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Journal Articles

Theory of macroscopic quantum tunneling with Josephson-Leggett collective excitations in multiband superconducting Josephson junctions

Asai, Hidehiro*; Ota, Yukihiro; Kawabata, Shiro*; Machida, Masahiko; Nori, F.*

Physical Review B, 89(22), p.224507_1 - 224507_7, 2014/06

 Times Cited Count:9 Percentile:39.47(Materials Science, Multidisciplinary)

Collective excitations reveal fundamental properties and potential applications of superconducting states. We study macroscopic quantum tunneling (MQT) in a Josephson junction composed of multiband superconductors, focusing on a phase mode induced by interband fluctuations: the Josephson-Leggett (JL) collective excitation mode. We derive a formula for the MQT escape rate for low-temperature switching events, carefully treating this collective mode. We clarify that the JL mode has two major effects on the MQT: (1) the zero-point fluctuations enhance the escape rate, and (2) the quantum dissipation induced by the couplings to the gauge-invariant phase difference suppresses the quantum tunneling. We show that the enhancement exceeds the suppression for a wide range of junction parameters. This enhancement originates from the single-mode interaction between the tunneling variable and the interband fluctuations.

Journal Articles

Massless collective excitations in frustrated multiband superconductors

Kobayashi, Keita; Machida, Masahiko; Ota, Yukihiro*; Nori, F.*

Physical Review B, 88(22), p.224516_1 - 224516_5, 2013/12

 Times Cited Count:17 Percentile:59.84(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Direct numerical simulation for non-equilibrium transport phenomena in superconducting detectors

Ota, Yukihiro*; Kobayashi, Keita; Machida, Masahiko; Koyama, Tomio*; Nori, F.*

Physics Procedia, 27, p.352 - 355, 2012/00

 Times Cited Count:3 Percentile:74.04

no abstracts in English

Journal Articles

Simulation of logic gate using d-dot's

Nakajima, Susumu*; Kato, Masaru*; Koyama, Tomio*; Machida, Masahiko; Ishida, Takekazu*; Nori, F.*

Physica C, 468(7-10), p.769 - 772, 2008/04

 Times Cited Count:4 Percentile:22.01(Physics, Applied)

A d-dot is a superconducting composite structure of d- and s-wave superconductors, which shows spontaneous half-quantized magnetic fluxes. We developed numerical method to analyze the time development of these spontaneous magnetic fluxes, based on the twocomponents Ginzburg-Landau equation. The d-dot can be used as an element of quantum dot cellular automata logic gates. We show a simulation, which demonstrates the transfer of information between two d-dot's.

Oral presentation

Inter-band frustration in multi-band superconductors

Ota, Yukihiro; Kobayashi, Keita; Machida, Masahiko; Nori, F.*

no journal, , 

Multi-band superconductors including iron-based superconductors are attractive superconducting materials, since they have many preferable properties such as different chemical compounds and high transition temperature. This study is to reveal their curious ground-state and excited-state features, towards understanding the inter-band interaction. In particular, we focus on the inter-band frustration originating from competitive inter-band interactions. Our theory and numerical calculations show that (1) a time-reversal-symmetry-breaking state occurs when the number of the bands is greater than 2 and (2) the disappearance of the energy difference between this peculiar state and a conventional one is characterized well by the occurrence of an acoustic mode, i.e., massless Leggett mode. We stress that the appearance of the massless Leggett mode is remarkable since typical collective excitations in multi-band superconductors are massive.

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