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IEEE Transactions on Applied Superconductivity

Experimental implications of intrinsic quasiparticle mechanisms of dissipation in the switching dynamics of Josephson devices

B. Ruggiero, P. Silvestrini

Published Mar 1, 1993• 0 citations
Publisher

Abstract

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.>

DOI
10.1109/77.233562

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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.>
The effect of the voltage-dependent junction resistance R/sub j/ on the critical current decay of Josephson junctions was studied. In the thermal regime, the author has obtained a simple expression for the lifetime of the zero-voltage state which takes into account the nonlinear dependence of R/sub j/ on the voltage, removing an assumption of the resistivity shunted junction model (RSJ), which assumes an ohmic shunt resistance to describe the junction dissipation. These results are confirmed by experimental data which also show the relevance of the intrinsic dissipation due to the presence of quasiparticles. This latter aspect is relevant, because it allows extremely underdamped systems to be obtained at low temperatures. These conditions are particularly interesting to study a macroscopic quantum aspect, namely the effects of the presence of quantized energy levels on the supercurrent decay. The dependence of this latter effect on the junction resistance is also briefly discussed.
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.
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 Nb/AlOx/Nb{\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}\mathrm{O}}_{x}/\mathrm{Nb}Nb/AlOx​/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.
Macroscopic quantum effects in Josephson systems have recently attracted interest in the scientific community both for the physics involved and in view of applications. Here we present experiments on the presence of energy level quantization at temperatures above the classical-quantum regime crossover temperature in different Josephson junctions. This has been possible by extending the measurements of the escape rate out of the zero-voltage state at higher sweeping bias rate (dI/dt up to 100 A/sec) in order to induce nonstationary conditions in the energy potential describing the junction dynamics. Our measurements allow us to obtain as fitting parameter the "effective" resistance to be used in the RSJ model which is compatible with the subgap resistance at very low voltage. The very low dissipation level obtained at low temperatures renders our results quite interesting in view of applications in the quantum limit.
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.
The escape rate of an underdamped ($Q\ensuremath{\approx}30$), current-biased Josephson junction from the zero-voltage state has been measured. The relevant parameters of the junction were determined in situ in the thermal regime from the dependence of the escape rate on bias current and from resonant activation in the presence of microwaves. At low temperatures, the escape rate became independent of temperature with a value that, with no adjustable parameters, was in excellent agreement with the zero-temperature prediction for macroscopic quantum tunneling.