Showing papers by "Takashi Mizokawa published in 2014"

Excitonic Bose-Einstein condensation in Ta 2 NiSe 5 above room temperature

Abstract: We show that finite temperature variational cluster approximation (VCA) calculations on an extended Falicov-Kimball model can reproduce angle-resolved photoemission spectroscopy (ARPES) results on ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ across a semiconductor-to-semiconductor structural phase transition at 325 K. We demonstrate that the characteristic temperature dependence of the flat-top valence band observed by ARPES is reproduced by the VCA calculation on the realistic model for an excitonic insulator only when the strong excitonic fluctuation is taken into account. The present calculations indicate that ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ falls in the Bose-Einstein condensation regime of the excitonic insulator state.

Role of the Ce valence in the coexistence of superconductivity and ferromagnetism of CeO 1 − x F x BiS 2 revealed by Ce L 3 -edge x-ray absorption spectroscopy

Abstract: We have performed Ce ${L}_{3}$-edge x-ray absorption spectroscopy (XAS) measurements on ${\mathrm{CeO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiS}}_{2}$, in which the superconductivity of the BiS${}_{2}$ layer and the ferromagnetism of the ${\mathrm{CeO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}$ layer are induced by the F doping, in order to investigate the impact of the F doping on the local electronic and lattice structures. The Ce ${L}_{3}$-edge XAS spectrum of CeOBiS${}_{2}$ exhibits coexistence of $4{f}^{1}$ (Ce${}^{3+}$) and $4{f}^{0}$ (Ce${}^{4+}$) state transitions revealing Ce mixed valency in this system. The spectral weight of the $4{f}^{0}$ state decreases with the F doping and completely disappears for $xg0.4$ where the system shows the superconductivity and the ferromagnetism. The results suggest that suppression of the Ce-S-Bi coupling channel by the F doping appears to drive the system from the valence fluctuation regime to the Kondo-like regime, leading to the coexistence of the superconducting BiS${}_{2}$ layer and the ferromagnetic ${\mathrm{CeO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}$ layer.

Determination of local atomic displacements in CeO1-xFxBiS2 system

Abstract: We have used Bi and Ce L3-edges extended x-ray absorption fine structure measurements to study local structure of CeO1−xFxBiS2 system as a function of F-substitution. The local structure of both BiS2 active layer and CeO1−xFx spacer layer changes systematically. The in-plane Bi–S1 distance decreases (ΔRmax ~ 0.08 A) and the out-of-plane Bi–S2 distance increases (ΔRmax ~ 0.12 A) with increasing F-content. On the other hand, the Ce-O/F distance increases (ΔRmax ~ 0.2 A) with a concomitant decrease of the Ce–S2 distance (ΔRmax ~ 0.15 A). Interestingly, the Bi–S1 distance is characterized by a large disorder that increases with F-content. The results provide useful information on the local atomic displacements in CeO1−xFxBiS2, that should be important for the understanding of the coexistence of superconductivity and low temperature ferromagnetism in this system.

Spectromicroscopy of electronic phase separation in K$_x$Fe$_{2-y}$Se$_2$ superconductor

Abstract: Structural phase separation in A$_x$Fe$_{2-y}$Se$_2$ system has been studied by different experimental techniques, however, it should be important to know how the electronic uniformity is influenced, on which length scale the electronic phases coexist, and what is their spatial distribution. Here, we have used novel scanning photoelectron microscopy (SPEM) to study the electronic phase separation in K$_x$Fe$_{2-y}$Se$_2$, providing a direct measurement of the topological spatial distribution of the different electronic phases. The SPEM results reveal a peculiar interconnected conducting filamentary phase that is embedded in the insulating texture. The filamentary structure with a particular topological geometry could be important for the high T$_c$ superconductivity in the presence of a phase with a large magnetic moment in A$_x$Fe$_{2-y}$Se$_2$ materials.

Bond order and the role of ligand states in stripe-modulated IrTe2

Abstract: The coupled electronic-structural modulations of the ligand states in ${\mathrm{IrTe}}_{2}$ have been studied by x-ray absorption spectroscopy and resonant elastic x-ray scattering (REXS). Distinctive preedge structures are observed at the Te-${M}_{4,5} (3d \ensuremath{\rightarrow} 5p)$ absorption edge, indicating the presence of a Te $5p--\mathrm{Ir} 5d$ covalent state near the Fermi level. An enhancement of the REXS signal near the Te $3d \ensuremath{\rightarrow} 5p$ resonance at the $Q=(1/5,0,\ensuremath{-}1/5)$ superlattice reflection is observed below the structural transition temperature ${T}_{s}\ensuremath{\sim}280$ K. The analysis of the energy-dependent REXS line shape reveals the key role played by the spatial modulation of the covalent Te $5p--\mathrm{Ir} 5d$ bond density in driving the stripelike order in ${\mathrm{IrTe}}_{2}$, and uncovers its coupling with the charge and/or orbital order at the Ir sites. The similarity between these findings and the charge-ordering phenomenology recently observed in the high-temperature superconducting cuprates suggests that the iridates may harbor similar exotic phases.

Important Roles of Te 5p and Ir 5d Spin–Orbit Interactions on the Multi-band Electronic Structure of Triangular Lattice Superconductor Ir1−xPtxTe2

Abstract: We report an angle-resolved photoemission spectroscopy (ARPES) study on a triangular lattice superconductor Ir1−xPtxTe2 in which the Ir–Ir or Te–Te bond formation, the band Jahn–Teller effect, and the spin–orbit interaction are cooperating and competing with one another. The Fermi surfaces of the substituted system are qualitatively similar to the band structure calculations for the undistorted IrTe2 with an upward chemical potential shift due to electron doping. A combination of the ARPES and the band structure calculations indicates that the Te 5p spin–orbit interaction removes the px/py orbital degeneracy and induces px ± ipy type spin–orbit coupling near the A point. The inner and outer Fermi surfaces are entangled by the Te 5p and Ir 5d spin–orbit interactions which may provide exotic superconductivity with singlet–triplet mixing.

Electronic structure of LaO 1 − x F x BiSe 2 ( x = 0.18 ) revealed by photoelectron spectromicroscopy

Abstract: We report an electronic structure study on ${\mathrm{LaO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiSe}}_{2}$ $(x=0.18)$ by means of photoelectron spectromicroscopy. The Fermi surfaces and band dispersions are basically consistent with the band-structure calculations on ${\mathrm{BiS}}_{2}$-based materials, indicating that the electron correlation effects may be irrelevant to describe physics of the new ${\mathrm{BiSe}}_{2}$ system. In ${\mathrm{LaO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiSe}}_{2}$ $(x=0.18)$, the area of the Fermi pockets is estimated to be $0.16\ifmmode\pm\else\textpm\fi{}0.02$ per Bi, consistent with the amount of F substitution. Although the spectromicroscopy technique avoids the effect of microscale inhomogeneity for angle-resolved photoemission spectroscopy (ARPES), the ARPES spectral features are rather broad in the momentum space, indicating the likely effect of local disorder in the ${\mathrm{BiSe}}_{2}$ layer.

Te 5 p orbitals bring three-dimensional electronic structure to two-dimensional Ir 0.95 Pt 0.05 Te 2

Abstract: We have studied the nature of the three-dimensional multiband electronic structure in the two-dimensional triangular lattice Ir${}_{1\ensuremath{-}x}$Pt${}_{x}$Te${}_{2}$ ($x=0.05$) superconductor using angle-resolved photoemission spectroscopy (ARPES), x-ray photoemission spectroscopy (XPS), and a band structure calculation. ARPES results clearly show a cylindrical (almost two-dimensional) Fermi surface around the zone center. Near the zone boundary, the cylindrical Fermi surface is truncated into several pieces in a complicated manner with strong three dimensionality. The XPS result and the band structure calculation indicate that the strong Te 5$p$-Te 5$p$ hybridization between the IrTe${}_{2}$ triangular lattice layers is responsible for the three dimensionality of the Fermi surfaces and the intervening of the Fermi surfaces observed by ARPES.

Temperature dependence of iron local magnetic moment in phase-separated superconducting chalcogenide

Abstract: We have studied local magnetic moment and electronic phase separation in superconducting ${\mathrm{K}}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{2}$ by x-ray emission and absorption spectroscopy. Detailed temperature-dependent measurements at the Fe K-edge have revealed coexisting electronic phases and their correlation with the transport properties. By cooling down, the local magnetic moment of Fe shows a sharp drop across the superconducting transition temperature (${T}_{c}$) and the coexisting phases exchange spectral weights with the low-spin state, gaining intensity at the expense of the higher-spin state. After annealing the sample across the iron-vacancy order temperature, the system does not recover the initial state and the spectral weight anomaly at ${T}_{c}$ as well as superconductivity disappear. The results clearly underline that the coexistence of the low-spin and high-spin phases and the transitions between them provide unusual magnetic fluctuations and have a fundamental role in the superconducting mechanism of the electronically inhomogeneous ${\mathrm{K}}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{2}$ system.

Model Selection of NiGa2S4 Triangular Lattice by Bayesian Inference

Abstract: We propose a novel method for extracting effective classical spin Hamiltonians from mean-field type electronic structural calculations by means of the Bayesian inference. We apply the method for a NiS2 triangular lattice in NiGa2S4, where the ground state is a spin disordered state. Starting from unrestricted Hartree–Fock calculations for the spin configurations of 16 Ni sites, we estimated that not only the strongest superexchange interaction between the third nearest neighbor-sites but also those between the nearest and the second nearest neighbor-sites should be considered for the effective classical spin Hamiltonian for NiGa2S4.

Determination of temperature-dependent atomic displacements in the Ca10Ir4As8(Fe2As2)5 superconductor with a metallic spacer layer

Abstract: We have used the combination of Fe $K$-edge and Ir ${L}_{3}$-edge x-ray absorption measurements as a function of temperature to investigate local atomic displacements in the newly discovered ${\mathrm{Ca}}_{10}{\mathrm{Ir}}_{4}{\mathrm{As}}_{8}{({\mathrm{Fe}}_{2}{\mathrm{As}}_{2})}_{5}$ superconducting system. We find relatively relaxed Fe-Fe atomic pair correlations with large displacements in the ${\mathrm{FeAs}}_{4}$ tetrahedra, revealed by Fe $K$-edge extended x-ray absorption fine structure (EXAFS) analysis. Similarly, the temperature dependence of Ir ${L}_{3}$-edge EXAFS shows nanoscale disorder in the IrAs layer that should have a significant effect on the active FeAs-layer characteristics. Furthermore, x-ray absorption near edge structure data are presented to discuss the evolution of the unoccupied electronic states revealing the marginal role of spin-orbit coupling, while the interlayer interactions and disorder should be important for describing the physics of the ${\mathrm{Ca}}_{10}{\mathrm{Ir}}_{4}{\mathrm{As}}_{8}{({\mathrm{Fe}}_{2}{\mathrm{As}}_{2})}_{5}$ system.

Effect of Pt substitution on the electronic structure of AuTe2

Abstract: We report a photoemission and x-ray absorption study on a ${\mathrm{Au}}_{1\ensuremath{-}x}{\mathrm{Pt}}_{x}{\mathrm{Te}}_{2}$ ($x=0$ and 0.35) triangular lattice in which superconductivity is induced by Pt substitution for Au. Au $4f$ and Te $3d$ core-level spectra of ${\mathrm{AuTe}}_{2}$ suggest a valence state of ${\mathrm{Au}}^{2+}$(${\mathrm{Te}}_{2}$)${}^{2\ensuremath{-}}$, which is consistent with its distorted crystal structure with Te-Te dimers and compressed ${\mathrm{AuTe}}_{6}$ octahedra. On the other hand, valence-band photoemission spectra and preedge peaks of the Te $3d$ absorption edge indicate that Au $5d$ bands are almost fully occupied and that Te $5p$ holes govern the transport properties and the lattice distortion. The two apparently conflicting pictures can be reconciled by strong Au $5d$/Au $6s$--Te $5p$ hybridization. The absence of a core-level energy shift with Pt substitution is inconsistent with the simple rigid band picture for hole doping. The Au $4f$ core-level spectrum gets slightly narrow with Pt substitution, indicating that the small Au $5d$ charge modulation in distorted ${\mathrm{AuTe}}_{2}$ is partially suppressed.

Synchrotron X-ray Diffraction Study of Structural Phase Transition in Ca10(Ir4As8)(Fe2-xIrxAs2)

Abstract: We report a structural transition found in Ca10(Ir4As8)(Fe2-xIrxAs2)5, which exhibits superconductivity at 16 K. The c-axis parameter is doubled below a structural transition temperature of approximately 100 K, while the tetragonal symmetry with space group P4/n (No.85) is unchanged at all temperatures measured. Our synchrotron x-ray diffraction study clearly shows iridium ions at a non-coplanar position shift along the z-direction at the structural phase transition. We discuss that the iridium displacements affect superconductivity in Fe2As2 layers.

Excitonic Bose-Einstein condensation in Ta2NiSe5

Abstract: Some semimetals or small-gap semiconductors with small carrier densities exhibit an electronic and structural phase transition due to interaction between conduction band electrons and valence band holes. Such family of materials are known as excitonic insulators and the phase transition can be described in Bardeen-Cooper-Schrieffer (BCS) or Bose-Einstein condensate (BEC) frameworks. In the present work, a semiconductor-semiconductor structural phase transition in Ta2NiSe5 at 325 K is shown to be excitonic BEC transition by means of angle-resolved photoemission spectroscopy (ARPES) and variational cluster approximation (VCA) calculation on an appropriate model. Characteristic temperature dependence of the flat-top valence band, which is observed by ARPES across the phase transition of Ta2NiSe5, is reproduced by the VCA calculation across the excitonic BEC transition in an extended Falicov-Kimball model for Ta2NiSe5. PACS numbers: 71.35.Lk, 71.20.-b, 67.85.Jk, 03.75.Nt

Synchrotron X-ray Diffraction Study of Structural Phase Transition in Ca10(Ir4As8)(Fe2−xIrxAs2)5

Abstract: We report on the structural phase transition found in Ca10(Ir4As8)(Fe2−xIrxAs2)5, which exhibits superconductivity at 16 K. The c-axis parameter is doubled below a structural transition temperature of approximately 100 K, while the tetragonal symmetry with the space group P4/n (No. 85) is unchanged at all temperatures measured. Our synchrotron X-ray diffraction study clearly shows iridium ions at a non-coplanar position shift along the z-direction at the structural phase transition. We discuss the fact that iridium displacements affect superconductivity in Fe2As2 layers.

Coexistence of Bloch electrons and glassy electrons in Ca10(Ir4As8)(Fe2- xIrxAs2)5 revealed by angle-resolved photoemission spectroscopy

Abstract: Angle-resolved photoemission spectroscopy of Ca10(Ir4As8)(Fe2_xIrxAs2)5 shows that the Fe 3d electrons in the FeAs layer form the hole-like Fermi pocket at the zone center and the electron-like Fermi pockets at the zone corners as commonly seen in various Fe-based superconductors. The FeAs layer is heavily electron doped and has relatively good two dimensionality. On the other hand, the Ir 5d electrons are metallic and glassy probably due to atomic disorder related to the Ir 5d orbital instability. Ca10(Ir4As8)(Fe2_xIrxAs2)5 exhibits a unique electronic state where the Bloch electrons in the FeAs layer coexist with the glassy electrons in the Ir4As8 layer.

Temperature dependent nanoscale atomic correlations in Ir1−xPtxTe2 (x = 0.0, 0.03 and 0.04) system

Abstract: X-ray absorption near-edge structure (XANES) spectroscopy has been used to investigate the unoccupied electronic states and local geometry of Ir1?xPtxTe2(x?=?0.0, 0.03 and 0.04) as a function of temperature. The Ir L3-edge absorption white line, as well as high energy XANES features due to the photoelectron multiple scatterings with near neighbours, reveal clear changes in the unoccupied 5d-electronic states and the local geometry with Pt substitution. We find an anomalous spectral weight transfer across the known first-order structural phase transition from the trigonal to monoclinic phase in IrTe2, which characterizes the reduced atomic structure symmetry below the transition temperature. No such changes with temperature are seen in the Pt substituted superconducting samples. In addition, a gradual increase of the spectral weight transfer is observed in IrTe2 with a further decrease in temperature below the transition, indicating that the low temperature phase is likely to have a symmetry lower than the monoclinic one. The results suggest that the interplay between inter-layer and intra-layer atomic correlations should have a significant role in the properties of an Ir1?xPtxTe2 system.

Electronic Structure of Quantum Spin-Liquid Coupound Ba3CuSb2O9

Abstract: We have investigated the electronic structure of a new spin-liquid compound, hexagonal Ba3CuSb2O9 [3] by means of unrestricted Hartree-Fock calculation. We first set six CuO6 octahedra on a honeycomb lattice and consider Cu 3d and oxygen 2p electrons. A multi-orbital d-p type Hamiltonian is selected as a model Hamiltonian, and we take the Cu 3d O 2p charge-transfer energy ∆ as an adjustable parameter and fix all the others. Then our calculation suggests that the Cu-O-OCu superexchange interaction J of Ba3CuSb2O9 strongly depends on the Cu 3d orbital configuration and the value of ∆. The orbital dependence of J would be related to the spin-liquid behavior of the system.

X-ray Photoemission and X-ray Absorption Spectroscopy of Hexagonal Ba 3 CuSb 2 O 9

Abstract: We investigate the electronic structure of a new candidate of quantum spin liquid (QSL) compound, hexagonal Ba3CuSb2O9 by X-ray photoemission spectroscopy and X-ray absorption spectroscopy (XPS and XAS). The figure below shows the O 1s XAS spectra of orthorhombic and hexagonal Ba3CuSb2O9, indicating the existence of oxygen hole in the hexagonal sample. We estimate that the amount of oxygen hole per Cu site is approximately 1/6. We will discuss the origin of QSL state on the basis of XPS and XAS results.