Publications (97)226.35 Total impact
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ABSTRACT: The isostructural transition in the tetragonal phase with a sizable change in the anion height, is realized in heavily Hdoped LaFeAsO and (La,P) codoped CaFe_{2}As_{2}. In these compounds, the superconductivity with higher T_{c} (4050 K) is realized near the isostructural transition. To find the origin of the anionheight instability and the role in realizing the higherT_{c} state, we develop the orbitalspin fluctuation theory by including the vertex correction. We analyze LaFeAsO_{1x}H_{x} and find that the nonnematic orbital fluctuations, which induce the anionheight instability, are automatically obtained at x∼0.5, in addition to the conventional nematic orbital fluctuations at x∼0. The nonnematic orbital order triggers the isostructural transition, and its fluctuation would be a key ingredient to realize higherT_{c} superconductivity of order 50 K.Physical Review Letters 05/2014; 112(18):187001. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
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 meanfieldlevel approximations, strong orbital and spin fluctuations at $Q=(2\pi/3,2\pi/3)$ emerge in the quasi onedimensional 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 selfconsistent VC method.05/2014;  [Show abstract] [Hide abstract]
ABSTRACT: The absence of nesting between electron and holepockets in LiFeAs with $T_c=18$K attracts great attention, as an important hint to understand the pairing mechanism of Febased superconductors. Here, we study the fiveorbital model of LiFeAs based on the recentlydeveloped orbitalspin fluctuation theories. It is found that the experimentally observed gap structure of LiFeAs is quantitatively reproduced in terms of the orbitalfluctuationmediated $s_{++}$wave state without signreversal. Especially, the largest gap observed on the small two holepockets composed of ($d_{xz}, d_{yz}$) orbitals can be explained, and this is a hallmark of the orbitalfluctuationmediated 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 holepocket made of $d_{xy}$orbital is signreversed, due to the cooperation of orbital and spin fluctuations. This gap structure with ``signreversal between holepockets'' is similar to that recently reported in (Ba,K)Fe$_2$As$_2$.02/2014;  [Show abstract] [Hide abstract]
ABSTRACT: The isostructural transition in the tetragonal ($C_4$) phase, with sizable change in the Asheight, is realized in heavily Hdoped LaFeAsO, Prdoped CaFe$_2$As$_2$, and Nadoped BaFe$_2$As$_2$. Here, we explain the overall phase diagram of LaFeAsO$_{1x}$H$_x$ by considering the vertex correction (VC) due to spin fluctuations. In heavilydoped case ($x\sim0.5$), the nonnematic orbital order is caused by the VC due to $d_{xy}$orbital spin fluctuations, and triggers the $C_4$ isostructural transition. In lightlydoped case ($x\sim0$), the orthorhombic phase is realized by the orbitalnematic order, which originates from the VC due to ($d_{xz}$, $d_{yz}$)orbital spin fluctuations. The nonnematic orbital fluctuations that couple to the Asheight change would be essential for the secondTc dome in LaFeAsO$_{1x}$H$_x$.12/2013;  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by the nematic electronic fluid phase in Sr_{3}Ru_{2}O_{7}, we develop a combined scheme of the renormalizationgroup method and the randomphaseapproximationtype 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 ferroorbital instability near the magnetic or superconducting quantum criticality, due to the AslamazovLarkintype 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 Febased superconductors.Physical Review Letters 08/2013; 111(5):057003. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: To understand the amazing variety of the superconducting states of Febased superconductors, we analyze the multiorbital Hubbard models for LaFeAsO and LiFeAs going beyond the randomphase approximation (RPA), by calculating the vertex correction (VC) and selfenergy 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 swave gap function with signreversal ($s_{\pm}$wave), without signreversal ($s_{++}$wave), and nodal swave 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;  [Show abstract] [Hide abstract]
ABSTRACT: To understand the recently established unique magnetic and superconducting phase diagram of LaFeAsO$_{1x}$H$_x$, we analyze the realistic multiorbital tightbinding 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$_{1x}$H$_x$. Based on this result, we discuss the experimental phase diagram, especially the doubledome 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  [Show abstract] [Hide abstract]
ABSTRACT: To understand the origin of the nodal gap structure realized in BaFe$_2$(As,P)$_2$, we study the threedimensional gap structure based on the threedimensional tenorbital Hubbard model with quadrupole interaction. In this model, strong spin and orbital fluctuations develop by using the randomphaseapproximation. By solving the Eliashberg gap equation, we obtain the fullygapped swave state with (without) sign reversal between holelike and electronlike 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 swave state in the crossover region between $s_{++}$wave and $s_\pm$wave states. The obtained nodal swave state possesses the loopshape nodes on electronlike Fermi surfaces, due to the competition between attractive and repulsive interactions in kspace. In contrast, the SC gaps on the holelike Fermi surfaces are fullygapped 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  [Show abstract] [Hide abstract]
ABSTRACT: We present a systematic study of the impurity effect on Tc in Febased superconductors, assuming that the signreversal swave state due to interpocket repulsion ($s_\pm$wave state) is realized. For this purpose, we introduce several realistic impurity models with nonlocal modifications of potentials and hopping integrals around the impurity site. When we use the impurity model parameters for 3d and 4dimpurity 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 massenhancement factor), consistently with the previous theoretical study for the onsite impurity model by Onari and Kontani. This result is essentially unchanged for different nonlocal impurity models with realistic parameters. We also discuss the effect of the impurityinduced nonlocal orbital order on the superconducting state.Physical review. B, Condensed matter 03/2013; 87(19). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Recently, nematic electronic states had been discovered in various strongly correlated metals such as ironbased superconductors, Sr3Ru2O7 and heavy fermions. These phenomena originate from the electronelectron 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 meanfield approximation. Here, we study this issue beyond the meanfield approximation. We calculate the vertex correction (VC) for the irreducible susceptibility in various multiorbital Hubbard models, and derive the spin and orbital fluctuations selfconsistently [1,2]. Near the magnetic quantum critical point, it is found that strong ferro and antiferroorbital fluctuations are induced by the VC in both ironbased superconductors and Sr3Ru2O7. The divergence of the ferroorbital 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;  [Show abstract] [Hide abstract]
ABSTRACT: Recently, the phase diagram of LaFeAsO1xHx is reported and twodome 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 tightbinding 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 electronhole Fermi surfaces is monotonically weakened with x and spin density wave order with momentum q=(,) disappears. In the overdoped regime for x>0.2, however, the nesting between electronelectron Fermi surfaces increases, and an incommensurate spin density wave order emerges. The orbital order also shows a reentrant 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 Hoverdoped 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;  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by the nematic electronic fluid phase in Sr3Ru2O7, we analyze the (dxz, dyz)orbital Hubbard model by the oneloop renormalizationgroup method [1]. We find that, in the weakinteraction case, the q=0 component of the orbital susceptibility \qcirc(q) is critically enhanced by the AslamazovLarkin (AL) type vertex correction due to the superconducting fluctuations. In the stronginteraction case, we also find the development of \qcirc(q) driven by the ALtype 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 Febased 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;  [Show abstract] [Hide abstract]
ABSTRACT: The mechanism and symmetry of the superconducting (SC) gap in Febased superconductors have been studied actively, and both the spin fluctuationmediated s±wave SC state and orbital fluctuationmediated s++wave SC state had been proposed. To obtain important information on the pairing mechanism, we analyze the Eliashberg gap equation using the 3dimensional 10orbital model. When we perform the RPA by considering only the Coulomb interaction, only the spin fluctuations develop, and the SC gap of z^2orbital 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 loopnode 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;  [Show abstract] [Hide abstract]
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 momentumdependent anisotropy in the superconducting gap from angleresolved photoemission spectroscopy (ARPES) in BaFe2(As1xPx)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 highTc superconductivity in this system.01/2013;  [Show abstract] [Hide abstract]
ABSTRACT: The symmetry of the phole state of a superconductor of MgB2 was studied by the observation of inelastic scattering anisotropy of fast electrons accompanied by boron Kshell excitation. A series of the energyselected 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 energyloss 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  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the electronic state and structure transition of BaNi2As2, which shows a similar superconducting phase diagram as Febased superconductors. We construct the tenorbital tightbinding model for BaNi2As2 by using the maximally localized Wannier function method. The Coulomb and quadrupolequadrupole interactions are treated within the randomphase approximation. We obtain the strong developments of charge quadrupole susceptibilities driven by the inplane and outofplane oscillations of Ni ions. The largest susceptibility is either O_{X^2Y^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^2Y^2} and d_{XZ(YZ)}. These antiferroquadrupole fluctuations would then be the origin of the strong coupling superconductivity in Nibased superconductors. Also, we propose that the antiferroquadrupole O_{X^2Y^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 Febased superconductors.Journal of the Physical Society of Japan 11/2012; 82(9). · 2.09 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the mechanism of orbital or spin fluctuations due to multiorbital Coulomb interaction in ironbased superconductors, going beyond the randomphase approximation. For this purpose, we develop a selfconsistent 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 antiferroorbital and ferroorbital (=nematic) fluctuations simultaneously develop for J/U∼0.1, both of which contribute to the swave superconductivity. Especially, the ferroorbital 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  [Show abstract] [Hide abstract]
ABSTRACT: To reveal the origin of the "nematic electronic fluid phase" in Sr$_3$Ru$_2$O$_7$, we apply the selfconsistent vertex correction analysis to the ($d_{xz},d_{yz}$)orbital Hubbard model. It is found that the AslamazovLarkin type vertex correction causes the strong coupling between spin and orbital fluctuations, which corresponds to the KugelKhomskii spinorbital 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 randomphaseapproximation. 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 Febased superconductors.Journal of the Physical Society of Japan 09/2012; 82(1). · 2.09 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In order to study the pairing symmetry in noncentrosymmetric superconductors, we solve the linearized Eliashberg's equation on the twodimensional extended Hubbard model in the presence of the Rashbatype spinorbit 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 onsite Coulomb repulsion U and offsite one V. Each of pairing symmetries is admixture of spinsinglet and triplet ones. On the basis of analytical study, it is found that the admixture of spinsinglet 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  [Show abstract] [Hide abstract]
ABSTRACT: In order to investigate the symmetry of a superconducting (SC) pairing state in a staggered field, we solve the linearized Éliashberg's equation based on the random phase approximation and the fluctuation exchange approximation in the Hubbard model under a staggered field. We show that an oddfrequency spintriplet (equalspin) swave pairing state can be realized in the system. It is caused by the fact that the staggered field suppresses the inplane spin susceptibility and enhances the charge susceptibility. The latter is due to a spindependent modulation in the electron density. The result suggests that the antiferromagnetic (AF) background gives a contribution inherently different from the AF spin fluctuation to SC pairing state.Physical review. B, Condensed matter 06/2012; 85(22). · 3.77 Impact Factor
Publication Stats
2k  Citations  
226.35  Total Impact Points  
Top Journals
Institutions

2005–2014

Nagoya University
 Department of Quantum Engineering
Nagoya, Aichi, Japan


2005–2008

The University of ElectroCommunications
 Department of Applied Physics and Chemistry
Tokyo, Tokyoto, Japan


2007

Tohoku University
 Department of Physics
Sendai, Kagoshimaken, Japan


2002–2005

The University of Tokyo
 Department of Physics
Edo, Tōkyō, Japan
