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IEEE Transactions on Applied Superconductivity

Parametric Characterization and Macroscopic Quantum Tunneling of $\hbox{Nb}/\hbox{AlO}_{\rm x}/\hbox{Nb}$ Josephson Junctions

Hao Li, Gang Li, Jianshe Liu, Tiefu Li, Wei Chen

Published Aug 20, 2014• 0 citations
Publisher

Abstract

Parameters of$\hbox{Nb}/\hbox{AlO}_{\rm x}/\hbox{Nb}$Josephson including its critical current and capacitance which determine the dynamic energy consumption and speed of the superconducting electronics have been measured. The critical current of a$22 \times 22 \ \mu\hbox{m}^{2}$junction is$1801 \pm 52 \ \mu\hbox{A}$calculated from the dependent curve between the lifetime of zero-voltage and switching current. The specific capacitance is$0.051 \pm 0.008 \ \hbox{pF}/\mu\hbox{m}^{2}$obtained by observing Fiske self-resonant step when applied several Gauss magnetic field. Critical current distributions had been measured as a function of temperature from 900 mK to 40 mK. The distribution widths become independent of temperature below 200 mK, which indicates the transition between thermal activation and macroscopic quantum tunneling of junction phase.

DOI
10.1109/tasc.2014.2349900

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Parametric Characterization and Macroscopic Quantum Tunneling of Nb/AlOx/Nb\hbox{Nb}/\hbox{AlO}_{\rm x}/\hbox{Nb}Nb/AlOx​/Nb Josephson Junctions
Parameters ofNb/AlOx/Nb\hbox{Nb}/\hbox{AlO}_{\rm x}/\hbox{Nb}Nb/AlOx​/NbJosephson including its critical current and capacitance which determine the dynamic energy consumption and speed of the superconducting electronics have been measured. The critical current of a22×22 μm222 \times 22 \ \mu\hbox{m}^{2}22×22 μm2junction is1801±52 μA1801 \pm 52 \ \mu\hbox{A}1801±52 μAcalculated from the dependent curve between the lifetime of zero-voltage and switching current. The specific capacitance is0.051±0.008 pF/μm20.051 \pm 0.008 \ \hbox{pF}/\mu\hbox{m}^{2}0.051±0.008 pF/μm2obtained by observing Fiske self-resonant step when applied several Gauss magnetic field. Critical current distributions had been measured as a function of temperature from 900 mK to 40 mK. The distribution widths become independent of temperature below 200 mK, which indicates the transition between thermal activation and macroscopic quantum tunneling of junction phase.
We fabricated high quality Josephson tunneling junctions by self-aligned lift-off process. Parameters including its critical current, normal resistance and capacitance which determine the dynamic energy consumption and speed of the superconducting electronics had been measured. The critical current of a 22 × 22 μm2Junction is 1796 ± 52 μA calculated from the dependent curve between the lifetime of zero-voltage and switching current. The junction capacitance is 24.7 ± 3.9 pF obtained by observing Fiske self-resonant step when applied several Gauss magnetic field.
The probability distributions for switching out of the superconducting state of low-cur-rent-density 1-\ensuremath{\mu}m Nb Josephson junctions with capacitance \ensuremath{\approx} 0.1 pF have been measured as a function of temperature TTT down to 3 mK. Below 100 mK the distribution widths become independent of TTT. The results are in excellent agreement with predictions for the quantum tunneling of the (macroscopic) junction phase that include the reduction of tunneling rates due to dissipation.
We study $\mathrm{Nb}\ensuremath{-}{\mathrm{AlO}}_{x}$-Nb Josephson junctions (JJs) with a phase-discontinuity $\ensuremath{\kappa}$ created by a pair of current injectors attached to one of the Nb electrodes. For $\ensuremath{\kappa}\ensuremath{\approx}\ensuremath{\pi}$ the Josephson potential energy UUU as a function of the average phase $\ensuremath{\psi}$ across the JJ has the form of a $2\ensuremath{\pi}$-periodic double-well potential. Thus, the device behaves as a $\ensuremath{\varphi}$ JJ with degenerate ground state phases $\ensuremath{\psi}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\varphi}$ (the value of $\ensuremath{\varphi}$ depends on the system parameters). Experimentally, the existence of two wells of the potential is confirmed by the observation of two different critical currents ${I}_{\mathrm{c}\ifmmode\pm\else\textpm\fi{}}$, corresponding to the escape from different wells. We investigate the escape of the Josephson phase from both wells by collecting statistics of the switching currents. The histogram of switching current exhibits two peaks corresponding to ${I}_{\mathrm{c}\ifmmode\pm\else\textpm\fi{}}$. The dependence of the width ${\ensuremath{\sigma}}_{+}$ and ${\ensuremath{\sigma}}_{\ensuremath{-}}$ of each peak on the bath temperature TTT indicates the transition from thermal activation to macroscopic quantum tunneling (MQT) at ${T}^{★}\ensuremath{\approx}260\phantom{\rule{0.28em}{0ex}}\mathrm{mK}$ as TTT decreases. We argue that the observed saturation value of ${\ensuremath{\sigma}}_{+}$ and ${\ensuremath{\sigma}}_{\ensuremath{-}}$ below T★{T}^{★}T★ is indeed related to quantum tunneling rather than to parasitic noise in the system, as the histogram width can be reduced by tuning the value of $\ensuremath{\kappa}$ away from $\ensuremath{\pi}$. The comparison of the experimental escape rate $\mathrm{\ensuremath{\Gamma}}$ with theoretical predictions further confirms MQT of the phase $\ensuremath{\psi}$ from both wells.
Switching current distributions of an Nb/Al–AlO x /Nb Josephson junction are measured in a temperature range from 25 mK to 800 mK. We analyse the phase escape properties by using the theory of Larkin and Ovchinnikov (LO) which takes discrete energy levels into account. Our results show that the phase escape can be well described by the LO approach for temperatures near and below the crossover from thermal activation to macroscopic quantum tunneling. These results are helpful for further study of macroscopic quantum phenomena in Josephson junctions where discrete energy levels need to be considered.
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.
We have developed a scheme for high resolution measurements of the switching current distribution of a current-biased Josephson tunnel junction using a timing technique. In the measurement setup digital control and read-out electronics are galvanically isolated from the analog sample bias electronics by an optical fiber link. We have successfully used this technique to investigate thermal activation and macroscopic quantum tunneling of the phase in a high-quality 5×5 μm2 Nb–Al/AlOx–Nb Josephson tunnel junction with a critical current of Ic≈325 μA. At temperatures above the cross over temperature of T★≈280 mK the escape is dominated by thermal activation. Due to the high quality factor of the junction (Q≈95), the escape temperature is noticeably affected by the thermal prefactor. At temperatures below T★, the value of which agrees well with the theoretical predictions, the escape of the phase by quantum mechanical tunneling is observed. The presented technique can be employed to characterize current-biased Josephson tunnel junctions for applications in quantum information processing.