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Journal of Physics Conference Series

Electrodynamic behaviour of a two-junction quantum interferometer enclosing a superconducting nanocylinder

Roberto De Luca

Published Mar 1, 2009• 0 citations
Publisher

Abstract

The electrodynamic response of a two-junction symmetric quantum interferometer, containing two equal 0-junctions and enclosing a superconducting nanocylinder, is studied by means of a first order perturbation analysis of the dynamical equations obtained for the composite system in the absence of noise. It is shown that, when an appropriate number of fluxons is trapped in the inner superconducting cylinder, the system behaves like a π-SQUID.

DOI
10.1088/1742-6596/153/1/012008

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The electrodynamic response of a two-junction symmetric quantum interferometer, containing two equal 0-junctions and enclosing a superconducting nanocylinder, is studied by means of a first order perturbation analysis of the dynamical equations obtained for the composite system in the absence of noise. It is shown that, when an appropriate number of fluxons is trapped in the inner superconducting cylinder, the system behaves like a π-SQUID.
The superconducting quantum interferometer with two quantization loops, the so-called double SQUID, or D-SQUID, is a modification of the well-known rf SQUID. The modification consists of a replacement of the one junction of the rf SQUID by a superconducting loop with two Josephson junctions. This device can be used to transform the weak magnetic flux applied to the loop with junctions to a voltage across the tank circuit inductively coupled to the other loop. As has previously been shown in experiments [G. S. Krivoy and H. Koch, J. Appl. Phys. 74, 2925 (1993)], D-SQUIDs have an extremely large output signal swing up to 1 mV and a transfer function of several mV/(h/2e). In this paper, a D-SQUID theory using a numerical and an analytical approach is developed. The theoretical results are compared with the above-mentioned experiments.
Integer and half-integer Shapiro step amplitudes in the current-voltage characteristics of symmetric superconducting quantum interference devices (SQUIDS), containing more than one Josephson junction on each arm, are analytically calculated for small values of the parameter β and in the absence of noise. This analysis is made possible by the reduction of the dynamical equations of the multiple-junction SQUID to the dynamical equation of a single Josephson junction possessing a nonconventional current-phase relation.
We report on experiments performed on a system consisting of a double SQUID (Superconducting Quantum Interference Device) built with gradiometer geometry. Two single-turn coils provide two independent control fluxes: one of these allows biasing the device and tilting the potential, while the other changes the barrier height of the potential. When the dynamics of the inner dc SQUID can be neglected, the free energy of the double SQUID, as a function of the internal magnetic flux, is just the corrugated parabola of an rf SQUID whose local minima represent metastable states for the system. Our analysis instead is substantially concerned with the interesting phenomenology generated by the static configurations of an internal two-junction interferometer and by the tunability of the internal loop inductance. Two readout systems are employed to thoroughly characterize the dynamics of our system. We investigate the dynamical response at temperatures low enough (tens of mK) to minimize the effects of thermal fluctuations concentrating the analysis on the aspects that could be relevant for macroscopic quantum coherence and computing. The results indicate that from the finite inductance of the inner loop originates a potential well generating competing processes with the tunneling between the two main wells of the rf-SQUID potential.
Multijunction superconducting quantum interferometers can be created spontaneously by intrinsic junctions in granular high-Tc materials, and as a by-product of step-edge technology used to fabricate high-Tc thin film dc superconducting quantum interference devices. The properties of such systems are relatively little known. The theory predicts multivalued dependence of critical current Ic on applied magnetic flux Φe and the appearance of different stationary ‘‘phase states’’ related to different possible relationships between the phases of superconducting order parameter in individual junctions. We report on the measurements performed on a (2×2) quantum interferometer, comprising a series array of two underdamped all-niobium tunnel junctions in each parallel arm. Experimental results closely follow the theoretical predictions and reveal several instability regions, in which noise generation takes place. These findings can possibly be related to the random telegraph noise observed in high-Tc materials.