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Journal Articles

Direct numerical demonstration of sign-preserving quasiparticle interference via an impurity inside a vortex core in an unconventional superconductor

Nagai, Yuki; Nakai, Noriyuki*; Machida, Masahiko

Physical Review B, 85(9), p.092505_1 - 092505_4, 2012/03

https://doi.org/10.1103/PhysRevB.85.092505

Times Cited Count：11 Percentile：44.07(Materials Science, Multidisciplinary)

We perform large-scale self-consistent numerical calculations, solving the Bogoliubov-de Gennes (BdG) equations in the magnetic field together with random impurities to directly demonstrate the typical quasiparticle interference (QPI) in the presence of vortices as observed by scanning tunneling microscopy and spectroscopy experiments in unconventional superconductors. The calculations reveal that the vortex itself never works as a scatter, causing the QPI pattern, but the vortex core containing impurity brings about the enhancement of the sign-preserving QPI peaks. Its origin is Andreev-bound states distorted by impurity, and all the measurement findings are consistently explained by the scenario based on the numerical results.

Journal Articles

Low-lying excitations induced by non-magnetic impurities in -wave superconductors

Nakai, Noriyuki; Nagai, Yuki; Machida, Masahiko

Physica C, 471(21-22), p.743 - 746, 2011/11

https://doi.org/10.1016/j.physc.2011.05.042

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

We have performed numerical calculations for low-lying excitations induced by a single non-magnetic impurity in a -wave superconductor on the basis of two different frameworks. One is the Bogoliubov-de Gennes theory in real coordinates, while the other is the Born approximation in wave-number coordinates. This paper shows that these results for low-lying excitations are identical by using Fourier transformations.

Journal Articles

Microwave surface impedance measurements of LiFeAs single crystals

Imai, Yoshinori*; Takahashi, Hideyuki*; Kitagawa, Kentaro*; Matsubayashi, Kazuyuki*; Nakai, Noriyuki*; Nagai, Yuki; Uwatoko, Yoshiya*; Machida, Masahiko; Maeda, Atsutaka*

Journal of the Physical Society of Japan, 80(1), p.013704_1 - 013704_4, 2011/01

https://doi.org/10.1143/JPSJ.80.013704

Times Cited Count：36 Percentile：81.82(Physics, Multidisciplinary)

no abstracts in English

Journal Articles

Phenomenological theory for s-wave superconducting states of iron-based superconductors

Machida, Masahiko; Nagai, Yuki*; Ota, Yukihiro; Nakai, Noriyuki; Nakamura, Hiroki; Hayashi, Nobuhiko*

Physica C, 470(Suppl.1), p.S372 - S374, 2010/12

https://doi.org/10.1016/j.physc.2009.10.087

Times Cited Count：2 Percentile：12.12(Physics, Applied)

We briefly present three phenomenological theoretical works for superconducting properties in iron-based superconductors. The first is a suggestion of multi-band s-wave pairing model consistently explaining different measurement data, the second a proposal to detect s-wave signature, and the third an evaluation of the critical current in polycrystalline samples by considering s-wave symmetry.

Journal Articles

Small jump of specific heat and small gap in iron pnictide superconductors

Nakai, Noriyuki; Nakamura, Hiroki; Ota, Yukihiro; Nagai, Yuki*; Hayashi, Nobuhiko*; Machida, Masahiko

Physica C, 470(Suppl.1), p.S368 - S369, 2010/12

https://doi.org/10.1016/j.physc.2010.01.055

Times Cited Count：1 Percentile：6.48(Physics, Applied)

Journal Articles

Simulation study on the vortex penetration in the presence of the square antidot array

Nakai, Noriyuki; Machida, Masahiko

Physica C, 470(20), p.1148 - 1150, 2010/11

https://doi.org/10.1016/j.physc.2010.05.060

Times Cited Count：10 Percentile：43.49(Physics, Applied)

Journal Articles

Laser ceramic materials for subpicosecond solid-state lasers using Nd $^{3+}$ -doped mixed scandium garnets

Okada, Hajime; Tanaka, Momoko; Kiriyama, Hiromitsu; Nakai, Yoshiki*; Ochi, Yoshihiro; Sugiyama, Akira; Daido, Hiroyuki; Kimura, Toyoaki*; Yanagitani, Takagimi*; Yagi, Hideki*; et al.

Optics Letters, 35(18), p.3048 - 3050, 2010/09

https://doi.org/10.1364/OL.35.003048

Times Cited Count：13 Percentile：53.86(Optics)

Journal Articles

First-principles-based $^{pm}$ -wave modeling for iron-based superconductors; Specific heat and nuclear magnetic relaxation rate

Nakai, Noriyuki; Nakamura, Hiroki; Ota, Yukihiro; Nagai, Yuki; Hayashi, Nobuhiko*; Machida, Masahiko

Physical Review B, 82(9), p.094501_1 - 094501_5, 2010/09

https://doi.org/10.1103/PhysRevB.82.094501

Times Cited Count：4 Percentile：21.96(Materials Science, Multidisciplinary)

no abstracts in English

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

https://doi.org/10.1103/PhysRevB.81.214511

Times Cited Count：32 Percentile：75.43(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 -wave, and otherwise -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

Exact diagonalization studies on two-band minimal model for iron-based superconductors

Okumura, Masahiko; Nakai, Noriyuki; Nakamura, Hiroki; Hayashi, Nobuhiko; Yamada, Susumu; Machida, Masahiko

Physica C, 469(15-20), p.932 - 935, 2009/10

https://doi.org/10.1016/j.physc.2009.05.102

Times Cited Count：8 Percentile：36.66(Physics, Applied)

Recently the superconductor of LaFeAsO was found and it has attracted much attentions by both theorists and experimentalists. We studies the effective two-bands model proposed by Raghu et al. [arXiv:0804.1113] by parallelized density-matirx renormalization group method [S. Yamada, M. Okumura, and M. Machida, arXiv:0707.0159]. By using this method, we can evaluate strong correlation effects caused by not only on-site but also inter-band interactions. We discuss these effects on physical quantities. In addition, we plan to propose a two-bands model including inter-band interaction in the cold atom system, which related the effective model of LaFeAsO. This model enables us to study systematically effects of inter-bands interaction because many parameters can be controlled precisely in this system. We expect that this system is useful to understand some behaviors of LaFeAsO.

Journal Articles

Simulation study for the orientation of the driven vortex lattice in an amorphous superconductor

Nakai, Noriyuki; Hayashi, Nobuhiko; Machida, Masahiko

Physica C, 469(15-20), p.1106 - 1109, 2009/10

https://doi.org/10.1016/j.physc.2009.05.208

Times Cited Count：10 Percentile：42.43(Physics, Applied)

We investigate the orientation of the vortex lattice driven by an applied current by means of numerical simulations based on the time-dependent Ginzburg-Landau (TDGL) theory. A lattice order is restored by a current driving of vortices under the influence of random vortex pinnings. The orientation of the moving vortex lattice is different between the presence and the absence of vortex pinnings. In the absence of vortex pinnings, the lattice vector is parallel to the direction of vortex motion. On the other hand, in the presence of vortex pinnings and in the collective vortex flow phase, we found that the lattice vector is perpendicular to the direction of vortex motion, which is consistent with the experimental observation under intermediate magnetic fields in an amorphous superconductor.

Journal Articles

First-principle calculation for the phonon structure on iron-based superconductors

Nakamura, Hiroki; Hayashi, Nobuhiko; Nakai, Noriyuki; Machida, Masahiko

Physica C, 469(15-20), p.1024 - 1026, 2009/10

https://doi.org/10.1016/j.physc.2009.05.178

Times Cited Count：8 Percentile：33.2(Physics, Applied)

We perform first-principle phonon calculations for three typical iron-based superconductors, i.e., LaFeAsO, BaFeAs, and FeSe. Though those crystals have different structures, we find that the optical modes associated with Fevibration have almost similar characters. Moreover, we examine the pressure effect on phonons in FeSe. By increasing the external pressure, the phonon mode frequency related to Fe vibration effectively rises up and the electronic density of states at Fermi level also increases. These results may correlate to the critical temperature enhancement under high pressure.

Journal Articles

First-principle electronic structure calculations for magnetic moment in iron-based superconductors; An LSDA + negative U study

Nakamura, Hiroki; Hayashi, Nobuhiko; Nakai, Noriyuki; Okumura, Masahiko; Machida, Masahiko

Physica C, 469(15-20), p.908 - 911, 2009/10

https://doi.org/10.1016/j.physc.2009.05.096

Times Cited Count：25 Percentile：68.57(Physics, Applied)

In order to resolve a discrepancy of the magnetic moment on Fe between the experimental and calculation results, we perform first-principle electronic structure calculations for iron-based superconductors LaFeAsO $_{1-x}$ F $_{x}$ in which and by using the LSDA+ framework. Consequently, we confirm in both the mother and doped compounds that negative correction is crucial in matching the calculated magnetic moment with the observed one. A reason of the negative correction is that the Coulomb interaction on Fe orbitals is unexpectedly screened than LSDA's expectation. We discuss which type of situation emerges when the negative is a good correction in these compounds.

Journal Articles

Ginzburg-Landau simulation for a vortex around a columnar defect in a superconducting film

Nakai, Noriyuki; Hayashi, Nobuhiko; Machida, Masahiko

Journal of Physics and Chemistry of Solids, 69(12), p.3301 - 3303, 2008/12

https://doi.org/10.1016/j.jpcs.2008.06.075

Times Cited Count：1 Percentile：7.45(Chemistry, Multidisciplinary)

By means of numerical simulations based on Ginzburg-Landau theory, we study the vortex depinning from a columnar defect in a superconducting film. We evaluate the limiting thickness of the film, below which the depinning does not occur even under an application of the magnetic field perpendicular to the columnar defect. The limiting thickness is a measure of the pinning strength of the columnar defect. The dependence of this limiting thickness on the magnitude of the applied field is obtained for two types of columnar defect.

Journal Articles

Nuclear magnetic relaxation and superfluid density in Fe-pnictide superconductors; An Anisotropic -wave scenario

Nagai, Yuki*; Hayashi, Nobuhiko; Nakai, Noriyuki; Nakamura, Hiroki; Okumura, Masahiko; Machida, Masahiko

New Journal of Physics (Internet), 10(10), p.103026_1 - 103026_17, 2008/10

https://doi.org/10.1088/1367-2630/10/10/103026

Times Cited Count：76 Percentile：90.84(Physics, Multidisciplinary)

We discuss the nuclear magnetic relaxation rate and the superfluid density with the use of the effective five-band model by Kuroki et al (Phys. Rev. Lett. 101, 087004 (2008)) in Fe-based superconductors. We show that a fully gapped anisotropic s-wave superconductivity consistently explains experimental observations. In our phenomenological model, the gaps are assumed to be anisotropic on the electron-like Fermi surfaces around the M point, where the maximum of the anisotropic gap is about four times larger than the minimum.

Journal Articles

Superconducting Pb island nanostructures studied by scanning tunneling microscopy and spectroscopy

Nishio, Takahiro*; An, Toshu*; Nomura, Atsushi*; Miyachi, Kosuke*; Eguchi, Toyoaki*; Sakata, Hideaki*; Lin, S.*; Hayashi, Nobuhiko; Nakai, Noriyuki; Machida, Masahiko; et al.

Physical Review Letters, 101(16), p.167001_1 - 167001_4, 2008/10

https://doi.org/10.1103/PhysRevLett.101.167001

Times Cited Count：98 Percentile：93.22(Physics, Multidisciplinary)

Superconductivity of nanosized Pb-island structures whose radius is 0.8 to 2.5 times their coherence length was studied under magnetic fields using low-temperature scanning tunneling microscopy and spectroscopy. Spatial profiles of superconductivity were obtained by conductance measurements at zero-bias voltage. Critical magnetic fields for vortex penetration and expulsion and for superconductivity breaking were measured for each island. The critical fields depending on the lateral size of the islands and existence of the minimum lateral size for vortex formation were observed.

Journal Articles