimeLI_{c}< \Phi_{0}/2\pi. ForLI_{c} \ll \Phi_{0}/2\pi, the curves of tank voltage versus drive current for various values of external flux\Phi_{x}through the ring agree with the calculations of Hansma. For higher critical currents, there is an asymmetry in these curves for frequencies above and below ω0, the resonant frequency of the circuit. Calculations using an expanded theory including the effects of a conductanceG_{o}(1+\alpha \cos \phi)and of screening flux give good agreement with the data. In principle α can be determined from these measurements, however the\cos \phiterm and the screening flux produce qualitatively similar effects. A quantitative determination of α requires a precise knowledge of Go/Ic. Using values of Goand Icfrom independent impedance measurements, the data yield a negative value of α in agreement with another experiment.
A superconducting ring containing a Nb point contact inductively coupled to a resonant circuit fed with an rf current source (typical rf SQUID configuration) was used to measure the tank voltage as a function of frequency in the regime LI/sub c/ less than PHOI/sub 0//2..pi... For LI/sub c/ much less than PHI/sub 0//2..pi.., the curves of tank voltage versus drive current for various values of external flux PHI/sub x/ through the ring agree with the calculations of Hansma. For higher critical currents, there is an asymmetry in these curves for frequencies above and below ..omega../sub 0/, the resonant frequency of the circuit. Calculations using an expanded theory including the effects of a conductance G/sub 0/(1 + ..cap alpha.. cos phi) and of screening flux give good agreement with the data. In principle, ..cap alpha.. can be determined from these measurements, but the cos phi term and the screening flux produce qualitatively similar effects. A quantitative determination of ..cap alpha.. requires a precise knowledge of G/sub 0//I/sub c/. Using values of G/sub 0/ and I/sub c/ from independent impedance measurements, the data yield a negative value of ..cap alpha.., in agreement with another experiment. (auth)
A simple method is developed for analysis of the RF SQUID in regime, where the bias frequency ω is comparable to the natural R/L frequency of the SQUID ring. The input impedance of the RF SQUID is calculated for various values of the parametersq = \omega L/R\leq \leq \pi/4, signal frequency and detuning of the rf bias frequency from tank circuit resonant frequency. It is shown, that both the input resistance and the input inductance may be positive or negative. Measurement of the real part of the input impedance of a hysteretic RF SQUID are described. The experimental data are in qualitative agreement with the prediction of the theory.
A method for determining the superconducting-quantum-interference-device (SQUID) parameter β = LsIc/Φo and the coupling coefficient K between the SQUID loop and tank circuit based on the general theory for the nonhysteretic rf SQUID is derived. Analysis of the periodic dependence of the amplitude of the tank-circuit voltage on applied dc flux leads to explicit expressions for β and K. Important advantages of the method are that, during measurements, the SQUID loop has neither to be opened nor decoupled from the tank circuit, and the temperature of the sample does not have to be changed. Measurements with a Nb-point-contact SQUID at 4.2 K to verify the method in practice are reported. Intrinsic errors were estimated to be less than 10%.
The electrical equations of a resonant circuit fed with a radio-frequency current and inductively coupled to a weakly superconducting loop have been derived. The current-voltage characteristic of the rf superconducting quantum interference device can then be calculated and studied as a function of the junction resistance or of the resonance frequency, enabling its behavior to be better understood. As shown by Kurkijärvi, the large positive slope of the steps and the rounding of the characteristic result from thermal noise. However, in order to explain quantitatively the experimental results concerning, namely, point contact weak links which are generally oxidized or which have several contact points, it is useful to remark that the connection between the two superconductors constituting the Josephson junction may be made from n resistive paths. The mean thermal energy of the fluctuating flux is then close to nkbT instead of kbT if the inductances linking the contact points are large enough, that is, on the order of 10−11 H.