Seiichiro Onari

Publications (54)52.65 Total impact

• Article: Nodal gap structure in Fe-based superconductors due to the competition between orbital and spin fluctuations

  Tetsuro Saito, Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: To understand the origin of the nodal gap structure realized in BaFe$_2$(As,P)$_2$, we study the three-dimensional gap structure based on the three-dimensional ten-orbital Hubbard model with quadrupole interaction. In this model, strong spin and orbital fluctuations develop by using the random-phase-approximation. By solving the Eliashberg gap equation, we obtain the fully-gapped s-wave state with (without) sign reversal between hole-like and electron-like Fermi surfaces due to strong spin (orbital) fluctuations, so called the $s_\pm$-wave ($s_{++}$-wave) state. When both spin and orbital fluctuations strongly develop, which will be realized near the orthorhombic phase, we obtain the nodal s-wave state in the crossover region between $s_{++}$-wave and $s_\pm$-wave states. The obtained nodal s-wave state possesses the loop-shape nodes on electron-like Fermi surfaces, due to the competition between attractive and repulsive interactions in k-space. In contrast, the SC gaps on the hole-like Fermi surfaces are fully-gapped due to orbital fluctuations. The present study explains the main characters of the anisotropic gap structure in BaFe$_2$(As,P)$_2$ observed experimentally.
  03/2013;
• Article: Effect of realistic finite-size impurities on Tc in Fe-based superconductors based on the five-orbital tight-binding model

  Youichi Yamakawa, Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: We present a systematic study of the impurity effect on Tc in Fe-based superconductors, assuming that the sign-reversal s-wave state due to inter-pocket repulsion ($s_\pm$-wave state) is realized. For this purpose, we introduce several realistic impurity models with non-local modifications of potentials and hopping integrals around the impurity site. When we use the impurity model parameters for 3d- and 4d-impurity atoms derived from the recent first principle study by Nakamura et al., we find that the $s_\pm$-wave state is very fragile against impurities: The superconductivity with $T_{c0}=30K$ is destroyed by introducing small residual resistivity $\rho_0^{cr} = 5z^{-1} ~ 10z^{-1} [\mu\Omega cm]$ ($z^{-1} = m^*/m$ being the mass-enhancement factor), consistently with the previous theoretical study for the on-site impurity model by Onari and Kontani. This result is essentially unchanged for different non-local impurity models with realistic parameters. We also discuss the effect of the impurity-induced non-local orbital order on the superconducting state.
  03/2013;
• Article: Zigzag chain structure transition and orbital fluctuations in Ni-based superconductors

  Youichi Yamakawa, Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: We investigate the electronic state and structure transition of BaNi2As2, which shows a similar superconducting phase diagram as Fe-based superconductors. We construct the ten-orbital tight-binding model for BaNi2As2 by using the maximally localized Wannier function method. The Coulomb and quadrupole-quadrupole interactions are treated within the random-phase approximation. We find that the charge susceptibility of quadrupole O_{X2-Y2} has the largest peak structure at q = (pi, 0, pi) which is derived from both the in-plane and out-of-plane oscillations of Ni ions. We propose that the antiferro-quadrupole O_{X2-Y2} order with q = (pi, 0, pi) is the origin of the zigzag chain structure reported in experiments. The obtained antiferro-quadrupole fluctuations would then be the origin of the strong coupling superconductivity in Ni-based superconductors. We identify similarities and differences between Ni- and Fe-based superconductors.
  11/2012;
• Article: Self-consistent Vertex Correction Analysis for Iron-based Superconductors: Mechanism of Coulomb Interaction-Driven Orbital Fluctuations.

  Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: We study the mechanism of orbital or spin fluctuations due to multiorbital Coulomb interaction in iron-based superconductors, going beyond the random-phase approximation. For this purpose, we develop a self-consistent vertex correction (VC) method, and find that multiple orbital fluctuations in addition to spin fluctuations are mutually emphasized by the "multimode interference effect" described by the VC. Then, both antiferro-orbital and ferro-orbital (=nematic) fluctuations simultaneously develop for J/U∼0.1, both of which contribute to the s-wave superconductivity. Especially, the ferro-orbital fluctuations give the orthorhombic structure transition as well as the softening of shear modulus C_{66}.
  Physical Review Letters 09/2012; 109(13):137001. · 7.37 Impact Factor
• Article: Orbital Nematic Instability in Two-Orbital Hubbard Model: Renormalization-Group + Constrained RPA Analysis

  Masahisa Tsuchiizu, Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: Motivated by the nematic electronic fluid phase in Sr_{3}Ru_{2}O_{7}, we develop a combined scheme of the renormalization-group method and the random-phase approximation (RPA), and analyze orbital susceptibilities of the (d_{xz}, d_{yz})-orbital Hubbard model with high accuracy. It is confirmed that the present model exhibits the ferro-orbital instability near the magnetic or superconducting quantum criticality, due to the Aslamazov-Larkin type vertex corrections. This mechanism of orbital nematic order presents a natural explanation for the nematic order in Sr_{3}Ru_{2}O_{7}, and is expected to be realized in various multiorbital systems, such as Fe-based superconductors.
  09/2012;
• Article: Spin-Fluctuation-Driven Orbital Nematic Order in Ru-Oxides: Self-Consistent Vertex Correction Analysis for Two-Orbital Model

  Yusuke Ohno, Masahisa Tsuchiizu, Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: To reveal the origin of the "nematic electronic fluid phase" in Sr$_3$Ru$_2$O$_7$, we apply the self-consistent vertex correction analysis to the ($d_{xz},d_{yz}$)-orbital Hubbard model. It is found that the Aslamazov-Larkin type vertex correction causes the strong coupling between spin and orbital fluctuations, which corresponds to the Kugel-Khomskii spin-orbital coupling in the local picture. Due to this mechanism, orbital nematic order with $C_2$ symmetry is induced by the magnetic quantum criticality in multiorbital systems, while this mechanism is ignored in the random-phase-approximation. The present study naturally explains the intimate relation between the magnetic quantum criticality and the nematic state in Sr$_3$Ru$_2$O$_7$ and Fe-based superconductors.
  09/2012;
• Article: Effect of inelastic scattering on the nuclear magnetic relaxation rate 1/T1T in iron-based superconductors

  Youichi Yamakawa, Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: We present a microscopic study of the nuclear magnetic relaxation rate 1/T1 based on the five-orbital model for iron-based superconductors. We mainly discuss the effect of the "inelastic" quasi-particle damping rate {\gamma} due to many-body interaction on the size of the coherence peak, for both s++ and s+- wave superconducting states. We focus on Ba(Fe1-xCox)2As2, and systematically evaluate {\gamma} in the normal state from the experimental resistivity, from optimally to over-doped compounds. Next, {\gamma} in the superconducting state is calculated microscopically based on the second order perturbation theory. In optimally doped compounds (Tc ~ 30 K), it is revealed that the coherence peak on 1/T1T is completely suppressed due to large {\gamma} for both s++ and s+- wave states. On the other hand, in heavily over doped compounds with Tc < 10 K, the coherence peak could appear for both pairing states, since {\gamma} at Tc is quickly suppressed in proportion to Tc^2. By making careful comparison between theoretical and experimental results, we conclude that it is difficult to discriminate between s++ and s+- wave states from the present experimental results.
  06/2012;
• Article: Non-Fermi-Liquid Transport Phenomena and Superconductivity Driven by Orbital Fluctuations in Iron Pnictides: Analysis by Fluctuation-Exchange Approximation

  Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: We study the five-orbital Hubbard model including the charge quadrupole interaction for iron pnictides. Using the fluctuation-exchange (FLEX) approximation, orbital fluctuations evolve inversely proportional to the temperature, and therefore the resistivity shows linear or convex T-dependence for wide range of temperatures. We also analyze the Eliashberg gap equation, and show that s-wave superconducting state without sign reversal (s++ -wave state) emerges when the orbital fluctuations dominate over the spin fluctuations. When both fluctuations are comparable, their competition gives rise to a nodal s-wave state. The present study offers us a unified explanation for both the normal and superconducting states.
  03/2012;
• Source

  Available from: ArXiv

  Article: Reply to Comment on "Neutron-Inelastic-Scattering Peak by Dissipationless Mechanism in the $s_{++}$-wave State in Iron-based Superconductors" [arXiv:1106.2376] by Y. Nagai and K. Kuroki

  Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: This article is a reply to the comment by Nagai and Kuroki [arXiv:1106.2376] for our unpublished paper in arXiv [arXiv:1105.6233]. Contrary to their claim, we certainly obtain a large hump structure at $q=(\pi,0)$ in the $s_{++}$-wave state in iron pnictides due to the ``dissipationless mechanism'', even when similar parameters used by Nagai and Kuroki are employed.
  07/2011;
• Article: Onari and Kontani Reply:

  Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: A Reply to the Comment by Yunkyu Bang.
  Physical Review Letters 06/2011; 106(25):259702. · 7.37 Impact Factor
• Source

  Available from: ArXiv

  Article: Neutron-Inelastic-Scattering Peak by Dissipationless Mechanism in the s++ -wave State in Iron-based Superconductors

  Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: We investigate the neutron scattering spectrum in iron pnictides based on the random-phase approximation in the five-orbital model with a realistic superconducting (SC) gap, Delta=5meV. In the normal state, the neutron spectrum is suppressed by large inelastic quasi-particle (QP) scattering rate gamma* ~ Delta. In the fully-gapped s-wave state without sign reversal (s++), a hump-shaped enhancement appears in the neutron spectrum just above 2Delta, since the inelastic QP scattering is prohibited by the SC gap. That is, the hump structure is produced by the dissipationless QPs for Ek<3Delta. The obtained result is more consistent with experimental spectra, compared to the results of our previous paper with Delta=50meV. On the other hand, both height and weight of the resonance peak in the fully-gapped s-wave states with sign reversal (s+-) are much larger than those observed in experiments. We conclude that experimentally observed broad spectral peak in iron pnictides is created by the present "dissipationless mechanism" in the s++ -wave state.
  05/2011;
• Source

  Available from: ArXiv

  Article: Pairing symmetry of superconductivity coexisting with antiferromagnetism

  Keisuke Shigeta, Seiichiro Onari, Yukio Tanaka
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  ABSTRACT: Pairing symmetry in the superconducting state coexisting with antiferromagnetic order is studied based on a microscopic theory. We calculate the linearized Eliashberg's equation within the random phase approximation in the Hubbard model with a staggered field. We find that odd-frequency spin-triplet (equal-spin) s-wave pairing state can be realized. This result contradicts a naive expectation that antiferromagnetic order induces antiferromagnetic spin fluctuation and favors spin-singlet d-wave pairing as in the standard strongly correlated systems.
  04/2011;
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  Available from: ArXiv

  Article: Origin of Orthorhombic Transition, Magnetic Transition, and Shear Modulus Softening in Iron Pnictide Superconductors: Analysis based on the Orbital Fluctuation Theory

  Hiroshi Kontani, Tetsuro Saito, Seiichiro Onari
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  ABSTRACT: The main features in iron-pnictide superconductors are summarized as (i) the orthorhombic transition accompanied by remarkable softening of shear modulus, (ii) high-Tc superconductivity close to the orthorhombic phase, and (iii) stripe-type magnetic order induced by orthorhombicity. To present a unified explanation for them, we analyze the multiorbital Hubbard-Holstein model with Fe-ion optical phonons based on the orbital fluctuation theory. In the random-phase-approximation (RPA), a small electron-phonon coupling constant ($\lambda ~ 0.2$) is enough to produce large orbital (=charge quadrupole) fluctuations. The most divergent susceptibility is the $O_{xz}$-antiferro-quadrupole (AFQ) susceptibility, which causes the s-wave superconductivity without sign reversal (s_{++}-wave state). At the same time, divergent development of $O_{x2-y2}$-ferro-quadrupole (FQ) susceptibility is brought by the "two-orbiton process" with respect to the AFQ fluctuations, which is absent in the RPA. The derived FQ fluctuations cause the softening of $C_{66}$ shear modulus, and its long-range-order not only triggers the orthorhombic structure transition, but also induces the instability of stripe-type antiferro-magnetic state. In other words, the condensation of composite bosons made of two orbitons gives rise to the FQ order and structure transition. The theoretically predicted multi-orbital-criticality presents a unified explanation for abovementioned features of iron pnictide superconductors.
  03/2011;
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  Available from: ArXiv

  Article: Emergence of fully gapped s_ {++}-wave and nodal d-wave states mediated by orbital and spin fluctuations in a ten-orbital model of KFe_ {2} Se_ {2}

  Tetsuro Saito, Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: We study the superconducting state in recently discovered high-Tc superconductor KxFe2Se2 based on the ten-orbital Hubbard-Holstein model without hole pockets. When the Coulomb interaction is large, a spin-fluctuation-mediated d-wave state appears due to the nesting between electron pockets. Interestingly, the symmetry of the body-centered tetragonal structure in KxFe2Se2 requires the existence of nodes in the d-wave gap, although a fully gapped d-wave state is realized in the case of a simple tetragonal structure. In the presence of moderate electron-phonon interaction due to Fe-ion optical modes, however, orbital fluctuations give rise to the fully gapped s++-wave state without sign reversal. Therefore, both superconducting states are distinguishable by careful measurements of the gap structure or the impurity effect on Tc.
  Phys. Rev. B. 02/2011; 83(14).
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  Available from: ArXiv

  Article: Comment on "Orbital Order, Structural Transition and Superconductivity in Iron Pnictides" [arXiv:1010.0129] by Yanagi et al

  Hiroshi Kontani, Seiichiro Onari
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  ABSTRACT: In contrast to the claim by Yanagi et al. in Ref. [1], the development of orbital fluctuations at $q ~ 0$ due to the "orthorhombic phonon" is unable to explain the high-Tc superconductivity in iron pnictides. The reason is that the orthorhombic-phonon is acoustic with the energy $\omega_q \propto q$, although the authors in Ref. [1] treated it as optical phonon ($\omega_o=0.02 eV$) inconsistently.
  01/2011;
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  Available from: ArXiv

  Article: Reply to Comment on "Violation of Anderson's Theorem for the sign-reversing s-wave state of Iron-Pnictide Superconductors" [arXiv:1012.0414] by Y. Bang

  Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: We explain that the study of impurity effect in iron pnictides are correctly calculated based on the T-matrix approximation, contrary to the Comment by Bang. The replacement T with T-I proposed by Bang breaks the perturbation theory and is therefore erroneous. Comment: 2 pages, 1 figure, response to arXiv:1012.0414
  12/2010;
• Article: Reply to Comment on

  Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: We explain that the study of impurity effect in iron pnictides are correctly calculated based on the T-matrix approximation, contrary to the Comment by Bang. The replacement T with T-I proposed by Bang breaks the perturbation theory and is therefore erroneous.
  11/2010;
• Source

  Available from: ArXiv

  Article: Non-Fermi-Liquid-Like Behaviors and Superconductivity Driven by Orbital Fluctuations in Iron Pnictides: Analysis by Fluctuation-Exchange Approximation

  Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: We study the five-orbital Hubbard-Holstein model for iron pnictides with small electron-phonon interaction due to Fe-ion Einstein oscillators. Using the fluctuation-exchange (FLEX) approximation, orbital fluctuations evolve inversely proportional to the temperature, and therefore the resistivity shows linear or convex T-dependence for wide range of temperatures. We also analyze the Eliashberg gap equation, and show that s-wave superconducting state without sign reversal (s_{++}-wave state) emerges when the orbital fluctuations dominate the spin fluctuations. When both fluctuations are comparable, their competition gives rise to a nodal s-wave state. The present study offers us a unified explanation for both the normal and superconducting states. Comment: 5 pages, 4 figures
  09/2010;
• Source

  Available from: ArXiv

  Article: Theory of the beta-type Organic Superconductivity under Uniaxial Compression

  Takeo Suzuki, Seiichiro Onari, Hiroshi Ito, Yukio Tanaka
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  ABSTRACT: We study theoretically the shift of the superconducting transition temperature (Tc) under uniaxial compression in beta-type organic superconductors, beta-(BEDT-TTF)2I3 and beta-(BDA-TTP)2X[X=SbF6,AsF6], in order to clarify the electron correlation, the spin frustration and the effect of dimerization. The transfer integrals are calculated by the extended Huckel method assuming the uniaxial strain and the superconducting state mediated by the spin fluctuation is solved using Eliashberg's equation with the fluctuation-exchange approximation. The calculation is carried out on both the dimerized (one-band) and nondimerized (two-band) Hubbard models. We have found that (i) the behavior of Tc in beta-(BEDT-TTF)2I3 with a stronger dimerization is well reproduced by the dimer model, while that in weakly dimerized beta-BDA-TTP salts is rather well reproduced by the two-band model, and (ii) the competition between the spin frustration and the effect induced by the fluctuation is important in these materials, which causes nonmonotonic shift of Tc against uniaxial compression.
  08/2010;
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  Available from: ArXiv

  Article: Orbital Fluctuation Theory in Iron Pnictides: Effects of As-Fe-As Bond Angle, Isotope Substitution, and $Z^2$-Orbital Pocket on the Superconductivity

  Tetsuro Saito, Seiichiro Onari, Hiroshi Kontani
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  ABSTRACT: We study the pairing mechanism in iron pnictide superconductors based on the five-orbital Hubbard-Holstein model. Due to Fe-ion oscillations, the s-wave superconducting (SC) state without sign reversal (s_{++}-wave state) is induced by orbital fluctuations by using realistic model parameters. The virtue of the present theory is that the famous empirical relation between Tc and the As-Fe-As bond angle is automatically explained, since the electron-phonon (e-ph) coupling that creates the orbital fluctuations is the strongest when the As$_4$-tetrahedron is regular. The negative iron isotope effect is also reproduced. In addition, the magnitude of the SC gap on hole-pockets is predicted to be rather insensitive to the corresponding d-orbital (xz/yz- or z^2-orbital), which is consistent with the recent bulk-sensitive angle-resolved photoemission spectroscopy (ARPES) measurement for (Ba,K)Fe$_2$As$_2$ and BaFe$_2$(As,P)$_2$. These obtained results indicate that the orbital-fluctuation-mediated s_{++}-wave state is a plausible candidate for iron pnictides. Comment: 11 pages, 10 figures, New discussions and figures had been added
  08/2010;