Seiichiro Onari

Nagoya University, Nagoya, Aichi, Japan

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Publications (98)226.53 Total impact

  • Seiichiro Onari, Youichi Yamakawa, Hiroshi Kontani
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    ABSTRACT: The isostructural transition in the tetragonal phase with a sizable change in the anion height, is realized in heavily H-doped LaFeAsO and (La,P) codoped CaFe_{2}As_{2}. In these compounds, the superconductivity with higher T_{c} (40-50 K) is realized near the isostructural transition. To find the origin of the anion-height instability and the role in realizing the higher-T_{c} state, we develop the orbital-spin fluctuation theory by including the vertex correction. We analyze LaFeAsO_{1-x}H_{x} and find that the non-nematic orbital fluctuations, which induce the anion-height instability, are automatically obtained at x∼0.5, in addition to the conventional nematic orbital fluctuations at x∼0. The non-nematic orbital order triggers the isostructural transition, and its fluctuation would be a key ingredient to realize higher-T_{c} superconductivity of order 50 K.
    Physical Review Letters 05/2014; 112(18):187001. · 7.73 Impact Factor
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    ABSTRACT: We study the mechanism of the triplet superconductivity in Sr2RuO4 based on the multiorbital Hubbard model. The electronic states are studied using the renormalization group method. Thanks to the vertex correction (VC) for the susceptibility, which is dropped in the mean-field-level approximations, strong orbital and spin fluctuations at $Q=(2\pi/3,2\pi/3)$ emerge in the quasi one-dimensional Fermi surfaces composed of $d_{xz}$ and $d_{yz}$ orbitals. Due to the cooperation of both fluctuations, we obtain the triplet superconductivity in the $E_u$ representation, in which the superconducting gap is given by the linear combination of $(\Delta_x(k),\Delta_y(k))=(\sin 3k_x,\sin 3k_y)$. These results are confirmed by a diagrammatic calculation called the self-consistent VC method.
    05/2014;
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    ABSTRACT: The absence of nesting between electron and hole-pockets in LiFeAs with $T_c=18$K attracts great attention, as an important hint to understand the pairing mechanism of Fe-based superconductors. Here, we study the five-orbital model of LiFeAs based on the recently-developed orbital-spin fluctuation theories. It is found that the experimentally observed gap structure of LiFeAs is quantitatively reproduced in terms of the orbital-fluctuation-mediated $s_{++}$-wave state without sign-reversal. Especially, the largest gap observed on the small two hole-pockets composed of ($d_{xz}, d_{yz}$) orbitals can be explained, and this is a hallmark of the orbital-fluctuation-mediated superconductivity. The $s_{++}$-wave gap structure becomes more anisotropic in the presence of weak spin fluctuations. As the spin fluctuations increase, we obtain the ``$s_\pm^{h}$-wave state'', in which only the gap of the large hole-pocket made of $d_{xy}$-orbital is sign-reversed, due to the cooperation of orbital and spin fluctuations. This gap structure with ``sign-reversal between hole-pockets'' is similar to that recently reported in (Ba,K)Fe$_2$As$_2$.
    02/2014;
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    ABSTRACT: We report peculiar momentum-dependent anisotropy in the superconducting gap observed by angle-resolved photoemission spectroscopy in BaFe2(As1-xPx)2 (x = 0.30, Tc = 30 K). Strongly anisotropic gap has been found only in the electron Fermi surface while the gap on the entire hole Fermi surfaces are nearly isotropic. These results are inconsistent with horizontal nodes but are consistent with modified s± gap with nodal loops. We have shown that the complicated gap modulation can be theoretically reproduced by considering both spin and orbital fluctuations.
    Scientific reports. 01/2014; 4:7292.
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    Seiichiro Onari, Youichi Yamakawa, Hiroshi Kontani
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    ABSTRACT: The isostructural transition in the tetragonal ($C_4$) phase, with sizable change in the As-height, is realized in heavily H-doped LaFeAsO, Pr-doped CaFe$_2$As$_2$, and Na-doped BaFe$_2$As$_2$. Here, we explain the overall phase diagram of LaFeAsO$_{1-x}$H$_x$ by considering the vertex correction (VC) due to spin fluctuations. In heavily-doped case ($x\sim0.5$), the non-nematic orbital order is caused by the VC due to $d_{xy}$-orbital spin fluctuations, and triggers the $C_4$ isostructural transition. In lightly-doped case ($x\sim0$), the orthorhombic phase is realized by the orbital-nematic order, which originates from the VC due to ($d_{xz}$, $d_{yz}$)-orbital spin fluctuations. The non-nematic orbital fluctuations that couple to the As-height change would be essential for the second-Tc dome in LaFeAsO$_{1-x}$H$_x$.
    12/2013;
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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-type method, and analyze orbital susceptibilities of the (d_{xz}, d_{yz})-orbital Hubbard model with high accuracy. It is confirmed that the present model exhibits a 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.
    Physical Review Letters 08/2013; 111(5):057003. · 7.73 Impact Factor
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    ABSTRACT: To understand the amazing variety of the superconducting states of Fe-based superconductors, we analyze the multiorbital Hubbard models for LaFeAsO and LiFeAs going beyond the random-phase approximation (RPA), by calculating the vertex correction (VC) and self-energy correction. Due to the spin+orbital mode coupling described by the VC, both orbital and spin fluctuations mutually develop, consistently with the experimental phase diagram with the orbital and magnetic orders. Due to both fluctuations, the s-wave gap function with sign-reversal ($s_{\pm}$-wave), without sign-reversal ($s_{++}$-wave), and nodal s-wave states are obtained, compatible with the experimental wide variety of the gap structure. Thus, the present theory provides a microscopic explanation of the normal and superconducting phase diagram based on the realistic Hubbard model.
    07/2013;
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    ABSTRACT: To understand the recently established unique magnetic and superconducting phase diagram of LaFeAsO$_{1-x}$H$_x$, we analyze the realistic multiorbital tight-binding model for $x=0 \sim 0.4$ beyond the rigid band approximation. Both the spin and orbital susceptibilities are calculated in the presence of the Coulomb and charge quadrupole interactions. It is found that both orbital and spin fluctuations strongly develop at both $x \sim 0$ and 0.4, due to the strong violation of the rigid band picture in LaFeAsO$_{1-x}$H$_x$. Based on this result, we discuss the experimental phase diagram, especially the double-dome superconducting phase. Moreover, we show that the quadrupole interaction is effectively produced by the vertex correction due to Coulomb interaction, resulting in the mutual development of spin and orbital fluctuations.
    Physical Review B 04/2013; 88(4). · 3.66 Impact Factor
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    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.
    Physical review. B, Condensed matter 03/2013; 88(4). · 3.77 Impact Factor
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    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.
    Physical review. B, Condensed matter 03/2013; 87(19). · 3.77 Impact Factor
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    ABSTRACT: Recently, nematic electronic states had been discovered in various strongly correlated metals such as iron-based superconductors, Sr3Ru2O7 and heavy fermions. These phenomena originate from the electron-electron correlation, since the lattice distortions are very small. Interestingly, many of these materials exhibit unconventional superconductivity, suggesting that the fluctuations of the nematic order parameter would cause the superconductivity. The origin of the nematic states had been unsolved since they cannot be explained by the mean-field approximation. Here, we study this issue beyond the mean-field approximation. We calculate the vertex correction (VC) for the irreducible susceptibility in various multiorbital Hubbard models, and derive the spin and orbital fluctuations self-consistently [1,2]. Near the magnetic quantum critical point, it is found that strong ferro- and antiferro-orbital fluctuations are induced by the VC in both iron-based superconductors and Sr3Ru2O7. The divergence of the ferro-orbital fluctuations presents the orbital nematic state in these materials. [1] S. Onari and H. Kontani, Phys. Rev. Lett. 109, 137001 (2012). [2] Y. Ohno, M. Tsuchiizu, S. Onari, and H. Kontani, arXiv:1209.3629.
    03/2013;
  • Youichi Yamakawa, Seiichiro Onari, Hiroshi Kontani
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    ABSTRACT: Recently, the phase diagram of LaFeAsO1-xHx is reported and two-dome structure of superconducting state, first dome for x<0.2 with Tc^max=29K and second dome for 0.2< x<0.5 with Tc^max=36K, has attract great attention[1]. To clarify the origin of the second superconducting dome, we construct tight-binding models for each doping level x and investigate the spin and orbital fluctuations based on the random phase approximation. We fined that the nesting between electron-hole Fermi surfaces is monotonically weakened with x and spin density wave order with momentum q=(,) disappears. In the over-doped regime for x>0.2, however, the nesting between electron-electron Fermi surfaces increases, and an incommensurate spin density wave order emerges. The orbital order also shows a re-entrant phase diagram. The spin and orbital fluctuations due to the incommensurate nesting would then be the origin of the second superconducting dome reported in the H-over-doped LaFeAsO. The obtained electronic states for x=0.5 are very similar to that for KFe2Se2[2], which is a heavily electron doped system(0.5 electron/Fe). [1] S. Iimura, et al., Nat. Commumn. 3, 943 (2012). [2] T. Saito, et al., Phys. Rev. B 83, 140512 (2011).
    03/2013;
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    ABSTRACT: Motivated by the nematic electronic fluid phase in Sr3Ru2O7, we analyze the (dxz, dyz)-orbital Hubbard model by the one-loop renormalization-group method [1]. We find that, in the weak-interaction case, the q=0 component of the orbital susceptibility \qcirc(q) is critically enhanced by the Aslamazov-Larkin (AL) type vertex correction due to the superconducting fluctuations. In the strong-interaction case, we also find the development of \qcirc(q) driven by the AL-type vertex correction due to spin fluctuations, consistently with the perturbation analysis [2]. Thus the strong orbital nematic fluctuation, i.e., orbital Pomeranchuk instability, emerges near the magnetic or superconducting quantum criticality. This mechanism of orbital nematic order presents a natural explanation for the nematic order in Sr3Ru2O7, and is expected to be realized in various multiorbital systems, such as Fe-based superconductors [3]. [1] M. Tsuchiizu, S. Onari, and H. Kontani, arXiv:1209.3664. [2] Y. Ohno, M. Tsuchiizu, S. Onari, and H. Kontani, arXiv:1209.3629. [3] S. Onari and H. Kontani, Phys. Rev. Lett. 109, 137001 (2012).
    03/2013;
  • Tetusro Saito, Seiichiro Onari, Hiroshi Kontani
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    ABSTRACT: The mechanism and symmetry of the superconducting (SC) gap in Fe-based superconductors have been studied actively, and both the spin fluctuation-mediated s±-wave SC state and orbital fluctuation-mediated s++-wave SC state had been proposed. To obtain important information on the pairing mechanism, we analyze the Eliashberg gap equation using the 3-dimensional 10-orbital model. When we perform the RPA by considering only the Coulomb interaction, only the spin fluctuations develop, and the SC gap of z^2-orbital dominant part on the hole pockets is almost zero. The resultant horizontal node is inconsistent with several measurements. However, the orbital fluctuations develop by introducing the quadrupole interaction g (due to the vertex correction) and it is found that (i) the horizontal node disappears and (ii) the crossover from s±-state to s++-state is realized. During the crossover, we obtained the loop-node structures on the electron pockets, which are actually observed by ARPES measurements in BaFe2(As,P)2. We expect that optimally doped BaFe2(As,P)2 is in the crossover regime between s++-state and s±-state.
    03/2013;
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    ABSTRACT: In the iron pnictide superconductors, two distinct unconventional mechanisms of superconductivity have been put forth: One is mediated by spin fluctuations leading to the s+- state with sign change of superconducting gap between the hole and electron bands, and the other is orbital fluctuations which favor the s++ state without sign reversal. Here we report direct observation of peculiar momentum-dependent anisotropy in the superconducting gap from angle-resolved photoemission spectroscopy (ARPES) in BaFe2(As1-xPx)2 (Tc=30 K). The large anisotropy found only in the electron Fermi surface (FS) and the nearly isotropic gap on the entire hole FSs are together consistent with modified s+- gap with nodal loops, which can be theoretically reproduced by considering both spin and orbital fluctuations whose competition generates the gap modulation. This indicates that these two fluctuations are nearly equally important to the high-Tc superconductivity in this system.
    01/2013;
  • Koh Saitoh, Kota Momonoi, Nobuo Tanaka, Seiichiro Onari
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    ABSTRACT: The symmetry of the p-hole state of a superconductor of MgB2 was studied by the observation of inelastic scattering anisotropy of fast electrons accompanied by boron K-shell excitation. A series of the energy-selected diffraction patterns taken at successive energy losses were decomposed into two components; the σ and π components, respectively. The magnitudes of the σ and π components as a function of energy loss, or partial electron energy-loss spectroscopy spectra, show a good correspondence to the partial density of states obtained by a theoretical calculation. A significant occupation of the σ state at just above the Fermi level, which is considered to play an important role for the formation of the superconductive state, was confirmed.
    Journal of Applied Physics 12/2012; 112(11). · 2.21 Impact Factor
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    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 obtain the strong developments of charge quadrupole susceptibilities driven by the in-plane and out-of-plane oscillations of Ni ions. The largest susceptibility is either O_{X^2-Y^2}-quadrupole susceptibility at q = (pi, 0, pi) or O_{XZ(YZ)}-quadrupole susceptibility at q = (pi, pi, pi), depending on the level splitting between d_{X^2-Y^2} and d_{XZ(YZ)}. These antiferro-quadrupole fluctuations would then be the origin of the strong coupling superconductivity in Ni-based superconductors. Also, we propose that the antiferro-quadrupole O_{X^2-Y^2} order with q = (pi, 0, pi) is the origin of the zigzag chain structure reported in experiments. We identify similarities and differences between Ni- and Fe-based superconductors.
    Journal of the Physical Society of Japan 11/2012; 82(9). · 2.09 Impact Factor
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    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. · 7.73 Impact Factor
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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.
    Journal of the Physical Society of Japan 09/2012; 82(1). · 2.09 Impact Factor
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    K. Shigeta, S. Onari, Y. Tanaka
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    ABSTRACT: In order to study the pairing symmetry in non-centrosymmetric superconductors, we solve the linearized Eliashberg's equation on the two-dimensional extended Hubbard model in the presence of the Rashba-type spin-orbit coupling (RSOC) within the random phase approximation. In the presence of the RSOC, three types of pairing symmetries appear in the phase diagram with respect to the on-site Coulomb repulsion U and off-site one V. Each of pairing symmetries is admixture of spin-singlet and -triplet ones. On the basis of analytical study, it is found that the admixture of spin-singlet and -triplet components depends on not only the predominant pairing symmetry but also dispersion relation and pairing interaction.
    Journal of the Physical Society of Japan 08/2012; 82(1). · 2.09 Impact Factor

Publication Stats

2k Citations
226.53 Total Impact Points

Institutions

  • 2005–2014
    • Nagoya University
      • Department of Quantum Engineering
      Nagoya, Aichi, Japan
  • 2005–2008
    • The University of Electro-Communications
      • Department of Applied Physics and Chemistry
      Tokyo, Tokyo-to, Japan
  • 2007
    • Tohoku University
      • Department of Physics
      Sendai, Kagoshima-ken, Japan
  • 2002–2005
    • The University of Tokyo
      • Department of Physics
      Edo, Tōkyō, Japan