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IEEE Transactions on Magnetics

Millimeter wave behavior of superconducting point contact SQUID

Helmut Kanter, F. Vernon

Published Jan 1, 1977• 9 citations
Publisher

Abstract

A superconducting point contact waveguide SQUID has been operated at 89 GHz. The performance is qualitatively the same as at lower frequencies in agreement with the SQUID model. With the Nb point located across a 0.15 mm high E band waveguide, reflection coefficient measurements were made as a function of both supercurrent phase as controlled by an externally applied magnetic field and millimeter wave power. Large parametric reactance variations were produced by changing the supercurrent phase. As the millimeter wave power is increased flux cycling was observed which involved up to four steps. This behavior corresponds to a flux transition time less than 10-12sec. Implications of the\cos \phiterm and negative resistance effects in the observed behavior of the reflection coefficient will be discussed.

DOI
10.1109/tmag.1977.1059461

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Soviet Journal of Low Temperature Physics•Jan 1, 1984•0 citations
A superconducting point contact waveguide SQUID has been operated at 89 GHz. The performance is qualitatively the same as at lower frequencies in agreement with the SQUID model. With the Nb point located across a 0.15 mm high E band waveguide, reflection coefficient measurements were made as a function of both supercurrent phase as controlled by an externally applied magnetic field and millimeter wave power. Large parametric reactance variations were produced by changing the supercurrent phase. As the millimeter wave power is increased flux cycling was observed which involved up to four steps. This behavior corresponds to a flux transition time less than 10-12sec. Implications of the\cos \phiterm and negative resistance effects in the observed behavior of the reflection coefficient will be discussed.
We have measured the characteristics of a novel SQUID magnetometer pumped at 9 GHz. The device comprises a niobium point-contact shunting a low impedance superconducting microwave transmission line which isolates the junction at high frequencies from a toroidal input transformer. The junction is matched to a 50 Ω coaxial transmission line and the SQUID is operated in a reflection mode. For low values of the weak link critical current we observe a modulation of the phase of the reflected microwave signal periodic in the applied flux. The modulation persists at high microwave power levels indicating that the junction response is maintained to at least 200 GHz. For large values of critical current, amplitude modulation is observed. We have obtained a differential sideband power of 230 pW/(\Phi_{0})^{2}which is close to the value predicted by a simple model. In flux-locked loop operation we have obtained an energy sensitivity referred to the input terminals of 7 × 10-31J/Hz, which is superior to that so far reported for any other SQUID.
The Josephson frequency conversion in the millimeter and submillimeter wave region is studied with superconducting Nb-Nb point contact junctions at 4.2 K, using the contact both as the local oscillator and the mixer with intermediate frequencies of 0.1 and 9.2 GHz. Frequency conversion signals are only detected in the latter case with a microwave receiver at 9.2 GHz in a range of Josephson frequencies between 145 and 1 600 GHz. The experiments with intermediate frequency fi of 0.1 GHz indicate for this small value of f i that the observed signals are not related to frequency conversion but noise effects. An analysis of the frequency conversion mechanism in a Josephson junction operated as an oscillator-mixer is proposed which includes the noise effects and also the feedback effects induced in a junction driven by a constant current source. We have then investigated the noise properties up to 450 GHz of superconducting point-contacts using the junction as an oscillator-mixer where the Josephson current is mixed with harmonics of an externally applied signal. The emitted intermediate frequency is detected at 9.2 GHz and its linewidth is measured. In the very high frequency range the dc quasiparticle current dominates in a Josephson weak-link and we compare the corresponding theoretical linewidth calculated by Scalapino with our experimental results.
The Josephson frequency conversion in the millimeter and submillimeter wave region is studied with superconducting Nb–Nb point-contact junctions at 4.2 °K, using the contact both as the local oscillator and the mixer with intermediate frequencies of 0.1, 0.3, and 9.2 GHz. The point contact is in a nonresonant structure and fed by a constant current source: neither a strong rf coupling of the contact with a cavity nor a stabilizing shunt resistance was proved necessary. The direct observation of frequency conversion was performed with a microwave receiver at 9.2 GHz in a range of Josephson frequencies between 145 and 1600 GHz. The experiments with intermediate frequencies fi of 0.1 and 0.3 GHz indicate that conventional frequency conversion does not occur but that the observed signals are related to noise effects. An analysis of the frequency conversion mechanism in a Josephson junction operated as an oscillator-mixer is proposed which includes the noise effects and also the feedback effects induced in a junction driven by a constant current source. The results show that low-frequency current components are induced in the junction, which can give large spurious signals when small i.f. are used.
At the present time there is considerable interest in the use of superconducting point contacts (SPC's) for various measurement purposes, in particular the measurement of millimeter and submillimeter band electromagnetic radiation. One problem facing the development of SPC detectors is ensuring their stability and resistance to external influences and noise and the ability to match them to the radiation source over a wide band. A design was developed with a stable uncontrolled SPC in an oversize quasi-optical waveguide for use within a wide band of wavelengths, and some of its characteristics in the 3 cm and 4 mm bands were investigated in the case of ''high resistance'' SPC's with an asymptotic resistance of 10 to 30 ..cap omega.. and critical currents of 5 to 40 ..mu..A.
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.
The behavior of a superconducting ring incorporating a point contact, coupled inductively to an oscillatory circuit (hf SQUID), with the rate of change of the magnetic flux in the ring being comparable, to the inverse energy relaxation time of excitations in the superconductor τϵ−1, is examined. For T ∽ Tc the results of the microscopic theory of the nonstationary behavior of point contacts, in which the coupling between the phase difference and the current of the Josephson junction differs from the usual expression I = V/R + Icsinφ in the resistive model, are used. It is shown that the nonequilibrium behavior of a weak link for ω≳τϵ−1 significantly changes the amplitude–frequency and signal characteristics of the SQUID as compared with the case ωτϵ−1≪1, for which the results based on the resistive model are valid. The characteristics of the SQUID in the nonequilibrium situation are sensitive to the ratio between the size of the contact and the mean free path of electrons relative to scattering by impurities.