Publications (166)391.25 Total impact
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Article: Pseudogap formation above the superconducting dome in iron-pnictides
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ABSTRACT: In the normal state above the superconducting transition temperature (Tc), the momentum-resolved electronic structure of the high-Tc cuprate shows a depletion dubbed the psudogap, which becomes less pronounced with doping. Its relation to the high-Tc superconductivity has been a key issue, and the possible rotational symmetry breaking in this state has been reported suggesting a peculiar electronic ordering preceding the superconducting pairing. Here we provide direct evidence for a systematic evolution of the pseudogap phase via angle-resolved photoemission spectroscopy on another family of high-Tc superconductor, iron-pnictides. We observe the P-substitution evolution of pseudogap in BaFe2(As1-xPx)2, which develops well above the magnetostructural transitions and persists above the nonmagnetic superconducting dome, showing a notable similarity with cuprates. In addition, the pseudogap formation is accompanied by inequivalent energy shifts in zx/yz orbitals of iron atoms, indicative of a peculiar iron orbital ordering which breaks the fourfold rotational symmetry.05/2013; -
Article: Unoccupied topological surface state in Bi$_{2}$Te$_{2}$Se
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ABSTRACT: Bias voltage dependent scattering of the topological surface state is studied by scanning tunneling microscopy/spectroscopy for a clean surface of the topological insulator Bi$_2$Te$_2$Se. A strong warping of constant energy contours in the unoccupied part of the spectrum is found to lead to a spin-selective scattering. The topological surface state persists to higher energies in the unoccupied range far beyond the Dirac point, where it coexists with the bulk conduction band. This finding sheds light on the spin and charge dynamics over the wide energy range and opens a way to designing opto-spintronic devices.05/2013; -
Article: Importance of both spin and orbital fluctuations in BaFe2(As1-xPx)2 : Evidence from superconducting gap anisotropy
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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; -
Article: Relation between the nodal and antinodal gap and critical temperature in superconducting Bi2212.
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ABSTRACT: An energy gap is, in principle, a dominant parameter in superconductivity. However, this view has been challenged for the case of high-Tc cuprates, because anisotropic evolution of a d-wave-like superconducting gap with underdoping has been difficult to formulate along with a critical temperature Tc. Here we show that a nodal-gap energy 2ΔN closely follows 8.5 kBTc with underdoping and is also proportional to the product of an antinodal gap energy Δ(*) and a square-root superfluid density √Ps for Bi2Sr2CaCu2O8+δ, using low-energy synchrotron-radiation angle-resolved photoemission. The quantitative relations imply that the distinction between the nodal and antinodal gaps stems from the separation of the condensation and formation of electron pairs, and that the nodal-gap suppression represents the substantial phase incoherence inherent in a strong-coupling superconducting state. These simple gap-based formulae reasonably describe a crucial part of the unconventional mechanism governing Tc.Nature Communications 01/2013; 4:1815. · 7.40 Impact Factor -
Dataset: Takeda PhysicaB 2004 at
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Dataset: PhysicaB 2004 takeda
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Article: Topological Surface States with Persistent High Spin Polarization across the Dirac Point in Bi_{2}Te_{2}Se and Bi_{2}Se_{2}Te.
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ABSTRACT: Helical spin textures with marked spin polarizations of topological surface states have been unveiled for the first time by state-of-the-art spin- and angle-resolved photoemission spectroscopy for two promising topological insulators, Bi_{2}Te_{2}Se and Bi_{2}Se_{2}Te. Their highly spin-polarized natures are found to be persistent across the Dirac point in both compounds. This novel finding paves a pathway to extending the utilization of topological surface states of these compounds for future spintronic applications.Physical Review Letters 10/2012; 109(16):166802. · 7.37 Impact Factor -
Article: Effect of electron-phonon coupling in the ARPES spectra of the tri-layer cuprate Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$
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ABSTRACT: Angle-resolved photoemission spectroscopy using tunable low energy photons allows us to study the quasi-particle (QP) dispersions of the inner and outer CuO2 planes (IP and OP) separately in the tri-layer cuprate Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$ (Bi2223). The kink energy of the OP band is $\sim$ 70 meV, as observed in various high-$T_c$ cuprates, while that of the IP band is as large as 100 meV in the superconducting (SC) state. This large kink energy is attributed to the $\sim$ 35 meV buckling mode plus the large ($\sim$ 60 meV) SC gap of IP. The IP band also shows a weak kink feature at 70 meV in the SC state. The latter feature can be explained either by the 70 meV half-breathing mode or by the $\sim$ 35 meV buckling-phonon mode plus the $\sim$ 40 meV SC gap of OP if interlayer scattering of QP is involved.08/2012; -
Article: Two-fold symmetry flattens Dirac cone of surface state at W(110)
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ABSTRACT: The $C_{2v}$ symmetry of the W(110) surface influences strongly the spin-polarized Dirac-cone-like surface state within a spin-orbit-induced symmetry gap. We present a detailed angle-resolved photoemission study with $s$- and $p$-polarized light along three different symmetry lines. The Dirac-cone-like feature appears along $\bar{\Gamma}\bar{H}$ and $\bar{\Gamma}\bar{S}$, while it is strongly deformed along $\bar{\Gamma}\bar{N}$. A two-fold $\Sigma_{3}$ symmetry of the $d$-type surface state is identified from photoemission experiments using linear polarized light. Our results are well described by model calculations based on an effective Hamiltonian with $C_{2v}$ symmetry including Rashba parameters up to third order. The flattened Dirac cone of the surface state is caused by hybridization with bulk continuum states of $\Sigma_{1}$ and $\Sigma_{2}$ symmetry. The spin texture of this state obtained from the model calculations shows a quasi-one dimensional behavior. This finding opens a new avenue in the study of $d$-electron-based persistent spin helix systems and/or weak topological insulators.07/2012; -
Article: Electronic structure reconstruction by orbital symmetry breaking in IrTe2
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ABSTRACT: We report an angle-resolved photoemission spectroscopy (ARPES) study on IrTe2 which exhibits an interesting lattice distortion below 270 K and becomes triangular lattice superconductors by suppressing the distortion via chemical substitution or intercalation. ARPES results at 300 K show multi-band Fermi surfaces with six-fold symmetry which are basically consistent with band structure calculations. At 20 K in the distorted phase, whereas the flower shape of the outermost Fermi surface does not change from that at 300 K, topology of the inner Fermi surfaces is strongly modified by the lattice distortion. The Fermi surface reconstruction by the distortion depends on the orbital character of the Fermi surfaces, suggesting importance of Ir 5d and/or Te 5p orbital symmetry breaking.07/2012; -
Article: Observation of a Highly Spin Polarized Topological Surface State in GeBi$_{2}$Te$_{4}$
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ABSTRACT: Spin polarization of a topological surface state for GeBi$_2$Te$_4$, the newly discovered three-dimensional topological insulator, has been studied by means of the state of the art spin- and angle-resolved photoemission spectroscopy. It has been revealed that the disorder in the crystal has a minor effect on the surface state spin polarization and it exceeds 75% near the Dirac point in the bulk energy gap region ($\sim$180 meV). This new finding for GeBi$_{2}$Te$_{4}$ promises not only to realize a highly spin polarized surface isolated transport but to add new functionality to its thermoelectric and thermomagnetic properties.07/2012; -
Article: Doping-Dependent and Orbital-Dependent Band Renormalization in Ba(Fe_1-xCo_x)_2As_2 Superconductors
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ABSTRACT: Angle resolved photoemission spectroscopy of Ba(Fe1-xCox)2As2 (x = 0.06, 0.14, and 0.24) shows that the width of the Fe 3d yz/zx hole band depends on the doping level. In contrast, the Fe 3d x^2-y^2 and 3z^2-r^2 bands are rigid and shifted by the Co doping. The Fe 3d yz/zx hole band is flattened at the optimal doping level x = 0.06, indicating that the band renormalization of the Fe 3d yz/zx band correlates with the enhancement of the superconducting transition temperature. The orbital-dependent and doping-dependent band renormalization indicates that the fluctuations responsible for the superconductivity is deeply related to the Fe 3d orbital degeneracy.06/2012; -
Article: Quasiparticle interference on the surface of Bi_ {2} Se_ {3} induced by cobalt adatom in the absence of ferromagnetic ordering
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ABSTRACT: Quasiparticle interference induced by cobalt adatoms on the surface of the topological insulator Bi2Se3 is studied by scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and x-ray magnetic circular dichroism. It is found that Co atoms are selectively adsorbed on top of Se sites and act as strong scatterers at the surface, generating anisotropic standing waves. A long-range magnetic order is found to be absent, and the surface state Dirac cone remains gapless. The anisotropy of the standing wave is ascribed to the heavily warped iso-energy contour of unoccupied states, where the scattering is allowed due to a nonzero out-of-plane spin.Phys. Rev. B. 05/2012; 85(20). -
Article: Pseudogap behavior of RuP probed by photoemission spectroscopy
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ABSTRACT: We have studied the electronic structure of RuP and related Ru pnictides using photoemission spectroscopy. Ru 3d core-level and valence-band spectra of RuP show that the Ru valence is +3 with t_{2g}^5 configuration. The photoemisson spectral weight near the Fermi level is moderately suppressed in the pseudogap phase of RuP, consistent with the pseudogap opening of 2\Delta/k_BT_c ~ 3 (gap size \Delta ~ 50 meV and transition temperature T_c ~ 330 K). The Ru 3d peak remains sharp in the pseudogap phase and the insulating phase of RuP, suggesting that the electronic orderings responsible for the phase transitions are different from the conventional charge density wave.05/2012; -
Article: Electronic Structure of BaFe2−xCoxAs2 Revealed by Angle-Resolved Photoemission Spectroscopy
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ABSTRACT: We report an angle-resolved photoemission spectroscopy study of BaFe2−x Co x As2. For x=0, above the structural and magnetic transition temperature (T s ), the spectral weight near the Fermi level is considerably suppressed around the Γ point where the Fe 3d yz/zx orbital degeneracy is expected. This observation suggests that the Jahn–Teller type instability is playing an important role in the tetragonal phase aboveT s . Below T s , the spectral weight of 0–100 meV is reconstructed to form flat bands at 70–100meV and Fermi surfaces, consistent with the orbital-dependent excitonic coupling. In the optimally doped and overdoped regimes, the hole pocket around the Γ point and the electron pocket around the M point are apparently nested, indicating that the doping dependence of the superconducting transition temperature cannot be explained by the nesting scenario and that the unusual electron–lattice fluctuation due to the orbital degeneracy is important. KeywordsARPES–FeAs layer–Orbital degeneracy–Excitonic insulatorJournal of Superconductivity and Novel Magnetism 04/2012; 24(3):1133-1136. · 0.65 Impact Factor -
Article: Orbital Degeneracy and Peierls Instability in Triangular Lattice Superconductor Ir$_{1-x}$Pt$_x$Te$_2$
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ABSTRACT: We have studied electronic structure of triangular lattice Ir$_{1-x}$Pt$_x$Te$_2$ superconductor using photoemission spectroscopy and model calculations. Ir $4f$ core-level photoemission spectra show that Ir $5d$ $t_{2g}$ charge modulation established in the low temperature phase of IrTe$_2$ is suppressed by Pt doping. This observation indicates that the suppression of charge modulation is related to the emergence of superconductivity. Valence-band photoemission spectra of IrTe$_2$ suggest that the Ir $5d$ charge modulation is accompanied by Ir $5d$ orbital reconstruction. Based on the photoemission results and model calculations, we argue that the orbitally-induced Peierls effect governs the charge and orbital instability in the Ir$_{1-x}$Pt$_x$Te$_2$.Phys. Rev. B. 03/2012; 86(1). -
Article: Spin-polarized Dirac-cone-like surface state with d character at W(110).
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ABSTRACT: The surface of W(110) exhibits a Dirac-cone-like state with d character within a spin-orbit-induced symmetry gap. As a function of the wave vector parallel to the surface, it shows a nearly massless energy dispersion and a pronounced spin polarization, which is antisymmetric with respect to the Brillouin zone center. In addition, the observed constant energy contours are strongly anisotropic for all energies. This discovery opens new pathways to the study of surface spin-density waves arising from a strong Fermi surface nesting as well as d-electron-based topological properties.Physical Review Letters 02/2012; 108(6):066808. · 7.37 Impact Factor -
Article: Experimental Verification of PbBi_{2}Te_{4} as a 3D Topological Insulator
Physical Review Letters 01/2012; 108(20):206803. · 7.37 Impact Factor -
Article: Topological Surface States with Persistent High Spin Polarization across the Dirac Point in Bi2Te2Se and Bi2Se2Te
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ABSTRACT: Helical spin textures with marked spin polarizations of topological surface states have been unveiled for the first time by state-of-the-art spin-and angle-resolved photoemission spectroscopy for two promising topological insulators, Bi2Te2Se and Bi2Se2Te. Their highly spin-polarized natures are found to be persistent across the Dirac point in both compounds. This novel finding paves a pathway to extending the utilization of topological surface states of these compounds for future spintronic applications.Physical Review Letters 01/2012; 109(16). · 7.37 Impact Factor -
Article: Relocation of the topological surface state of Bi$_{2}$Se$_{3}$ beneath the surface by Ag intercalation
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ABSTRACT: We studied the Ag-intercalated 3D topological insulator Bi$_{2}$Se$_{3}$ by scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy, combined with a first principles calculations. We demonstrate that silver atoms deposited on the surface of Bi$_{2}$Se$_{3}$ are intercalated between the quintuple layer (QL) units of the crystal, causing a expansion of the van der Waals gaps and the detachment of topmost QLs from the bulk crystal. This leads to a relocation (in the real space) of the the topological state beneath the detached quintuple layers, accompanied by the emergence of parabolic and "M-shaped" trivial bands localized above the relocated topological states. These novel findings open a pathway to the engineering of Dirac fermions shielded from the ambient contamination and may facilitate the realization of fault-tolerant quantum devices.12/2011;
Top co-authors
Top Journals
Institutions
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1995–2011
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Hiroshima University
- • Graduate School of Science
- • Hiroshima Synchrotron Radiation Center (HiSOR)
- • Department of Quantum Matter
- • Department of Biomaterials Science
Hiroshima-shi, Hiroshima-ken, Japan
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2007–2010
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Fudan University
- Department of Physics
Shanghai, Shanghai Shi, China
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1990–2010
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The University of Tokyo
- • Department of Physics
- • Institute for Solid State Physics
Tokyo, Tokyo-to, Japan
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2006–2008
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Tokyo University of Science
- Department of Applied Physics
Tokyo, Tokyo-to, Japan
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2004
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RIKEN
Wako, Saitama-ken, Japan
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