T. Koyama

Multi-scale simulation for terahertz wave emission from the intrinsic Josephson junctions

A numerical method applicable to the analysis of the multi-scale electromagnetic (EM) excitations in intrinsic Josephson junctions (IJJs) is presented. Using this method, we investigate the EM wave emission from the IJJs observed in Bi2Sr2CaCu2O8 mesas. The IJJs have three length scales that are greatly different in magnitude, i.e.?the distance between superconducting layers (d ~ 10 ? 3??m), the Josephson length (?J ~ 1??m) and the c-axis penetration depth (?c ~ 102??m). The EM field excited in the IJJs generally shows spatial variation of these three length scales at the same time. In our numerical method the coupled dynamical equations for the phase differences and the EM field can be solved simultaneously in all the scales in the whole space composed of the IJJs and the surrounding vacuum. We clarify that the spatial symmetry of the EM field excited at the resonance with the ?-cavity-mode is different from that with the 2?-cavity-mode. The strong EM wave emission originating from the ?-cavity-mode resonance takes place in the region where the uniform branch becomes unstable in the I?V characteristics.

Theory and Simulation of Electromagnetic Wave Emission from Intrinsic Josephson Junctions

Concerning the spontaneous THz electromagnetic wave emission from intrinsic Josephson junctions, we revisit the theory for intrinsic Josephson effects from standpoints of the couplings between stacked junctions and consequent superconducting phase dynamics. After reviewing the previous simulation works based on the theory, we present recent multi-physics simulation studies suggested by authors to systematically reproduce the emission experiments. Some typical simulation results are exhibited and the future perspective of the simulation is also given.

Variety of c -Axis Collective Excitations in Layered Multigap Superconductors

We present a dynamical theory for the phase differences along a stacked direction of intrinsic Josephson junctions (IJJs) in layered multigap superconductors, motivated by the discovery of highly anisotropic iron-based superconductors with thick perovskite-type blocking layers. The dynamical equations describing ac and dc intrinsic Josephson effects peculiar to multigap IJJs are derived, and collective Leggett mode excitations in addition to the Josephson plasma established in single-gap IJJs are predicted. The dispersion relations of their collective modes are explicitly displayed, and the remarkable peculiarity of the Leggett mode is demonstrated.

Macroscopic quantum tunneling in multigap superconducting Josephson junctions: Enhancement of escape rate via quantum fluctuations of the Josephson-Leggett mode

We theoretically study the macroscopic quantum tunneling (MQT) in a hetero Josephson junction formed by a conventional single-gap superconductor and a multi-gap one such as

Theory of Josephson effects in iron-based multi-gap superconductor junctions

{MgB}_{2}

Electrodynamics and intrinsic Josephson effects in multi-gap superconductors

and iron-based superconductors. In such a Josephson junction more than one phase differences are defined. We clarify the quantum dynamics of the phase differences and construct a theory for the MQT in the multi-gap Josephson junctions. The dynamics of the phase differences are strongly affected by the Josephson-Leggett mode, which is the out-of-phase oscillation mode of the phase differences. The escape rate is calculated in terms of the effective action renormalized by the Josephson-Leggett mode. It is shown that the escape rate is drastically enhanced when the frequency of the Josephson-Leggett mode is less than the Josephson-plasma frequency.

Emission of terahertz electromagnetic waves from intrinsic Josephson junction arrays embedded in resonance LCR circuits

We revisit the theory for Josephson effects in Josephson junctions with multiple tunneling channels, which can be realized when multi-gap superconductors, e.g., iron-based superconductors or MgB2 are employed as the electrodes. We mainly re-examine a hetero (multi-band)superconductor- insulator-(single-band)superconductor junction. Deriving an effective Lagrangian density based on the time-dependent Ginzburg-Landau model, we discuss how the relative fluctuations between multiple gaps (i.e., Leggetts excitation modes) and the pairing symmetry modify the Shapiro steps.

I-V characteristics in multi-gap intrinsic Josephson junction stacks

We develop a theory for the Josephson effects in 2-gap intrinsic Josephson junction stacks (IJJs). The coupled dynamical equations for the phase differences are derived from the low-energy effective Lagrangian. The equations can describe the longitudinal Josephson plasma and the Josephson–Leggett (JL) mode propagating in the direction perpendicular to the junctions. Numerical results for the I – V characteristics are presented. The I – V characteristics shows multiple-branch structure similar to that in Bi-2212 IJJs. When the Josephson frequency is approached to the JL mode frequency in non-uniform voltage branches, the JL mode is resonantly excited. At the resonant voltage a step-like structure appears in the I – V curves in low-voltage branches.