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Physical review. B, Condensed matter

Supercurrent decay in extremely underdamped Josephson junctions

B. Ruggiero, C. Granata, Vittorio Giulio Palmieri, A. Esposito, M. Russo, P. Silvestrini

Published Jan 1, 1998• 32 citations
Publisher

Abstract

We present an experimental study of the effective dissipation relevant in the thermally activated supercurrent decay of extremely underdamped Josephson junctions. Data referring to the supercurrent decay of ${\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}\mathrm{O}}_{x}/\mathrm{Nb}$ Josephson junctions are compared with the Kramers theory. Our measurements allow us to obtain the ``effective'' resistance to be used in the resistively shunted junction model that results to be the subgap resistance due to the presence of thermally activated quasiparticles. The extremely low dissipation level obtained at low temperatures renders our result quite interesting in view of experiments in the quantum limit.

DOI
10.1103/physrevb.57.134

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We present a design of extremely underdamped Josephson junctions either for use in experiments in the quantum limit or for devices using the switching dynamics of the junction, when a low noise level is mandatory. High resistance molybdenum meander lines integrated with the junction allow one to increase the insulation of the sample from its electromagnetic environment. We report an experimental study of the effective dissipation relevant in the thermally activated supercurrent decay presenting evidence of a substantial reduction of the dissipation level at low temperature.
The decay of the zero-voltage state is studied, including quasi-particle tunneling and the interference cos phi term in the equivalent circuit model. As a result of the approach used, the effective resistance for describing the junction dissipation, which was an arbitrary parameter in the resistively shunted junction (RSJ) model, can now be obtained in terms of the measurable junction parameters and bias conditions. This allows a direct comparison of data with theory. The very good agreement between data and theory confirms the essential correctness of the authors' assumptions. The data also provide an excellent experimental test of the thermal activation theory in underdamped physical systems.>
Measurements of temperature dependence of the return current in high quality Nb/AlOx/Nb Josephson junctions are presented. From the experimental data, we obtain the effective resistance, i.e., the effective dissipation, for the retrapping process, according to the generalized resistively shunted junction model proposed by Chen, Fisher, and Leggett. We present a careful analysis, based on a comparison between the measured temperature dependencies of both the return and the quasiparticle tunneling current. We find that the junction subgap conductance, which includes the quasiparticle and the quasiparticle-pair interference terms, is responsible for the return process. The measurements have been performed on various samples, in a wide range of critical current densities from 50 to 2250 A/cm2, covering different damping regimes and spanning over the high and low temperature limits. Junctions with low critical current density show ideal dissipation which makes the return current scale with temperature according to the BCS exponential behavior without flattening out effects. This result may be relevant for the possible use of Nb/AlOx/Nb junctions in macroscopic quantum coherence experiments, which strongly require a very low dissipation.
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We study the decay of the zero-voltage state of an underdamped Josephson junction, including in the equivalent circuit model the interference cos\ensuremath{\varphi} term. From a theoretical point of view, we provide an expression for the lifetime of the metastable state, which takes into account the dependence on the voltage of both the quasiparticles tunneling and the interference-term conductances. We also present a comparison of the theory with our experimental results, which clearly indicates the importance of these intrinsic mechanisms of dissipation on the supercurrent decay. We show that the interference term must be included in the theory if agreement between theory and data is to be achieved within the experimental uncertainty.