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Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*; Matsumoto, Hideki*
Physica C, 470(Suppl.1), p.S882 - S883, 2010/12
Times Cited Count：1 Percentile：6.46(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 
-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 -wave superconductivity is more enlarged than that for s-wave without sign change between the order parameters.
Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*; Matsumoto, Hideki*
Physica C, 470(20), p.1137 - 1140, 2010/11
Times Cited Count：1 Percentile：6.46(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 
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.
-wave symmetry in multigap superconducting Josephson junctionsOta, Yukihiro; Machida, Masahiko; Koyama, Tomio*
Physical Review B, 82(14), p.140509_1 - 140509_4, 2010/10
Times Cited Count：20 Percentile：63.17(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 
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 -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 -wave case the ones with even index have larger values. The existence of the fractional Shapiro steps is also predicted.
Ota, Yukihiro; Kondo, Yasushi*
no journal, ,
We show that some composite pulses widely employed in nuclear magnetic resonance experiments are regarded as non-adiabatic geometric quantum gates with Aharonov-Anandan phases. Thus, we obtain an alternative understanding of such techniques from fundamental viewpoints in quantum mechanics. To examine the robustness of such composite pulses against fluctuations, we propose a practical noise model in a two-level system. Then, we find that the composite pulses have purely geometrical nature even under a certain type of fluctuations. Furthermore, we classify fluctuations in terms of the robustness of geometric quantum gates.
Yamada, Susumu; Onishi, Hiroaki; Igarashi, Ryo; Okumura, Masahiko*; Machida, Masahiko
no journal, ,
no abstracts in English
