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Journal of Applied Physics

Switching dynamics of Nb/AlOx/Nb Josephson junctions: Measurements for an experiment of macroscopic quantum coherence

Maria Gabriella Castellano, Roberto Leoni, G. Torrioli, F. Chiarello, C. Cosmelli, A. Costantini, G. Diambrini-Palazzi, P. Carelli, Roberto Cristiano, Luigi Frunzio

Published Sep 1, 1996• 30 citations
Publisher

Abstract

Josephson junctions based on Nb/AlOx/Nb trilayer technology have demonstrated excellent quality, exhibiting very low dissipation in the subgap region. This property is very important for all those experiments which have to deal with the quantum behavior of macroscopic variables in Josephson devices. In view of performing an experiment of macroscopic quantum coherence with a Josephson device, we measured the escape temperature and the return current of several junctions fabricated with different processes and having different characteristics, by cooling them from 4.2 to 0.3 K. We show that, for what concerns the process of escape from the zero-voltage state, no difference can be found among different junctions. On the other hand, the dissipation properties are strongly influenced by the current density, with lower current density resulting in lower intrinsic dissipation.

DOI
10.1063/1.363147

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Josephson junctions based on Nb/AlOx/Nb trilayer technology have demonstrated excellent quality, exhibiting very low dissipation in the subgap region. This property is very important for all those experiments which have to deal with the quantum behavior of macroscopic variables in Josephson devices. In view of performing an experiment of macroscopic quantum coherence with a Josephson device, we measured the escape temperature and the return current of several junctions fabricated with different processes and having different characteristics, by cooling them from 4.2 to 0.3 K. We show that, for what concerns the process of escape from the zero-voltage state, no difference can be found among different junctions. On the other hand, the dissipation properties are strongly influenced by the current density, with lower current density resulting in lower intrinsic dissipation.
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.
We present Nb/AlOx/Nb–Al/AlOx/Nb stacked Josephson junctions which we propose as three-terminal elements to control the quantum behavior of Josephson complex systems. A bias current can be independently injected in the “top” junction, providing a fine control of the critical current of the “bottom” junction. The reported characterization of the device refers to data in the classical limit including measurements of the switching dynamics between metastable states of the system at different temperatures and bias conditions. At low temperature, the effective dissipation of the bottom junction remains substantially unchanged when different injection currents flow in the top junction. These results are interesting to project new configurations of macroscopic quantum experiments.
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 macroscopic quantum tunnelling (MQT) in intrinsic Josephson junctions (IJJs) suggest that they may be suitable as qubits. An advantage of using IJJs over trilayers is the absence of any artificial barrier and hence the possible absence of decohering two-level fluctuators. A disadvantage is that it is difficult to fabricate single IJJs. Hence all IJJ MQT experiments have been carried out on arrays of many IJJs in series. Since MQT is suppressed by dissipation, it is important (a) to understand the role of dissipation on the dynamics of IJJs and (b) to consider how the dissipation and the dynamics change as a function of the number of junctions in the array which are in the voltage state. We have measured distributions of the nth-order switching current in TlBaCaCuO IJJ arrays, where n indicates switching from n to n+1 junctions being in the voltage state. The junctions show conventional underdamped behaviour at temperatures below some temperature T*, with a suppression of the width of the distribution above T*. We find that T* depends upon n. For n = 0 (i.e. switching from the supercurrent branch), T* is less than 4.2 K. For n > 0, we measured T* values between 20 K and 40 K. Calculations and simulations show that a change in T* results from a change in Q. Analysis suggests that the Q of the IJJs is determined by the environmental impedance when all junctions in the array are in the zero-voltage state, whereas having one junction or more in the voltage state isolates the array from the environment leading to a higher Q. We discuss our data in the context of possible qubit applications.
In an effort to determine the suitability of Josephson junctions for applications in quantum computation, we measured low-noise dc current-voltage characteristics of Nb/AlO/sub x//Nb and Al/AlO/sub x//Al junctions in the temperature range from 90 mK to 1 K. Nb-based samples were obtained from several different facilities with critical currents of a few /spl mu/A and critical current densities between 100 and 3000 A/cm/sup 2/. We fabricated the Al-based samples using double-angle evaporation and obtained critical currents of a few /spl mu/A with critical current densities of about 30 A/cm/sup 2/. We found that the sub-gap leakage current in the Nb-based samples does not depend on temperature in the range 90 mK to 1 K, whereas that for the Al-based samples follows the expected BCS behavior to about 150 mK. Our Al-based samples have a lower level of dissipation than Nb-based devices; however, both Al- and Nb-based samples achieved dissipation levels sufficiently low for some quantum computing applications.
We report on the fabrication of superconductor/insulator/ferromagnetic metal/superconductor (Nb/AlOx/PdNi/Nb) π-Josephson junctions. An analysis of Fiske modes up to about 400 GHz provides information on the junction quality factor and the relevant damping mechanisms. High quality factors of up to 30 around 200 GHz have been achieved. They allow for the study of the dynamic properties of the junctions, in particular the switching probability from the zero-voltage into the voltage state with and without microwave irradiation. The experiments with microwave irradiation are well explained within semi-classical models and numerical simulations. At mK temperature, the switching dynamics without applied microwaves clearly shows secondary quantum effects. Here, we could observe for the first time macroscopic quantum tunneling in Josephson junctions with ferromagnetic interlayer.