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Journal Articles

Time-reversal symmetry breaking phase and gapped surface states in $$d$$-wave nanoscale superconductors

Nagai, Yuki; Ota, Yukihiro*; Tanaka, Kaori*

Journal of Physics; Conference Series, 969, p.012039_1 - 012039_6, 2018/03

 Times Cited Count:0 Percentile:100

no abstracts in English

Journal Articles

Time-reversal symmetry breaking and gapped surface states due to spontaneous emergence of new order in $$d$$-wave nanoislands

Nagai, Yuki; Ota, Yukihiro*; Tanaka, Kaori*

Physical Review B, 96(6), p.060503_1 - 060503_5, 2017/08

 Times Cited Count:5 Percentile:56.63(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Nuclear magnetic relaxation rates of unconventional superconductivity in doped topological insulators

Nagai, Yuki; Ota, Yukihiro*

Physical Review B, 94(13), p.134516_1 - 134516_8, 2016/10

 Times Cited Count:6 Percentile:57.07(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Critical temperature enhancement of topological superconductors; A Dynamical mean-field study

Nagai, Yuki; Hoshino, Shintaro*; Ota, Yukihiro*

Physical Review B, 93(22), p.220505_1 - 220505_5, 2016/06

 Times Cited Count:7 Percentile:52.06(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

A Screened automated structural search with semiempirical methods

Ota, Yukihiro; Ruiz-Barragan, S.*; Machida, Masahiko; Shiga, Motoyuki

Chemical Physics Letters, 648, p.119 - 123, 2016/03

 Times Cited Count:2 Percentile:89.03(Chemistry, Physical)

We developed an interface program between a program suite for an automated search of chemical reaction pathways, GRRM, and a program package of semiempirical methods, MOPAC. A two-step structural search is proposed as an application of this interface program. A screening test is first performed by semiempirical calculations. Subsequently, a reoptimization procedure is done by ab initio or density functional calculations. We apply this approach to ion adsorption on cellulose. The computational efficiency is also shown for a GRRM search. The interface program is suitable for the structural search of large molecular systems for which semiempirical methods are applicable.

Journal Articles

Theory of low-energy behaviors in topological s-wave pairing superconductors

Ota, Yukihiro; Nagai, Yuki; Machida, Masahiko

Physica C, 518, p.5 - 9, 2015/11

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

Topological superconductors are notable materials, owing to the mathematical interest as topological matter and the application potential of quantum engineering. To clarify their characters relying on gap-function types, we build up a low-energy effective theory, focusing on a model of superconductor Cu$$_{x}$$Bi$$_{x}$$Se$$_{3}$$. The non-magnetic impurity effects in our previous numerical study indicates that the system has both p-wave and s-wave aspects. A perturbation study is insightful for explaining this peculiar behavior. Since the normal electrons are dominated by the Dirac-type dispersion relation, we assign a low-energy case as a large-Dirac-mass one. When the system has an odd-parity fully-gapped pair potential, our approach indicates that the effective gap function in the low-energy domain has not only a chiral p-wave-like component, but also an s-wave-like one. This peculiar mixture of p-wave and s-wave leads to intriguing responses in the present model.

Journal Articles

Inverse coherence effects in nuclear magnetic relaxation rates as a sign of topological superconductivity

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Physical Review B, 92(18), p.180502_1 - 180502_5, 2015/11

 Times Cited Count:3 Percentile:79.82(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Topological $$s$$-wave pairing superconductivity with spatial inhomogeneity; Midgap-state appearance and robustness of superconductivity

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Journal of the Physical Society of Japan, 84(3), p.034711_1 - 034711_8, 2015/03

 Times Cited Count:5 Percentile:50.64(Physics, Multidisciplinary)

no abstracts in English

Journal Articles

Robustness against non-magnetic impurities in topological superconductors

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Journal of Physics; Conference Series, 568(2), p.022030_1 - 022030_5, 2014/12

 Times Cited Count:0 Percentile:100

We study the robustness against non-magnetic impurities in a three-dimensional topological superconductor, focusing on an effective model (massive Dirac Bogoliubov-de Gennes (BdG) Hamiltonian with $$s$$-wave on-site pairing) of Cu$$_{x}$$Bi$$_{2}$$Se$$_{3}$$ with the parameter set determined by the first-principles calculation. With the use of the self-consistent $$T$$-matrix approximation for impurity scattering, we discuss the impurity-concentration dependence of the zero-energy density of states. We show that a single material variable, measuring relativistic effects in the Dirac-BdG Hamiltonian, well characterizes the numerical results. In the nonrelativistic limit, the odd-parity fully-gapped topological superconductivity is fragile against non-magnetic impurities, since this superconductivity can be mapped onto the $$p$$-wave superconductivity. On the other hand, in the ultrarelativistic limit, the superconductivity is robust against the non-magnetic impurities, since the effective model has the $$s$$-wave superconductivity. We derive the effective Hamiltonian in the both limit.

Journal Articles

Impurity effects in a two-dimensional topological superconductor; A Link of $$T_{rm c}$$-robustness with a topological number

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Journal of the Physical Society of Japan, 83(9), p.094722_1 - 094722_5, 2014/09

 Times Cited Count:9 Percentile:37.67(Physics, Multidisciplinary)

no abstracts in English

Journal Articles

Ferromagnetism in multi-orbital Fermi gas loaded on a one-dimensional optical lattice

Kobayashi, Keita; Okumura, Masahiko; Ota, Yukihiro*; Yamada, Susumu; Machida, Masahiko

JPS Conference Proceedings (Internet), 3, p.016006_1 - 016006_6, 2014/06

no abstracts in English

Journal Articles

Nonmagnetic impurity effects in a three-dimensional topological superconductor; From $$p$$- to $$s$$-wave behaviors

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Physical Review B, 89(21), p.214506_1 - 214506_6, 2014/06

 Times Cited Count:20 Percentile:26.57(Materials Science, Multidisciplinary)

Unconventional features in superconductivity are revealed by responses to impurity scattering. We study nonmagnetic impurity effects in a three-dimensional topological superconductor, focusing on an effective model (massive Dirac Hamiltonian with s-wave on-site pairing) of copper-doped bismuth-selenium compounds. Using a self-consistent T-matrix approach for impurity scattering, we examine in-gap states in density of states. We find that the results are well characterized by a single material variable, which measures relativistic effects in the Dirac Hamiltonian. In the nonrelativistic regime, an odd-parity superconducting state is fragile against nonmagnetic impurities. We show that this behavior is caused by a p-wave character involved in the topological superconducting state. In contrast, we show that in the relativistic regime the superconductivity is robust against nonmagnetic impurities, owing to an s-wave character. To summarize, the system has two aspects, p- and s-wave features, depending on the weight of relativistic effects.

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:8 Percentile:56.23(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

Quantum phases in degenerate-$$p$$-orbital attractive one-dimensional fermionic optical lattices

Kobayashi, Keita; Ota, Yukihiro; Okumura, Masahiko; Yamada, Susumu; Machida, Masahiko

Physical Review A, 89(2), p.023625_1 - 023625_6, 2014/02

 Times Cited Count:9 Percentile:48.66(Optics)

no abstracts in English

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:16 Percentile:36.71(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Analysis of collective excitation for multi band superconductor; Frustrated spin model approach

Kobayashi, Keita; Ota, Yukihiro*; Machida, Masahiko

Physica C, 494, p.13 - 16, 2013/11

 Times Cited Count:4 Percentile:74.83(Physics, Applied)

no abstracts in English

Journal Articles

Numerical construction of a low-energy effective Hamiltonian in a self-consistent Bogoliubov-de Gennes approach of superconductivity

Nagai, Yuki; Shinohara, Yasushi*; Futamura, Yasunori*; Ota, Yukihiro*; Sakurai, Tetsuya*

Journal of the Physical Society of Japan, 82(9), p.094701_1 - 094701_10, 2013/09

 Times Cited Count:17 Percentile:27.54(Physics, Multidisciplinary)

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:87.01(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

Nontrivial Haldane phase of an atomic two-component Fermi gas trapped in a 1D optical lattice

Kobayashi, Keita; Okumura, Masahiko; Ota, Yukihiro*; Yamada, Susumu; Machida, Masahiko

Physical Review Letters, 109(23), p.235302_1 - 235302_5, 2012/12

 Times Cited Count:22 Percentile:24.84(Physics, Multidisciplinary)

no abstracts in English

119 (Records 1-20 displayed on this page)