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IEEE Transactions on Applied Superconductivity

Microwave induced steps in RF-field-driven DC SQUID and its potential applications

Takaharu Kondo, Yoshinao Mizugaki, J. Chen, Kensuke Nakajima, Takashi Yamashita

Published Mar 1, 2001• 0 citations
Publisher

Abstract

We have been studying an RF-Field-driven DC SQUID (RFDS) that exhibits RF-induced-current-steps larger than those of single junctions at lower normalized frequency and a strong selection rule for the step orders which depends on the external DC magnetic field. In RFDS, even order steps are dominant in the absence of DC magnetic field. Contrarily, a DC magnetic field of a half flux quantum dominantly enhances odd order steps. Therefore the output of an RFDS can be switched between two voltage levels corresponding to the RF-induced-step by an external DC field input for a certain bias condition. We carried out numerical simulations to characterize the switching properties of the RFDS for various bias conditions, including RF circulating currents and DC bias current.

DOI
10.1109/77.919490

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We have been studying an RF-Field-driven DC SQUID (RFDS) that exhibits RF-induced-current-steps larger than those of single junctions at lower normalized frequency and a strong selection rule for the step orders which depends on the external DC magnetic field. In RFDS, even order steps are dominant in the absence of DC magnetic field. Contrarily, a DC magnetic field of a half flux quantum dominantly enhances odd order steps. Therefore the output of an RFDS can be switched between two voltage levels corresponding to the RF-induced-step by an external DC field input for a certain bias condition. We carried out numerical simulations to characterize the switching properties of the RFDS for various bias conditions, including RF circulating currents and DC bias current.
We present numerical investigation of the RF properties of a double-junction SQUID model, which is referred to as an RF-field-driven double-junction SQUID (RFDS). The RFDS generates large current steps, the order of which can be controlled by external dc field. We simulate the current-voltage characteristics of the RFDS in the normalized frequency range from 0.065 to 5.0. In comparison with the results for a single junction, the step heights of the RFDS are much larger for the low frequency region; e.g. the first step of the RFDS becomes nine times larger than that of a single junction when a dc flux of 0.5 0 is applied ( 0 is a flux quantum). For the high frequency region, the step height of the RFDS is comparable to that of a single junction. The results suggest that the RFDS should be applicable as a wide-band RF detector.
Shapiro steps in an RF-field-driven dc-SQUID (RFDS), where an RF signal is coupled to the SQUID loop magnetically, are numerically investigated. We calculate the current–voltage characteristics of RFDSs under no dc offset flux, which show the enhancement of the even-order Shapiro steps. First, the maximum second (the lowest even-order) step height is evaluated as a function of the normalized frequency Ω. In the case of the inductance parameter βL ≥ 1.0, the maximum second step height is maintained at a certain value in the low frequency region. In the case of βL < 1.0, on the other hand, the maximum step height decreases in the low frequency region of Ω < 0.10. The dependence of the current positions of the step upon the RF signal amplitude shows that the voltage transition at the non-superconductive area between the superconducting interference lobes limits the upper current position of the step.
The current height of RF-induced voltage steps in nonhysteretic single Josephson junctions is usually a small fraction of the critical current. It is demonstrated that when a resistively shunted DC superconducting quantum interference device (SQUID) is subjected to an RF magnetic flux, a voltage step with a large current height is obtained. In contrast to weak selection rules for single Josephson junctions, strong selection of the step harmonic number occurs in a shunted DC SQUID for appropriate choice of RF amplitude and DC flux offset, with odd harmonic numbers selected by half-integer flux quantum offsets and even harmonic numbers by integer flux quantum offsets. This pronounced effect has immediate implications for voltage standards and fast, high-precision digital-to-analog converters.>
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We propose a new type of Josephson switching device which utilizes quantum transitions in a superconducting quantum interference device (SQUID) loop. This device is driven by a DC bias current and RF magnetic field. The output voltage is determined by the transition rate and the AC-Josephson relationship. We investigate the desirable device parameters by simulation. Two parameters, the maximum step height and the overlapped current region for the 0th and 1st steps, are plotted as functions of frequency and power of the RF-field activation. We also derive the relationship between the gain and device parameters. The analytical relationship is in good agreement with the numerical results. Besides the simple switching operation, a flip-flop operation is also successfully simulated. The results indicate that the device with appropriate parameters is suitable for switching applications.
This paper describes the I-V characteristics (as a function of applied magnetic field) and magnetic field response of a dc SQUID which incorporates two weak-link Josephson junctions in a superconducting ring. These characteristics are derived by digital computer simulation, using the Stewart-McCumber model. The effects of asymmetry (dissimilarity between the junctions) on SQUID critical current I c, normal resistance R and capacitance C are also simulated. With microwave excitation applied, the Josephson oscillations of the dc SQUID are found to be “frequency-locked” to the applied microwave signal even when critical currents I c of the two junctions differed, Subharmonic voltage steps are obtained for a symmetric dc SQUID in a magnetic field even when the current-phase relationship is purely sinusoidal.