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Superconductor Science and Technology

Fluxon modes and microwave emission in BSCCO type Josephson stacks

N. F. Pedersen, S. Madsen

Published Feb 19, 2004• 6 citations
Publisher

Abstract

We consider a system of stacked Josephson junctions and investigate the different fluxon modes. Some of these modes can be used to construct an oscillator with a frequency in the range 200 GHz–1 THz. We discuss the output that can be expected from the different fluxon modes, and we calculate the theoretical maximum output numerically.
DOI
10.1088/0953-2048/17/5/005

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Flux-flow-type Josephson oscillators consisting of epitaxial NbCN/MgO/NbCN junctions were fabricated and their current-voltage (I-V) characteristics were measured in various magnetic fields to evaluate their oscillation characteristics. On the I-V curve for a fabricated oscillator, clear current steps were observed up to 3 mV, indicating that oscillation occurs up to 1.4 THz. Using a theoretical model, available output power for the oscillator was calculated to be in the range from 20 mu W to 40 mu W in the frequency range from 500 GHz to 1.2 THz.>
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We have investigated numerically a system consisting of a long Josephson junction embedded into a microstrip resonator. For such a configuration the Josephson vortex dynamics in the junction is driven by the oscillating currents in the resonator. We have calculated the complex impedance of the junction at the resonant frequency. The Q factor of the resonator and the change of the resonant frequency of the whole system can then be easily calculated knowing the parameters of the resonator without Josephson junction. The fluxon dynamics and the complex impedance of the junction are simulated for different values of the junction length, damping, dc bias current, and dc magnetic field. This allows us to explain the behavior of the impedance of the junction and different loss mechanisms. In particular, we predict a nonmonotonic dependence of the both real and imaginary parts of the impedance on the amplitude of the oscillating current in the resonator for some range of parameters. Our results show that such a combination of active and passive superconducting elements forms a rather interesting nonlinear physical system which might be useful for development of tunable/switchable superconducting resonators and suggests an experimental technique for detecting trapped vortices in long junctions.
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