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Journal Articles

Kinetic inductance neutron detector operated at near critical temperature

Vu, TheDang; Nishimura, Kazuma*; Shishido, Hiroaki*; Harada, Masahide; Oikawa, Kenichi; Miyajima, Shigeyuki*; Hidaka, Mutsuo*; Oku, Takayuki; Soyama, Kazuhiko; Aizawa, Kazuya; et al.

Journal of Physics; Conference Series, 1590, p.012036_1 - 012036_9, 2020/07

Journal Articles

Temperature dependent characteristics of neutron signals from a current-biased Nb nanowire detector with $$^{10}$$B converter

Vu, TheDang; Iizawa, Yuki*; Nishimura, Kazuma*; Shishido, Hiroaki*; Kojima, Kenji*; Oikawa, Kenichi; Harada, Masahide; Miyajima, Shigeyuki*; Hidaka, Mutsuo*; Oku, Takayuki; et al.

Journal of Physics; Conference Series, 1293, p.012051_1 - 012051_9, 2019/10

Journal Articles

Quantum phases in intrinsic Josephson junctions; Quantum magnetism analogy

Machida, Masahiko; Kobayashi, Keita; Koyama, Tomio*

Physica C, 491, p.44 - 46, 2013/08

 Times Cited Count:1 Percentile:93.54(Physics, Applied)

no abstracts in English

Journal Articles

Revisit to terahertz wave emission with motions of Josephson vortices

Matsumoto, Hideki*; Koyama, Tomio*; Machida, Masahiko; Ota, Yukihiro*; Yamada, Susumu

Physica C, 491, p.40 - 43, 2013/08

 Times Cited Count:2 Percentile:86.91(Physics, Applied)

no abstracts in English

Journal Articles

Numerical study for electromagnetic wave emission from intrinsic Josephson junction stacks with a dielectric cover

Koyama, Tomio*; Matsumoto, Hideki*; Ota, Yukihiro*; Machida, Masahiko

Physica C, 491, p.20 - 23, 2013/08

 Times Cited Count:0 Percentile:100(Physics, Applied)

no abstracts in English

Journal Articles

Nonlocal Ginzburg-Landau theory for superconductors

Koyama, Tomio*; Machida, Masahiko

Physica C, 484, p.100 - 103, 2013/01

 Times Cited Count:1 Percentile:93.54(Physics, Applied)

no abstracts in English

Journal Articles

Electromagnetic wave emission from intrinsic Josephson junction stacks; A Review of theory and simulation

Machida, Masahiko; Ota, Yukihiro*; Koyama, Tomio*; Matsumoto, Hideki*

Journal of Physics; Conference Series, 393, p.012013_1 - 012013_8, 2012/09

 Times Cited Count:1 Percentile:51.96

Since the discovery of intrinsic Josephson effects in High-Tc cuprate superconductors, a tremendous number of theoretical and experimental studies have been made to clarify the coupling nature between stacked junctions. The reason comes from an expectation that Josephson junctions stacked in atomic-scale synchronize and high-power electromagnetic wave radiates. Consequently, two types of couplings, called "inductive and capacitive couplings" have been so far established. On the basis of the inductive coupling, several papers suggested that THz high-power emission is possible by using Josephson vortex flow states in the presence of layer parallel magnetic field and c-axis transport current. However, there has been no clear evidence of the emission in vortex flow states, although a few works found out only the sign. Afterwards, Ozyuzer et al., reported that they successfully observe a clear emission from intrinsic Josephson junctions. The situation was rather an unexpected one, since the magnetic field is not applied and the employed size is much larger than the experimental trends at that time. In this presentation, we review a history of the quest of the high power electromagnetic wave emission from intrinsic Josephson stacks before and after the Ozyuzer's work from a theoretical and numerical view. Especially, we would like to emphasize that direct numerical simulation techniques on multi-scale dynamical problems between inside and outside the junction have fully developed by authors.

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:2 Percentile:31.02

no abstracts in English

Journal Articles

Theory of phase dynamics in intrinsic Josephson junctions with multigap superconducting layers

Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*

Physica C, 471(21-22), p.760 - 762, 2011/11

 Times Cited Count:0 Percentile:100(Physics, Applied)

We construct a theory of dynamical behavior in intrinsic Josephson junction stacks (IJJ's) with multi-gap superconducting layers, motivated by recent discovery rush of highly-anisotropic iron-based superconductors with a thick perovskite block layers. The theory predicts the existence of two kinds of phase modes, one of which is the Josephson plasma mode and other of which is the Leggett's mode. The IJJ's with multiple tunneling channels have a novel energy scale, inter-band Josephson coupling on each superconducting layer which generates a mass of the Leggett's mode. We discuss a cooperative phenomena induced by such a coupling in addition to capacitive and inductive couplings between the superconducting layers.

Journal Articles

Inter-grain Josephson currents in two-gap superconductors

Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*

Physica C, 470(Suppl.1), p.S489 - S490, 2010/12

 Times Cited Count:0 Percentile:100(Physics, Applied)

We microscopically study inter-grain Josephson current in iron-pnictide superconductors. The theoretical examination predicts that the inter-grain Josephson current is significantly reduced by $$pm s$$-wave symmetry as the incoherent tunneling process becomes dominant and the density of states and the gap amplitude between two bands are identical. In addition, we suggest important points to improve the inter-grain critical current density.

Journal Articles

Collective modes and Josephson vortices in a heterotic Josephson junction between single- and two-gap superconductors

Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*; Matsumoto, Hideki*

Physica C, 470(Suppl.1), p.S882 - S883, 2010/12

 Times Cited Count:0 Percentile:100(Physics, Applied)

We construct a theory of a heterotic Josephson junction between single- and two-gap superconductors. The theory predicts the presence of in-phase and out-of-phase collective modes of superconducting phases. The former is the Josephson plasma mode, and the latter is a counterpart of Leggett's mode in Josephson junctions. A signature for $$pm s$$-wave superconductivity is a drastic reduction of the frequency of the Josephson plasma mode. We also show that the magnetic field distribution of the Josephson vortex for $$pm s$$-wave superconductivity is more enlarged than that for s-wave without sign change between the order parameters.

Journal Articles

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

Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*

Journal of Physics; Conference Series, 248, p.012090_1 - 012090_8, 2010/11

Since the discovery of iron-based superconductors, Josephson junctions including such new materials have been intensively studied in order to explore their pairing symmetry and application potential. One of interesting features of this type of materials is the presence of multiple (more than two) bands on Fermi surface and their contributions to superconductivity. As a result, iron-based superconductor junctions are possible to have multiple tunneling channels between the superconducting electrodes. In this paper, we show a theory of Josephson junctions with multigap superconductors. We discuss how the relative fluctuations between multiple gaps and the pairing symmetry modify the Shapiro steps.

Journal Articles

Theory and simulation of electromagnetic wave emission from intrinsic Josephson junctions

Machida, Masahiko; Ota, Yukihiro; Sasa, Narimasa; Koyama, Tomio*; Matsumoto, Hideki*

Journal of Physics; Conference Series, 248, p.012037_1 - 012037_8, 2010/11

 Times Cited Count:9 Percentile:8.32

no abstracts in English

Journal Articles

Electrodynamics and intrinsic Josephson effects in multi-gap superconductors

Koyama, Tomio*; Ota, Yukihiro; Machida, Masahiko

Journal of Physics; Conference Series, 248, p.012036_1 - 012036_6, 2010/11

 Times Cited Count:2 Percentile:33.85

We develop a theory for the Josephson effects in 2-gap intrinsic Josephson junctions stacks (IJJs). The coupled dynamical equations for the phase differences are derived from the low-energy effective Lagrangian. The equations 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 structures similar to that in Bi-2212 IJJ's. When the Josephson frequency is approached to the JL mode frequency in non-uniform voltage branches, the JL mode is resonantly exited. At the resonant voltage a step-like structure appears in the $$I$$-$$V$$ curves in low-voltage branches.

Journal Articles

First-principles calculations for anisotropy of iron-based superconductors

Nakamura, Hiroki; Machida, Masahiko; Koyama, Tomio*; Hamada, Noriaki*

Physica C, 470(20), p.1066 - 1069, 2010/11

 Times Cited Count:2 Percentile:86.24(Physics, Applied)

Journal Articles

Anomalous Josephson vortex solutions in Josephson junctions with multiple tunneling channels

Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*; Matsumoto, Hideki*

Physica C, 470(20), p.1137 - 1140, 2010/11

 Times Cited Count:1 Percentile:92.56(Physics, Applied)

We construct a theory for Josephson junctions with multiple tunneling channels. We focus on two situations, i.e., a heterotic junction composed of two-gap-superconductor, insulator, and one-gap-superconductor, and a grain-boundary junction formed by two identical multi-gap superconductors. Then, we show that the magnetic field distribution of the Josephson vortex for $$pm$$s-wave superconductivity is more enlarged than that for s-wave without sign change between the order parameters. We display such anomalous vortices and suggest how to evaluate the enlargement.

Journal Articles

Shapiro steps as a direct probe of $$pm$$$$s$$-wave symmetry in multigap superconducting Josephson junctions

Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*

Physical Review B, 82(14), p.140509_1 - 140509_4, 2010/10

 Times Cited Count:17 Percentile:36.54(Materials Science, Multidisciplinary)

We theoretically study the Shapiro steps in a hetero-Josephson junction made of a single-gap superconductor and a two-gap one. We find that an anomalous dc Josephson current is induced by tuning the frequency of an applied microwave to the Josephson-Leggett mode frequency, which creates an extra step structure in the ${it I-V}$ characteristics besides the conventional Shapiro steps. The step heights at the resonance voltages exhibit an alternate structure of a large and small value reflecting the gap symmetry of the two-gap superconductor. In the $$pm$$$$s$$-wave case in which the two gaps have opposite signs in the two-gap superconductor the steps with odd index are enhanced, whereas in the $$s$$-wave case the ones with even index have larger values. The existence of the fractional Shapiro steps is also predicted.

Journal Articles

Ambegaokar-Baratoff relations for Josephson critical current in heterojunctions with multigap superconductors

Ota, Yukihiro; Nakai, Noriyuki; Nakamura, Hiroki; Machida, Masahiko; Inotani, Daisuke*; Ohashi, Yoji*; Koyama, Tomio*; Matsumoto, Hideki*

Physical Review B, 81(21), p.214511_1 - 214511_6, 2010/06

 Times Cited Count:25 Percentile:26.21(Materials Science, Multidisciplinary)

An extension of the Ambegaokar-Baratoff relation to a superconductor-insulator-superconductor (SIS) Josephson junction with multiple tunneling channels is derived. Applying the resultant relation to a SIS Josephson junction formed by an iron-based (five-band) and a single-band Bardeen-Cooper-Schrieffer (BCS) type superconductors, a theoretical bound of the Josephson critical current ($$I_{rm c}$$) multiplied by the resistance of the junction ($$R_{rm n}$$) is given. We reveal that such a bound is useful for identifying the pairing symmetry of iron-pnictide superconductors. One finds that if a measured value of $$I_{rm c}R_{rm n}$$ is smaller than the bound then the symmetry is $$pm s$$-wave, and otherwise $$s$$-wave without any sign changes. In addition, we stress that temperature dependence of $$I_{rm c}R_{rm n}$$ is sensitive to the difference of the gap functions from the BCS type gap formula in the above heterojunction.

Journal Articles

Theory of vortex structure in Josephson junctions with multiple tunneling channels; Vortex enlargement as a probe of $$pm$$$$s$$-wave superconductivity

Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*; Matsumoto, Hideki*

Physical Review B, 81(1), p.014502_1 - 014502_5, 2010/01

 Times Cited Count:18 Percentile:34.77(Materials Science, Multidisciplinary)

We theoretically study Josephson vortex structure in Josephson junctions which have multiple tunneling channels caused by multiple superconducting gaps. Deriving coupled sine-Gordon equation from the free energy taking account of the multiple tunneling channels, we examine two typical situations, a heterotic junctions composed of multi-gap superconductor, insulator, and single-gap superconductor, and a grain boundary junction formed by two identical multigap superconductors. Then, we reveal in both situations that the magnetic field distribution of the Josephson vortex for $$pm$$$$s$$-wave superconductivity is more enlarged than that for $$s$$-wave without sign change between the order parameters. Its mechanism is ascribed to a cancellation of the multiple Josephson currents. We display such an anomalous Josephson vortex and suggest how to evaluate the enlargement.

Journal Articles

First-principles study for the anisotropy of iron-based superconductors toward power and device applications

Nakamura, Hiroki; Machida, Masahiko; Koyama, Tomio*; Hamada, Noriaki*

Journal of the Physical Society of Japan, 78(12), p.123712_1 - 123712_4, 2009/12

 Times Cited Count:37 Percentile:15.41(Physics, Multidisciplinary)

93 (Records 1-20 displayed on this page)