Publications (109)258.17 Total impact

Article: Orbital Polarization with Nontrivial MomentumDependence in FeSe and other FeBased Superconductors
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ABSTRACT: Nontrivial kdependence of the orbital polarization ($\Delta E_{xz}(k)$, $\Delta E_{yz}(k)$) in the orthorhombic phase, such as the signreversal of the orbital splitting between $\Gamma$ and Xpoints in FeSe, provides significant information to understand the nematicity in Febased superconductors. To solve this problem, we analyze the multiorbital Hubbard models in the orbitalordered state by extending the orbitalspin fluctuation theory. The present theory describes the spontaneous symmetry breaking with respect to the orbital polarization and spin susceptibility selfconsistently. In the orbitalordered state in FeSe, we obtain the two Dirac cone Fermi pockets in addition to the sign reversal of the orbital polarization, consistently with experiments. The orbitalorder in Febased superconductors originates from the strong orbitalspin interplay due to the AslamazovLarkin processes.  [Show abstract] [Hide abstract]
ABSTRACT: Nematicity and magnetism are two key features in Febased superconductors, and their interplay is one of the most important unsolved problems. In FeSe, the magnetic order is absent below the structural transition temperature $T_{str}=90$K, in stark contrast that the magnetism emerges slightly below $T_{str}$ in other families. To understand such amazing material dependence, we investigate the emergence of the nematic orbitalorder ($n_{xz} \neq n_{yz}$) based on various firstprinciples Hubbard models. In Febased superconductors, spinfluctuationmediated large orbitalfluctuations appear because of the strong orbitalspin interplay due to the manybody effect. This effect is very significant in FeSe due to the small ratio between the Hund's and Coulomb interactions ($J/U$) and large $d_{xz},d_{yz}$orbitals weight at the Fermi level. For this reason, in FeSe, orbital order is established by weak spin fluctuations, so the magnetism is absent below $T_{\rm str}$. In contrast, in LaFeAsO, the magnetic order appears just below $T_{str}$ both experimentally and theoretically. Thus, the orbitalspin interplay is the key ingredient of the wide variety of the normalstate phase diagram in Febased superconductors.  [Show abstract] [Hide abstract]
ABSTRACT: A ternary compound, MgPtSi, was synthesized by solidstate reaction. An examination of the compound by powder Xray diffraction revealed that it crystallizes in the orthorhombic TiNiSitype structure with the Pnma space group. The structure comprises alternately stacked layers of Mg and PtSi honeycomb network, which is reminiscent of MgB2, and the buckling of the honeycomb network causes orthorhombic distortion. Electrical and magnetic studies revealed that MgPtSi exhibited superconductivity with a transition temperature of 2.5 K. However, its isostructural compounds, namely, MgRhSi and MgIrSi, were not found to exhibit superconductivity.Physical Review B 05/2015; 91(17). DOI:10.1103/PhysRevB.91.174514 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Precise gap structure in LiFeAs (Tc = 18 K) given by ARPES studies offers us significant information to understand the pairing mechanism in ironbased superconductors. The most remarkable characteristics in LiFeAs gap structure would be that "the largest gap emerges on the tiny holepockets around Z point". This result had been naturally explained in terms of the orbitalfluctuation scenario (T. Saito et al., Phys. Rev. B 90, 035104 (2014)), whereas an opposite result is obtained by the spinfluctuation scenario. In this paper, we study the gap structure in LiFeAs by taking the spinorbit interaction (SOI) into account, motivated by the recent ARPES studies that revealed the significant SOIinduced modification of the Fermi surface topology. For this purpose, we construct the two possible tightbinding models with finite SOI by referring the bandstructures given by different ARPES groups. In addition, we extend the gap equation for multiorbital systems with finite SOI, and calculate the gap functions by applying the orbitalspin fluctuation theory. On the basis of both SOIinduced band structures, main characteristics of the gap structure in LiFeAs are naturally reproduced only in the presence of strong interorbital interactions between (xz/yz  xy) orbitals. Thus, the experimental gap structure in LiFeAs is a strong evidence for the orbitalfluctuation pairing mechanism.Physical Review B 04/2015; 92(13). DOI:10.1103/PhysRevB.92.134522 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the mechanism of the triplet superconductivity (TSC) in Sr2RuO4 based on the multiorbital Hubbard model. The electronic states are studied using the recently developed renormalization group method combined with the constrained randomphaseapproximation, called the RG+cRPA 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 (FSs) composed of xz and yzorbitals. 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 $(\Delta_x(k),\Delta_y(k))\sim (\sin 3k_x,\sin 3k_y)$. Very similar results are obtained by applying the diagrammatic calculation called the selfconsistent VC method. Thus, the idea of "orbital+spin fluctuation mediated TSC" is confirmed by both RG+cRPA method and the selfconsistent VC method. We also reveal that a substantial superconducting gap on the xyorbital FS is induced from the gaps on the quasi onedimensional FSs, in consequence of the large orbitalmixture due to the 4d spinorbit interaction.Physical Review B 03/2015; 91(15). DOI:10.1103/PhysRevB.91.155103 · 3.74 Impact Factor 
Article: Eilenberger theory for nuclear magnetic relaxation rate in superconducting vortex lattice state
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ABSTRACT: On the basis of the Eilenberger theory, spatial variation of the local NMR relaxation rate ${T}_{1}^{$${}1}$ is quantitatively estimated in the vortex lattice state, to clarify the differences between the $s$wave and the $d$wave superconductors. We study the temperature and the magnetic field dependencies of ${T}_{1}^{$${}1}$ inside and outside of the vortex core, including influences of nonmagnetic impurity scatterings in the Born limit and in the unitary limit. These results are helpful to detect detailed characters of local electronic structures in the vortex lattice states via siteselective NMR experiments.Physical Review B 01/2015; 91(1). DOI:10.1103/PhysRevB.91.014509 · 3.74 Impact Factor  Springer Series in Materials Science 01/2015; 211:331376. DOI:10.1007/9783319112541_9
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ABSTRACT: We report peculiar momentumdependent anisotropy in the superconducting gap observed by angleresolved photoemission spectroscopy in BaFe2(As1xPx)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 12/2014; 4:7292. DOI:10.1038/srep07292 · 5.58 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The absence of nesting between electron and hole pockets in LiFeAs with ${T}_{\mathrm{c}}=18$ 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 recently developed orbitalspin fluctuation theories. It is found that the experimentally observed gap structure of LiFeAs, which is a ``fingerprint'' of the pairing mechanism, is quantitatively reproduced in terms of the orbitalfluctuationmediated ${s}_{++}$wave state. Specifically, the largest gap observed on the two small hole pockets 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 ``hole${s}_{\ifmmode\pm\else\textpm\fi{}}$wave state,'' in which only the gap of the large hole pocket made of the ${d}_{xy}$ 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 $(\mathrm{Ba},\mathrm{K}){\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$.Physical Review B 07/2014; 90(3). DOI:10.1103/PhysRevB.90.035104 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
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. · 7.51 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.  [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$_2$.  [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$_x$. 
Article: Superconductivity in Ca1xLaxFeAs2: A Novel 112Type Iron Pnictide with Arsenic Zigzag Bonds
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ABSTRACT: We report superconductivity in the novel 112type ironbased compound Ca1xLaxFeAs2. Singlecrystal Xray diffraction analysis revealed that the compound crystallizes in a monoclinic structure (space group P21), in which Fe2As2 layers alternate with Ca2As2 spacer layers such that monovalent arsenic forms zigzag chains. Superconductivity with a transition temperature (Tc) of 34 K was observed for the x = 0.1 sample, while the x = 0.21 sample exhibited trace superconductivity at 45 K. Firstprinciples band calculations demonstrated the presence of almost cylindrical Fermi surfaces, favorable for the high Tc in Ladoped CaFeAs2.Journal of the Physical Society of Japan 11/2013; 82(12). DOI:10.7566/JPSJ.82.123702 · 1.59 Impact Factor 
Article: Possible OddFrequency Pairing in QuasiOneDimensional Organic Superconductors (TMTSF) 2 X
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ABSTRACT: In order to clarify a pairing symmetry in quasionedimensional organic superconductors (TMTSF)(2)X, we investigate the superconducting state in the extended Hubbard model on a quarterfilled square lattice within the random phase approximation. We find that a competition among four pairing states including oddfrequency pairing occurs in the quasionedimensional system where the 2k(F) spin fluctuation coexists with the 2k(F) charge fluctuation. We suggest that the oddfrequency pairing can appear in a realistic parameter.Journal of the Physical Society of Japan 10/2013; 82(10):104702. DOI:10.7566/JPSJ.82.104702 · 1.59 Impact Factor  [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. DOI:10.1103/PhysRevLett.111.057003 · 7.51 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.  [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). DOI:10.1103/PhysRevB.88.041106 · 3.74 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). DOI:10.1103/PhysRevB.88.045115 · 3.66 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). DOI:10.1103/PhysRevB.87.195121 · 3.66 Impact Factor
Publication Stats
3k  Citations  
258.17  Total Impact Points  
Top Journals
Institutions

2015

Okayama University
 Department of Physics
Okayama, Okayama, Japan


20052014

Nagoya University
 Department of Quantum Engineering
Nagoya, Aichi, Japan 
The University of ElectroCommunications
 Department of Applied Physics and Chemistry
Tokyo, Tokyoto, Japan


2007

Japan Science and Technology Agency (JST)
Edo, Tōkyō, Japan


20022005

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