研究者詳細 - 大橋　洋士

論文 - 大橋　洋士

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Stability of π-Phase in Atomic-Scale Superconductor/Magnet Multilayered System

Kanegae Y., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 70 （ 7 ） 2124 - 2133 2001年07月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 00319015

Atomic-scale superconductor/ferromagnet (S/F) and superconductor/antiferromagnet (S/AF) multilayered systems are investigated in order to clarify the stability of the π-phase where the phase difference of the superconducting (SC) order parameter between neighboring SC layers is π. We clarify the stable region of this phase in the phase diagram with respect to temperature and exchange field (B) in magnetic layers. In the phase diagram, we show that the π-phase appears in both the S/F and S/AF cases. In particular, although a previous work reported that the π-phase is not obtained for any reasonable value of B in the S/AF case where the band structure in the normal state of SC and magnetic layers are metallic and identical with each other, we find in this case that the stable region of the π-phase is as wide as that in the S/F case under a certain condition. We also investigate how the π-phase is affected by detailed band structures and the symmetry of SC order parameter, and discuss the possibility of this phase in the layered magnetic superconductor, RuSr2GdCu2O8.

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Local Enhancement of Spin Susceptibility around a Nonmagnetic Impurity in the Normal State of High-Tc Cuprate Superconductors

Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 70 （ 7 ） 2054 - 2060 2001年07月

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00319015

We investigate local characters of spin susceptibility around a nonmagnetic impurity in the normal state of high-Tc cuprate superconductors. In a model two-dimensional system with strong antiferromagnetic (AF) spin fluctuations and with an extended impurity potential which is finite not only at the impurity site but also around it, we find that the AF susceptibility and the uniform one are strongly enhanced around the impurity. In particular, the uniform susceptibility which is almost temperature-independent in the clean system remarkably increases with decreasing temperature. We also show that the local enhancement of the susceptibility is due to (1) an increase of the local density of states at the Fermi level near the impurity site, and (2) a mode-mode coupling effect which is caused by the impurity potential. Our results can explain the recent Knight shift measurements in YBa2(Cu1-xZnx)3O6+y and YBa2(Cu1-xLix)3O6+y showing a Curie-Weiss temperature dependence of the uniform susceptibility at (around) the impurities, and also agree with the recent experiments on the nuclear spin-lattice relaxation rate in Zn-doped YBa2Cu3O6+y and YBa2Cu4O8 which observed anomalous enhancement of the low-energy AF spin fluctuations.

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Possible mechanism of Semiconductive temperature dependence of c-axis resistivity in the normal state of high-Tc superconductors: Renormalization effect of fermi surface by antiferromagnetic spin fluctuations

Terao M., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 70 （ 1 ） 233 - 240 2001年01月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00319015

We propose a mechanism of the semiconductive temperature dependence of the c-axis resistivity (ρc) in the normal state of high-Tc superconductors. We show that antiferromagnetic spin fluctuations in the CuO2-layer affect the temperature dependence of the Fermi surface shape and that they decrease (increase) the conductive carrier density (band mass) in the c-direction. Thus, when this effect dominates the temperature dependence of ρc, ρc increases at low temperatures. On the other hand, in the in-plane direction, the AF spin fluctuations do not affect the carrier density while they decrease the so-called k-mass at low temperatures. As a consequence, taking into account the damping rate which is caused by the electron-electron interaction, we obtain the in-plane resistivity (ρab) which decreases with decreasing temperature. Thus, the present mechanism can explain the semiconductive temperature dependence of ρc satisfying the metallic behavior of ρab.

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Nonmagnetic impurity effects on antiferromagnetic spin fluctuations in high-Tc superconductivity

大橋洋士

Physica C: Superconductivity and its Applications （Physica C: Superconductivity and its Applications） 357-360 （ SUPPL. 2 ） 74-77 - 77 2001年

研究論文（国際会議プロシーディングス）, 単著, 筆頭著者, 査読有り, ISSN 09214534

We investigate nonmagnetic impurity effects on antiferromagnetic (AF) spin fluctuations in dx2-y2-wave superconductivity. We show that, far below the transition temperature (Tc), the low-energy AF spin fluctuations are enhanced by impurity scatterings in the unitarity limit owing to the formation of low-energy bound states around the impurities. As a result, the nuclear-spin lattice relaxation rate on Cu (1/(T1T)Cu) increases at low temperatures even below Tc. On the other hand, the enhancement of the AF spin fluctuations does not occur in the case of the Born scattering because of the absent of the bound states. The result in the unitarity limit can explain the anomalous increase of 1/(T1T)Cuwhich was recently observed in Zn-doped YBa2Cu4O8. © 2001 Elsevier Science B.V. All rights reserved.

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Nonmagnetic impurity effects on spectrum of antiferromagnetic spin fluctuations and nuclear spin-lattice relaxation rate in high-Tc superconductivity

Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 69 （ 9 ） 2977 - 2992 2000年09月

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り, ISSN 00319015

We theoretically investigate antiferromagnetic (AF) spin fluctuations in impurity-doped high-Tcsuperconductivity. We assume a dx2-y2-wave superconductor with strong AF spin fluctuations, and take into account nonmagnetic impurities within the t-matrix approximation. In the clean system, the imaginary part of the AF spin susceptibility (Imχ(Q,ω)) is suppressed far below the excitation gap Eg= 2 × Max[Δp] (Δp: superconducting order parameter), while it has a peak slightly below Eg. On the other hand, in the presence of unitarity scatterers, it is shown that Imχ(Q,ω) far below Egincreases with decreasing temperature even in the superconducting state. As a result, at low temperatures, Imχ(Q,ω) has two peaks far and slightly below Egas observed in Zn-doped YBa2Cu3O6.97. In addition, because of the enhancement of the low-energy AF spin fluctuations, the nuclear spin-lattice relaxation rate on Cu ((1/T1T)Cu) also increases at low temperatures in contrast to the case of the clean superconductivity. This result can explain the recent experiment on (1/T1T)Cuin Zn-doped YBa2Cu4O8.

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Theory of Basov correlation between penetration depth and conductivity along the c-axis in high-Tc cuprate superconductors

Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 69 （ 3 ） 659 - 662 2000年03月

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り, ISSN 00319015

We present a theoretical explanation for the correlation between the c-axis penetration depth and the c-axis conductivity (σc) in high-Tccuprates which was experimentally discovered by Basov et al. [Phys. Rev. B 50 (1994) 3511]. Our model is based on the BCS theory for the quasi-two-dimensional d-wave superconductivity in which the strength of the incoherency in the c-axis charge dynamics is controlled by potential scatterings. We show that the Basov correlation is realized when the difference in the magnitude of σcamong various high-Tc cuprate materials is dominated not by the damping of electrons but by the carrier density and/or the band mass along the c-axis. It is also shown that there exists a correlation between the c-axis Josephson plasma frequency and σcin high-Tccuprates.

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Temperature dependence of c-axis Josephson plasma oscillation in high superconductors

Ohashi Y., Takada S.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics） 61 （ 6 ） 4276 - 4288 2000年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 10980121

We present a microscopic theory for the c-axis transverse Josephson plasma oscillation in quasi-two-dimensional d-wave superconductors. In our model, the incoherent motion of electrons along the c axis are introduced by potential scattering with arbitrary scattering strength, and their depairing effect on superconductivity is determined self-consistently. We show that the plasma frequency decreases with increasing temperature in the presence of the scattering. In particular, when the scattering strength exceeds a critical value, it vanishes at the transition temperature (Formula presented) as is observed in (Formula presented) In this case, when the scattering is in the Born limit, the V-shaped superconducting density of states (DOS) can be obtained in spite of the impure system. This result can explain why the V-shaped DOS is observed in this material by tunneling experiment although the temperature-dependent plasma indicates the presence of a scattering. Our model also produces a good fit to experimental data on (Formula presented) From the fit, we find that both the superfluid and the normal electrons contribute to the plasma oscillation in this material. This feature is different from the ordinary Josephson plasma, which is the oscillation of only the superfluid component. © 2000 The American Physical Society.

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Collective phase oscillation in two-dimensional d-wave superconductors

Ohashi Y., Takada S.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics） 62 （ 9 ） 5971-5983 - 5983 2000年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 01631829

We theoretically investigate the Carlson-Goldman (CG) mode in two-dimensional dx2-y2-wave superconductivity. In conventional s-wave superconductors, it is known that the CG mode is observed as a peak in the structure function of the pair field susceptibility S(q,ω) only just below the transition temperature Tc and only in dirty systems. On the other hand, in the d-wave state, we show that the peak structure can be observed down to T≁0.2Tc in clean systems. Furthermore, we also show that the peak splits into two when the direction of the momentum q deviates from the qx or qy axes, or nodal directions of the order parameter. These features originate from a singularity in the charge-density fluctuation due to the presence of the nodes of the d-wave order parameter: It enhances the screening of the long-range Coulomb interaction so that the CG mode can exist far below Tc. We also investigate a quasi-two-dimensional system, and show that the Josephson plasma can be observed in S(q,ω) when q is parallel to the c direction.

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Numerical renormalization group studies on single impurity Anderson model in superconductivity: A unified treatment of magnetic, nonmagnetic impurities, and resonance scattering

Yoshioka T., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 69 （ 6 ） 1812-1823 - 1823 2000年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 00319015

We investigate a single impurity Anderson model in superconducting state focusing on the localized excited states induced around the impurity site with use of the numerical renormalization group method. Using this model, we present a unified treatment of electronic states around various kinds of impurities embedded in conventional s-wave superconductivity that have been investigated independently so far: (1) magnetic impurity, (2) nonmagnetic impurity with potential scattering, and (3) nonmagnetic impurity with resonance scattering. In the case of the symmetric Anderson model, we clarify how the localized excited state induced around (1) is transformed into the one induced around (3) with a decrease of the strength of the on-site Coulomb repulsion at the impurity site. On the other hand, in the case of the asymmetric Anderson model, we show how the bound state induced by the magnetic impurity disappears when the impurity is gradually transformed from (1) into (2) with an increase of the energy of the impurity level. We also clarify the region where competition between the Kondo effect and superconductivity dominates the properties of the ground and the localized excited states in the parameter space of the Anderson model. In this region, we show that the bound state energy is scaled well by the single parameter TK/Δ (TK: Kondo temperature, Δ: superconducting order parameter) as in the case of the antiferromagnetic s-d interaction in superconductivity.

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Local antiferromagnetism in high-Tc cuprate superconductors and the magnetic phases of U-based heavy-fermion compounds

Ohashi Y.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics） 60 （ 22 ） 15388 - 15393 1999年

研究論文（学術雑誌）, 単著, 筆頭著者, 査読有り, ISSN 10980121

We investigate the possibility of the local induction of the antiferromagnetism (AF) in high-(Formula presented) superconductors and U-based heavy fermion compounds. In the former case, assuming the d-wave superconductivity in the bulk system, we find that AF can locally appear around surfaces and impurities and can coexist with the d-wave superconductivity. The induced AF is enhanced near the surface by the presence of superconductivity. Furthermore, the zero-energy bound state near the (110) surface of (Formula presented)-wave superconductors is split by the local AF, as was observed in (Formula presented) We also show in a model that this phenomenon is mathematically equivalent to (1) the surface (Formula presented)-wave superconductivity, which has been also proposed in the high-(Formula presented) cuprates, and (2) the local AF in the unconventional spin-density wave, which has been proposed as a candidate for the magnetism in (Formula presented) In particular, the case (2) can explain the recent experiment for (Formula presented) which observes that the temperature at which the antiferromagnetic moment appears is lower than the phase-transition temperature. © 1999 The American Physical Society.

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Ikeda and ohashi reply

Ikeda H., Ohashi Y.

Physical Review Letters （Physical Review Letters） 82 （ 25 ） 1999年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 00319007

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Monte Carlo Study on the Nuclear Magnetic Ordering in Scandium Crystal: Possibility of a Ferrimagnetic State

Kanegae Y., Ohashi Y., Ikeda H.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 68 （ 7 ） 2366-2372 - 2372 1999年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 00319015

Nuclear magnetic ordering is investigated in scandium crystal by the Monte Carlo simulation. We determine the phase diagram in the ground state with respect to interactions between the nuclear spins. In this phase diagram, we find a ferrimagnetic state with the total magnetization being smaller than the complete ferromagnetism by 1/3: This state changes into the ferromagnetism at a finite external magnetic field, which leads to a threefold jump in the magnetization as was observed by the experiment. Thus, this state is a strong candidate for the nuclear magnetic ordering in scandium below T ≲ 2[μK].

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Effect of the BCS supercurrent on the spontaneous surface flow in unconventional superconductivity with broken time-reversal-symmetry

Kusama Y., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 68 （ 3 ） 987-993 - 993 1999年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 00319015

A spontaneous surface current state is investigated in unconventional superconductivity with broken time-reversal-symmetry. It is shown that a BCS supercurrent lowers the energy of the system in this state. The lowest state has zero total current; it satisfies the Bloch theorem which states absence of the total current in any stable state with no external field. We also show that the magnitude and the direction of the spontaneous current are strongly affected by the detail of the Fermi surface. We explain this effect in connection with a low-energy bound state near the surface originating from the anisotropy of the order parameter.

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Theoretical studies on the observation of the low-frequency plasma oscillation and its coupling with the carlson-goldman mode in dirty quasi-two-dimensional superconductors

Ohashi Y., Takada S.

Physical Review B - Condensed Matter and Materials Physics （Physical Review B - Condensed Matter and Materials Physics） 59 （ 6 ） 4404-441 - 4411 1998年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 10980121

The plasma oscillation in a superconductor is investigated for the case when the plasma energy is smaller than the superconducting gap. We show that the plasma energy has a temperature dependence in dirty superconductors. When the strength of the impurity scattering exceeds a critical value, the plasma disappears at the transition temperature (Formula presented) as observed in high-(Formula presented) cuprates (Formula presented) We also study the coupling of the plasma with the Carlson-Goldman (CG) mode in a dirty superconductor. A pole analysis show that the coupling occurs just below (Formula presented) However, even in this situation, we find that two different modelike peak structures can be still observed in the spectra of the density-density correlation function (Formula presented) and the structure function of the pair field susceptibility (Formula presented) The peak in (Formula presented) is located at a finite energy at (Formula presented) corresponding to the low-frequency plasma, while the CG mode with a linear dispersion is found in (Formula presented) © 1999 The American Physical Society.

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On the Plasma Oscillation in Superconductivity

Ohashi Y., Takada S.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 67 （ 2 ） 551-559 - 559 1998年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00319015

The plasma mode in the superconducting state is studied within the random phase approximation. It is proved that the plasma oscillation in the limit of zero momentum (g) is not affected by the superconductivity. This statement itself has been obtained so far under some special conditions where energies of the plasma frequency and the superconducting order parameter are much different. In this paper, we clarify that it holds true at all temperatures irrespective of (1) the magnitude of the plasma frequency, (2) the symmetry of the order parameter, and (3) details of the system, e.g., dimensionality and the layered structure. We also show that the plasma sum-rule is satisfied even in the superconducting state, which states that the spectrum of the density-density correlation function in the limit q → 0 is dominated by the plasma mode only Although the phase fluctuation of the order parameter couples with the charge one, the collective phase oscillation, so-called the Carlson-Goldman mode, does not contribute to the spectrum of the density-density correlation function in this limit.

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Ground State Properties and Localized Excited States around a Magnetic Impurity Described by the Anisotropic s-d Interaction in Superconductivity

Yoshioka T., Ohashi Y.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 67 （ 4 ） 1332-1341 - 1341 1998年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 00319015

We investigate the electronic state around a magnetic impurity in the superconductivity in order to clarify how the anisotropy of the s-d interaction works in the presence of the superconducting energy gap. Using the numerical renormalization group method, we obtain regions induced by the anisotropy where two localized excited states with different energies appear at the same time; they cannot obtain as far as the isotropic interaction is considered. This means that the anisotropy of the s-d interaction works relevantly in some cases in the superconducting state. We also examine whether or not the bound state energy for the anisotropic and antiferromagnetic s-d interaction is scaled by TK/Δ (TK: Kondo temperature. Δ: superconducting order parameter), and find that it does not hold in the regions with two bound states.

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Theory of unconventional spin density wave: A possible mechanism of the micromagnetism in u-based heavy fermion compounds

Ikeda H., Ohashi Y.

Physical Review Letters （Physical Review Letters） 81 （ 17 ） 3723-3726 - 3726 1998年

研究論文（学術雑誌）, 共著, 最終著者, 査読有り, ISSN 00319007

We propose a novel spin density wave (SDW) state as a possible mechanism of the anomalous antiferromagnetism, the so called micromagnetism, in URu2Si2 below 17.5 K. In this new SDW, the electron-hole pair amplitude changes its sign in the momentum space as in the case of the unconventional superconductivity. It is shown that this state can be realized in an extended Hubbard model within the mean field theory. We also examine some characteristic properties of this SDW to compare with the experimental results. All these properties well explain the unsolved problem of the micromagnetism. © 1998 The American Physical Society.

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Quantum diffusion of the positive muon in superconducting tantalum

Kadono D., Kiefl R., Chakhalian J., Dunsiger S., Hitti B., MacFarlane W., Major J., Schimmele L., Matsumoto M., Ohashi Y.

Physical Review Letters （Physical Review Letters） 79 （ 1 ） 107-110 - 110 1997年

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00319007

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Goldstone Mode in Charged Superconductivity: Theoretical Studies of the Carlson-Goldman Mode and Effects of the Landau Damping in the Superconducting State

Ohashi Y., Takada S.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 66 （ 8 ） 2437-2458 - 2458 1997年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00319015

We investigate the collective phase oscillations in superconductivity, i.e., the Carlson-Goldman mode in the charged system and the phason in the neutral, focusing on their velocities and how they appear in the structure function of the phase fluctuation of the order parameter. Our microscopic theory can cover from the clean to the dirty system, and furthermore it can treat the Landau damping correctly. Based on our formulation, we obtain following results: (1) The velocity of the phason is proportional to (TC- T)1/6(T ≲ TC) in the clean system, though it has been believed as (TC- T)1/4(TC: superconducting transition temperature). (2) The structure function has a central peak which grows as T → TC. This phenomenon is related to the central peak problem in the field of the structural phase transition. (3) The Carlson-Goldman mode can be observed by a tunneling experiment in case of the dirty system, while not in the clean. It is in contrast to the previous prediction. All of these results are found to be deeply related to the presence of the Landau damping as well as the screening effect, both of which are caused by the quasi-particles excited thermally.

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Unified Description of the Mid Gap States in Inhomogeneous Superconductors

Ohashi Y., Takada S.

Journal of the Physical Society of Japan （Journal of the Physical Society of Japan） 65 （ 1 ） 246 - 254 1996年

研究論文（学術雑誌）, 共著, 筆頭著者, 査読有り, ISSN 00319015

Condition for the mid gap state to exist in inhomogeneous superconductor is investigated. On the basis of the Andreev equations, a general expression of the wavefunction of this bound state is derived. Then, the mid gap state is shown to exist only if signs of the pair potential is different, which an quasiparticle feels in the initial and final states in the scattering process described by the Andreev equations. It is also studied why some inhomogeneous systems have no mid gap state. Our results present a unified picture that the absence of the mid gap states results from their hybridization due to the spatial variation of the order parameter and the presence of the normal reflection.

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