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Acta Physica Sinica

FLUX TRANSITION PROPERTIES OF RF-SQUID IN THE DIS-SIPATIVE MODE IN THE PRESENCE OF LARGE FLUCTUATION

WANG XIAO-HUI, JIN XIN, YAO XI-XIAN, 南京大学物理系,南京,210008

Published Jan 1, 1991• 0 citations
Publisher

Abstract

In the presence of large thermal fluctuation, conditions that RF-SQUID in the dissipa-tive mode can be properly operated are analysed, the flux transition distribution densities are obtained, thus the mean value and the width of transition are derived. Finally we find that the-effective critical current of RF-SQUID loop is greatly reduced by large fluctuation and the transition width can be determined with a simple equation.

DOI
10.7498/aps.40.1689

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In the presence of large thermal fluctuation, conditions that RF-SQUID in the dissipa-tive mode can be properly operated are analysed, the flux transition distribution densities are obtained, thus the mean value and the width of transition are derived. Finally we find that the-effective critical current of RF-SQUID loop is greatly reduced by large fluctuation and the transition width can be determined with a simple equation.
We measured the amplitude–frequency characteristics of radio frequency superconducting quantum interference devices (rf SQUIDs) over a temperature range between 65 and 79 K. Using the expressions derived from the recently developed rf SQUID theory, valid also at large thermal fluctuations, we determined from these data the basic parameters of high-transition-temperature superconductor (HTS) rf SQUIDs. These parameters were: (a) the high-frequency coupling coefficient between the rf SQUID and the tank circuit resonator, k, (b) the SQUIDs hysteretic parameter, β, (c) the critical current of the Josephson junction, Ic, (d) its normal resistance, Rn, and (e) its noise parameter, Γ. We found a good agreement with the values of β(Ic) and Rn determined directly after destructively opening the SQUID loop. In accordance with the theoretical predictions, our experimental results show that at large thermal fluctuation levels (T≅77 K), rf SQUIDs with large loop inductance operate in nonhysteretic mode up to β values exceeding 3. Furthermore, we have shown that the optimal energy sensitivity is attained in the nonhysteretic mode at a value of β distinctly higher than 1. A quantitative comparison of white noise predicted by the theory with that obtained from the experiment showed a reasonable agreement. We also discussed the contribution of the phase information to the SQUID’s signal and noise at optimum operation conditions, when a mixer was used as a signal detector.
A simple method is developed for analysis of rf SQUIDs operating in the hysteretic mode, for arbitrary values of rf detuning and plateau number. For plateaus with large numbers, the reactive component of the interferometer flux is shown to be essential. The results of theoretical analysis are confirmed by experiments with a 10 MHz point-contact SQUID. The new method is applied to studies of a SQUID with a separate coil for driving flux. The response curve slope in such SQUID can be somewhat larger than that in the conventional rf SQUID. The balanced SQUID circuits are also discussed.
In this paper a discussion is given of the noise sources and sensitivity limits of the rf biased SQUID. Both the low critical current inductive and the higher critical current hysteretic devices are considered. The intrinsic SQUID noise is treated by considering the internal flux response of a SQUID ring to an applied rf flux. While approximate, this approach yields the most direct picture of the nature of the intrinsic noise. Circuit optimization procedures for maximizing SQUID performance under various operating conditions are also outlined. Finally the effects of non-ideal weak link behavior upon SQUID operation are treated. (Author)
The dc SQUID had been presented for use in the rf SQUID instead of a weak link. The new device, referred to as “double SQUID”, possesses a large output signal (hundreds of microvolts) in operating in a hysteretic mode. For making an operating mode a double SQUID is coupled to a circuit traditional for rf SQUIDs containing a tank circuit and an rf current pumping generator. The magnetic flux being measured is recognized by the dc SQUID quantization loop which results in changing its critical current. As a result the height of the flat part of the tank circuit I–V characteristics coupled to a double SQUID is modulated. It is this modulation that is the SQUID output signal. The experimental investigations of the double SQUID showed the validity of the assumptions under consideration. Output signals up to 690 μV, noise spectral density ≈2×10−5ϕ0/ Hz 1/2 (ϕ0 is the flux quantum) and energy resolution ≈1.4×10−29 J/Hz have been obtained.
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.