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arXiv:1410.4388

Resonant enhancement of MQT in Josephson junctions: the influence of coherent two-level systems

M. V. Fistul

Published Oct 16, 2014• 0 citations
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Abstract

We report a theoretical study of the macroscopic quantum tunneling (MQT) in small Josephson junctions containing randomly distributed two-level systems. We focus on the magnetic field dependent crossover temperature $T_{cr}$ between the thermal fluctuation and quantum regimes of switching from the superconducting (the zero-voltage) state to a resistive one. In the absence of two-levels systems the crossover temperature shows a smooth decrease with an applied magnetic field characterized by an external flux $Φ$. Beyond that we predict a narrow peak in the dependence of $T_{cr}(Φ)$ occurring in the intermediate range of $Φ$. The effect becomes more pronounced as the junction size increases. We explain this effect quantitatively by a strong resonant suppression of a potential barrier for the Josephson phase escape that is due to the coherent quantum Rabi oscillations in two-level systems present in the junction.

DOI
10.48550/arxiv.1410.4388

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We report a theoretical study of the macroscopic quantum tunneling (MQT) in small Josephson junctions containing randomly distributed two-level systems. We focus on a Josephson phase escape for switching from the superconducting (the zero-voltage) state to a resistive one. Above the crossover temperature Tcr{T}_{cr}Tcr​ the thermal fluctuations of the Josephson phase induce such a switching, and as T<TcrT<{T}_{cr}T<Tcr​ the regime of the MQT occurs. In the absence of two-level systems (TLSs) a magnetic field applied parallel to the junction plane results in a smooth reduction of ${T}_{cr}(\mathrm{\ensuremath{\Phi}})$, where $\mathrm{\ensuremath{\Phi}}$ is an applied magnetic flux. As the TLSs are present in Josephson junctions we obtain a resonant enhancement of the MQT. This phenomenon manifests itself by a narrow peak in the dependence of ${T}_{cr}(\mathrm{\ensuremath{\Phi}})$ occurring in the intermediate range of $\mathrm{\ensuremath{\Phi}}$, i.e., $0<\mathrm{\ensuremath{\Phi}}<{\ensuremath{\phi}}_{0}$ (${\ensuremath{\phi}}_{0}$ is the magnetic flux quantum). We explain this effect quantitatively by a strong resonant suppression of the potential barrier for the Josephson phase escape that is due to the coherent quantum Rabi oscillations in two-level systems present in the junction.
The study of the quantum behaviour of Josephson junctions is interesting not only by itself, but also for the technological importance in electronics employing superconducting weak links. Recently great attention has been devoted to the effects of Resonant Macroscopic Quantum Tunneling (RMQT) between levels the same energy in neighbouring wells of the potential shape describing the junction. This can produce voltage spikes along the supercurrent branch of the I-V curves and peaks in the current switching distributions. Schmidt, Clealand, and Clarke have presented a detailed analysis of the resonant tunnel dynamics for the T=0 case. The present paper is based on a different theoretical approach, first proposed by Larkin and Ovchinnikov, which allows one to include explicitly the temperature. We study the phenomenon in its time dependence and this allows us to consider the lifetime of the voltage spikes as well as the effect of the RMQT on the quantum escape rate out of the V=0 state.>
A nanometer-thick small mesa consiting of only two or three Bi 2 Sr 2 CaCu 2 O y intrinsic Josephson junctions (IJJs) is studied through the switching current distribution measurements down to 0.4 K. Experimental results clearly show that the first switching events from the zero-voltage state for 1 K < T < 4 K are successfully described by a conventional thermal activation (TA) theory for a single Josephson junction, and that they become independent of temperature below T* ∼ 0.7 K. We observe the microwave-induced peak in the switching distribution at 0.4 K, which is induced by the microwave irradiation at 55 GHz. These results strongly suggest that the system crossovers to macroscopic quantum tunneling (MQT) regime below T* , which is as high as the previously reported value for a stacked IJJs with several tens of junctions, in contrast to the recent result on a similar mesa-structured surface IJJ.
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We study macroscopic quantum tunneling (MQT) in ccc-axis twist Josephson junctions made of high-Tc{T}_{c}Tc​ superconductors in order to clarify the influence of the anisotropic order parameter symmetry (OPS) on MQT. The dependence of the MQT rate on the twist angle $\ensuremath{\gamma}$ about the ccc axis is calculated by using the functional integral and the bounce method. Due to the ddd-wave OPS, the $\ensuremath{\gamma}$ dependence of standard deviation of the switching current distribution and the crossover temperature from thermal activation to MQT are found to be given by $\mathrm{cos}\phantom{\rule{0.2em}{0ex}}2\ensuremath{\gamma}$ and $\sqrt{\mathrm{cos}\phantom{\rule{0.2em}{0ex}}2\ensuremath{\gamma}}$, respectively. We also show that a dissipative effect resulting from the nodal quasiparticle excitation on MQT is negligibly small, which is consistent with recent MQT experiments using ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ intrinsic junctions. These results indicate that MQT in ccc-axis twist junctions becomes a useful experimental tool for testing the OPS of high-Tc{T}_{c}Tc​ materials at low temperature, and suggest high potential of such junctions for qubit applications.