担当区分：最終著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

We present a hidden Markov model analysis for fluorescent time series of colloidal quantum dots. A fundamental quantity to measure optical performance of the quantum dots is a distribution function for the light-emission duration. So far, to estimate it, a threshold value for the fluorescent intensity was introduced, and the light-emission state was evaluated as a state above the threshold. With this definition, the light-emission duration was estimated, and its distribution function was derived as a blinking plot. Due to the noise in the fluorescent data, however, this treatment generates a large number of artificially short-lived emission states, thus leading to an erroneous blinking plot. In the present paper, we propose a hidden Markov model to eliminate these artifacts. The hidden Markov model introduces a hidden variable specifying the light-emission and quenching states behind the observed fluorescence. We found that it is possible to avoid the above artifacts by identifying the state from the hidden-variable time series. We found that, from the analysis of experimental and theoretical benchmark data, the accuracy of our hidden Markov model is beyond human cognitive ability.

DOI： 10.1103/PhysRevB.106.104305

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Strongly correlated electron systems, generally recognized as d- and f-electron systems, have attracted attention as a platform for the emergence of exotic properties such as high-Tc superconductivity. However, correlated electron behaviors have been recently observed in a group of novel materials, electrides, in which s-electrons are confined in subnanometer-sized spaces. Here, we present a trend of electronic correlation of electrides by evaluating the electronic correlation strength obtained from model parameters characterizing effective Hamiltonians of 19 electrides from first principles. The calculated strengths vary in the order 0D ≫ 1D > 2D ∼ 3D electrides, which corresponds to experimental trends, and exceed 10 (a measure for the emergence of exotic properties) in all of the 0D and some of the 1D electrides. We also found the electronic correlation depends on the cation species surrounding the s-electrons. The results indicate that low-dimensional electrides will be new research targets for studies of strongly correlated electron systems.

DOI： 10.1021/acs.jpclett.1c03637

Kyutacar

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We present an ab initio derivation method for effective low-energy Hamiltonians of material with strong spin-orbit interactions. The effective Hamiltonian is described in terms of the Wannier function in the spinor form, and effective interactions are derived with the constrained random phase approximation (cRPA) method. Based on this formalism and the developed code, we derive an effective Hamiltonian of a strong spin-orbit interaction material Ca5Ir3O12. This system consists of three edge-shared IrO6 octahedral chains arranged along the c axis, and the three Ir atoms in the ab plane compose a triangular lattice. For such a complicated structure, we need to set up the Wannier spinor function under the local coordinate system. We found that a density-functional band structure near the Fermi level is formed by local dxy and dyz orbitals. Then, we constructed the ab initio dxy/dyz model. The estimated nearest-neighbor transfer t is close to 0.2 eV, and the cRPA on-site U and neighboring V electronic interactions are found to be 2.4-2.5 eV and 1 eV, respectively. The resulting characteristic correlation strength defined by (U-V)/t is above 7, and thus this material is classified as a strongly correlated electron system. The on-site transfer integral involved in the spin-orbit interaction is 0.2 eV, which is comparable to the on-site exchange integrals ∼0.2 eV, indicating that the spin-orbit-interaction physics would compete with the Hund physics. Based on these calculated results, we discuss possible rich ground-state low-energy electronic structures of spin, charge, and orbitals with competing Hund, spin-orbit, and strong-correlation physics.

DOI： 10.1103/PhysRevB.104.075153

Kyutacar

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

RESPACK is a first-principles calculation software for evaluating the interaction parameters of materials and is able to calculate maximally localized Wannier functions, response functions based on the random phase approximation and related optical properties, and frequency-dependent electronic interaction parameters. RESPACK receives its input data from a band-calculation code using norm-conserving pseudopotentials with plane-wave basis sets. Automatic generation scripts that convert the band-structure results to the RESPACK inputs are prepared for xTAPP and Quantum ESPRESSO. An input file for specifying the RESPACK calculation conditions is designed pursuing simplicity and is given in the Fortran namelist format. RESPACK supports hybrid parallelization using OpenMP and MPI and can treat large systems including a few hundred atoms in the calculation cell. Program summary: Program Title: RESPACK CPC Library link to program files: https://dx.doi.org/10.17632/3cxb7474nj.1 Developer's repository link: https://sites.google.com/view/kazuma7k6r Licensing provisions: GNU General Public Licence v3.0 Programming language: FORTRAN, PYTHON External routines: LAPACK, BLAS, MPI Nature of problem: Ab initio calculations for maximally localized Wannier function, response function with random-phase approximation, and matrix-element evaluations of frequency-dependent screened direct and exchange interactions. With this code, an effective low-energy model of materials is derived from first principles. Solution method: Our method is based on ab initio many-body perturbation calculation and the maximally localized Wannier function calculation. The program employs the plane-wave basis set, and evaluations of matrix elements are performed with the fast Fourier transformation. The generalized tetrahedron method is used for the Brillouin Zone integral. Additional comments including restrictions and unusual features: RESPACK supports xTAPP and Quantum ESPRESSO packages, and automatic generation scripts for converting the band-calculation results to the RESPACK inputs are prepared for these software. The current RESPACK only supports band-calculation codes using norm-conserving pseudopotentials with plane-wave basis sets. RESPACK supports hybrid parallelization using OpenMP and MPI to treat large systems in which a few hundred atoms are contained in unit cell.

DOI： 10.1016/j.cpc.2020.107781

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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記述言語：英語   掲載種別：研究論文（学術雑誌）

In this study, we report the results of experiments and calculations of phonon dispersion of iridium oxide with a strong spin–orbit interaction (SOI). Using inelastic X-ray scattering (IXS), we measured the IXS spectra of Ca5Ir3O12 along Γ–A, Γ–M, and Γ–K–M directions in the Brillouin zone of a hexagonal lattice at room temperature. We also show ab initio density-functional phonon dispersions based on local density approximation and generalized gradient approximation (GGA) considering SOI. By comparing the experimental and calculated results, we found that the GGA phonon dispersion with SOI is in very good agreement with the experimental results. The phonon calculation was performed for supercells of 1 × 1 × 3 and preliminary 2 × 2 × 1. We found no phonon instability within these supercells. Low-energy phonon properties such as Debye temperature, specific heat, and sound velocity are also discussed.

DOI： 10.7566/JPSJ.89.053601

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We report a study of the second-order phase transition at 105 K in the geometrically frustrated iridate Ca5Ir3O12 using a Raman scattering method. The Raman scattering spectra of a single crystal were measured from 4 K to room temperature. Ab initio phonon calculations that consider the spin–orbit interaction were also conducted and compared with the experimental spectra. Agreement between the theoretical and experimental results at room temperature is reasonably good. At room temperature, 6A01 þ 9E0 þ 5E00 were assigned among the Raman active modes, 6A01 þ 13E0 þ 6E00, based on the reported P62 m crystal structure. Below Ts, 23 additional peaks were observed, suggesting the appearance of a superlattice structure. The polarization dependence of Raman spectra below Ts indicates the existence of 6 symmetry. We observed at least one additional mode as a broad weak-intensity peak at temperatures higher than Ts. This suggests possible local distortion around the Ir ions, which would be expected for Ir ions with mixed valence states.

DOI： 10.7566/JPSJ.89.054602

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記述言語：英語   掲載種別：研究論文（学術雑誌）

© 2019 Elsevier B.V. We have carried out harmonic voltage response experiments by application of AC current along the c-axis of Ca5Ir3O12, which has a nonlinear electrical conductivity in a non-ordered state. This AC current method can allow us to investigate a detail of the nonlinear conductivity by application of small current. We observed the harmonics up to 7th order below 200 K. The results reveal that the nonlinear conductivity exists even in application of current close to zero. In addition, the temperature dependence of the resistance R0 estimated at the zero current limit is expressed by ln R0∝T-2/3, which is explained by an adaptation of Efros-Shklovskii variable range hopping or Fogler-Teber-Shklovskii variable range hopping. As a field assisted motion of charge career occurs in hopping conduction, from this analysis result, the nonlinear conductivity comes form the field-assisted hopping conduction.

DOI： 10.1016/j.jmmm.2019.166203

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記述言語：日本語   掲載種別：研究論文（その他学術会議資料等）

DOI： 10.11316/jpsgaiyo.75.1.0_1710

CiNii Article

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）

Motivated by the recent experimental discovery of superconductivity in the infinite-layer nickelate Nd0.8Sr0.2NiO2 [Li et al., Nature (London) 572, 624 (2019)], we study how the correlated Ni 3dx2-y2 electrons in the NiO2 layer interact with the electrons in the Nd layer. We show that three orbitals are necessary to represent the electronic structure around the Fermi level: Ni 3dx2-y2, Nd 5d3z2-r2, and a bonding orbital made from an interstitial s orbital in the Nd layer and the Nd 5dxy orbital. By constructing a three-orbital model for these states, we find that the hybridization between the Ni 3dx2-y2 state and the states in the Nd layer is tiny. We also find that the metallic screening by the Nd layer is not so effective in that it reduces the Hubbard U between the Ni 3dx2-y2 electrons just by 10%-20%. On the other hand, the electron-phonon coupling is not strong enough to mediate superconductivity of Tc ∼ 10 K. These results indicate that NdNiO2 hosts an almost isolated correlated 3dx2-y2 orbital system. We further study the possibility of realizing a more ideal single-orbital system in the Mott-Hubbard regime. We find that the Fermi pockets formed by the Nd-layer states dramatically shrink when the hybridization between the interstitial s state and Nd 5dxy state becomes small. By an extensive materials search, we find that the Fermi pockets almost disappear in NaNd2NiO4 and NaCa2NiO3

DOI： 10.1103/PhysRevB.100.205138

Kyutacar

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記述言語：英語   掲載種別：研究論文（学術雑誌）

© 2019 Elsevier B.V. Nickel oxide (NiO) with bulk crystalline size is an antiferromagnetic insulator correlated strongly with crystal structure. A reduction in crystalline size causes a change in the magnetic sublattice, resulting in the appearance of ferromagnetic moments. Furthermore, in the nano crystals fabricated in mesoporous silica, there is a lattice distortion at the unit cell level that brings about the change in the magnetic property, suggesting a prominent magneto-structural correlation. We investigate anisotropic compression effects on nanocrystalline NiO with a crystalline size (D) of 11.4 nm to observe this remarkable magneto-structural correlation. Magneto-crystalline anisotropy is at its maximum when negative contraction occurs. The negative contraction appears even in a bulk crystal under non-ideal hydrostatic compression, and its anomalous structural change is suppressed with decreasing D.

DOI： 10.1016/j.jmmm.2019.165407

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記述言語：英語   掲載種別：研究論文（学術雑誌）

© 2019 Author(s). We study the effects of hydrostatic pressure (HP) compression on the superconducting transition of severely strained Nb samples, whose grain sizes are reduced to the submicrometer level. Engineered granularity by high-pressure torsion (HPT) treatment changes the strength of coupling between submicrometer-scale grains and introduces lattice strain. We attempt to utilize the initially accumulated shear strain in the starting material for increasing the superconducting transition temperature T c under HP compression. The HP effects on non-strained Nb have already been investigated in the pressure regime over 100 GPa by Struzhkin et al. [Phys. Rev. Lett. 79, 4262 (1997)], and T c reportedly exhibited an increase from 9.2 to 9.9 K at approximately 10 GPa. (1) Slightly strained Nb in the HPT treatment exhibits the increase in T c under HP due to the strengthening of the intergrain coupling, so the pressure scale of the pressure response observed by Struzhkin et al. is reduced to approximately one-seventh at the maximum. (2) Prominently strained Nb in the HPT treatment exhibits the increase in T c under HP due to a reduction in structural symmetry at the unit-cell level: In a Nb sample subjected to HPT (6 GPa, 10 revolutions), T c exceeds 9.9 K at approximately 2 GPa. According to our first-principle calculations, the reduction in the structural symmetry affords an increase in the density of states at the Fermi energy, thereby yielding a prominent increase in T c at low pressures.

DOI： 10.1063/1.5083094

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記述言語：英語   掲載種別：記事・総説・解説・論説等（学術雑誌）

<p>In this article, uniaxial compression effects on cuprate superconductors such as YBa₂Cu₄O₈ [Y-124], Y_0.98Ca_0.02Ba₂Cu₄O₈ [Y_0.98Ca_0.02-124], and Hg_0.83Re_0.18Ba₂Ca_2.4Cu_3.6O₁₄ [Hg_0.83(Re_0.18)-1223] were reviewed. The uniaxial compression effects were compared with the hydrostatic compression effects via measuring the Meissner signal of their single crystals in the setup of using a diamond anvil cell. In Y-124 and Y_0.98Ca_0.02-124, the disappearance of the Meissner signal depended on the style of compression, and the difference in the change in the superconducting transition temperature <i>T</i>_c against initial compression are discussed with the structural symmetry of the CuO₅ pyramid. In Hg_0.83(Re_0.18)-1223, the mechanism of increasing <i>T</i>_c is discussed on the basis of the first principle calculation.</p>

DOI： 10.4131/jshpreview.29.262

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CiNii Research

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記述言語：英語   掲載種別：研究論文（学術雑誌）

© 2019 The Japan Institute of Metals and Materials In fifth-group element superconductors V, Nb, and Ta, the increase in superconducting transition temperature (Tc) was attempted by using both high-pressure torsion (HPT) and additional hydrostatic pressure (HP) compression. The former brings about the grain refinement and strain accumulation in the unit-cell level. The additional compression for severely strained superconductors triggers strengthening intergrain-contact and/or structural deformation in the unit-cell level. The manner of the appearance of the above two effects depends on the kind of elements: First, in V, there is no prominent effect of HPT, comparing to the hydrostatic compression effects on its non-strained material. Next, in Ta, the effect of strengthening intergrain-contact appears at small hydrostatic compression, resulting in temporal increase in Tc. Finally, Nb exhibits prominent increase in Tc by both effects and, in particular, the structural deformation in the unit-cell level promotes the increase in Tc. Thus, the accumulation of residual strain in the level of starting material can be a promising work to manipulate Tc under HP compression.

DOI： 10.2320/matertrans.MF201932

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担当区分：筆頭著者   記述言語：日本語   掲載種別：研究論文（その他学術会議資料等）

DOI： 10.11316/jpsgaiyo.74.1.0_2961

CiNii Article

CiNii Research

記述言語：日本語   掲載種別：研究論文（その他学術会議資料等）

DOI： 10.11316/jpsgaiyo.74.1.0_2311

CiNii Article

CiNii Research

記述言語：日本語   掲載種別：研究論文（その他学術会議資料等）

DOI： 10.11316/jpsgaiyo.74.2.0_1876

CiNii Article

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）

© 2018 The Physical Society of Japan. We report the discovery of nonlinear conductivity along the c-axis in a single crystal of Ca5Ir3O12, which indicates a semiconducting behavior with a narrow band gap of ∼0.2 eV. The resistivity decreases with increase in the applied current. This nonlinearity is reversible with the direction of current. We also show the ab initio density functional band structures and the Fermi surface. We found that the spin-orbit interactions result in an appreciable change in the lowenergy electronic structure; the interaction splits the metallic bands and leads to a pocket-like band structure, thus reducing the metallic trend. The size of the spin-orbit interaction is estimated as ∼0.3 eV, which is large enough to be comparable to the valence bandwidth of ∼0.5 eV. The Fermi surface exhibits a sheet structure along the c∗-axis, due to the 1D chain structure of edge-sharing IrO6.

DOI： 10.7566/JPSJ.87.013703

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記述言語：日本語   掲載種別：研究論文（研究会，シンポジウム資料等）

DOI： 10.11316/jpsgaiyo.73.1.0_2044

CiNii Article

CiNii Research

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記述言語：日本語   掲載種別：研究論文（研究会，シンポジウム資料等）

DOI： 10.11316/jpsgaiyo.73.2.0_1440

CiNii Article

CiNii Research

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担当区分：筆頭著者   記述言語：日本語   掲載種別：研究論文（その他学術会議資料等）

DOI： 10.11316/jpsgaiyo.73.1.0_2678

CiNii Article

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）

© 2017 American Physical Society. The effects of uniaxial strain and hydrostatic pressure on Hg0.83Re0.18Ba2Ca2.4Cu3.6O14[Hg0.83(Re0.18)-1223] were investigated by ac magnetic measurements under stress corresponding to a pressure of 20 GPa at maximum. According to a previous thermal study based on the Ehrenfest relation, in-plane contraction should increase the superconducting transition temperature Tc, whereas out-of-plane contraction should decrease Tc. This suggests that the increase in Tc under hydrostatic-pressure contraction must be smaller than that under in-plane contraction. However, the present uniaxial-strain experiments revealed enhancement of Tc under both in-plane and out-of-plane contraction, and the largest enhancement was observed under hydrostatic-pressure contraction. According to a band calculation, all contraction styles induce hole doping from the HgO blocks to the CuO2 blocks, and hydrostatic-pressure contraction yields the largest hole doping among three contractions. This behavior explains well a series of changes in Tc in the stress region of below 8 GPa. More specifically, under hydrostatic-pressure contraction, Tc exhibited an increase, a decrease, and another increase with increasing pressure, and this multistep change is similar to that observed in Bi-2223-type cuprate superconductors, suggesting that it is necessary to distinguish the effect of strain on the middle CuO2 plane in the three-CuO2-plane package from that on the outer planes.

DOI： 10.1103/PhysRevB.95.064503

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記述言語：日本語   掲載種別：研究論文（その他学術会議資料等）

DOI： 10.11316/jpsgaiyo.72.2.0_1788

CiNii Article

CiNii Research

その他リンク： https://ci.nii.ac.jp/naid/130007508009

記述言語：日本語   掲載種別：研究論文（その他学術会議資料等）

<p>イリジウム酸化物Ca_5_Ir_3_O_12_はIrが+4.67と中間価数状態にあり，IrO_6_の1次元的な辺共有からなる1次元鎖が三角格子をなした幾何学的にフラストレートした強相関5d電子系物質である。今回，Caサイトの元素置換によるIrサイトへのキャリアドープ（電子ドープおよびホールドープ）した電気抵抗および熱電能におけるキャリアドープ効果について報告する。また，最近得られた単結晶試料の輸送特性などについても報告する。</p>

DOI： 10.11316/jpsgaiyo.72.1.0_2327

CiNii Article

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）

© 2016 The Author(s). Finding a physical approach for increasing the superconducting transition temperature (T c) is a challenge in the field of material science. Shear strain effects on the superconductivity of rhenium were investigated using magnetic measurements, X-ray diffraction, transmission electron microscopy, and first-principles calculations. A large shear strain reduces the grain size and simultaneously expands the unit cells, resulting in an increase in T c. Here we show that this shear strain approach is a new method for enhancing T c and differs from that using hydrostatic strain. The enhancement of T c is explained by an increase in net electron-electron coupling rather than a change in the density of states near the Fermi level. The shear strain effect in rhenium could be a successful example of manipulating Bardeen-Cooper-Schrieffer-type Cooper pairing, in which the unit cell volumes are indeed a key parameter.

DOI： 10.1038/srep36337

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

We present ab initio GW plus cumulant-expansion calculations for an organic compound (TMTSF)2PF6 and a transition-metal oxide SrVO3. These materials exhibit characteristic low-energy band structures around the Fermi level, which bring about interesting low-energy properties; the low-energy bands near the Fermi level are isolated from the other bands, and, in the isolated bands, unusually low-energy plasmon excitations occur. To study the effect of this low-energy-plasmon fluctuation on the electronic structure, we calculate spectral functions and photoemission spectra using the ab initio cumulant expansion of the Green's function based on the GW self-energy. We found that the low-energy plasmon fluctuation leads to an appreciable renormalization of the low-energy bands and a transfer of the spectral weight into the incoherent part, thus resulting in an agreement with experimental photoemission data.

DOI： 10.1103/PhysRevB.93.085124

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記述言語：日本語   掲載種別：研究論文（その他学術会議資料等）

<p>イリジウム酸化物$\mathrm{Ca_{5} Ir_{3} O_{12}}$ (六方晶)はIrイオンの価数が+4.67と中間価数状態にあり，$\mathrm{Ir O_{6}}$の辺共有からなる1次元鎖が三角格子をなした幾何学的にフラストレートした強相関5d電子系物質である。今回，CaサイトのBiおよび La 置換によるIrサイトへのキャリアドープ（電子ドープ）を行った。磁化，電気抵抗および熱電能におけるキャリアドープ効果について報告する。</p>

DOI： 10.11316/jpsgaiyo.71.2.0_1955

CiNii Article

CiNii Research

担当区分：筆頭著者   記述言語：日本語   掲載種別：研究論文（学術雑誌）

<p>第一原理計算汎用コード「TAPP(東京大学 ab initio program package)」および「ポストTAPP」コードを用いた最近の進捗として, 超伝導転移温度パラメータの評価を実装した。計算アルゴリズムおよびコードの紹介と実証研究に関する結果報告を行う。</p>

DOI： 10.11316/jpsgaiyo.71.2.0_2669

CiNii Article

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）

To investigate the possibility of whether electron-phonon coupling can enhance orbital fluctuations in iron-based superconductors, we develop an ab initio method to construct the effective low-energy models including the phonon-related terms. With the derived effective electron-phonon interactions and phonon frequencies, we estimate the static part (ω=0) of the phonon-mediated effective on site intra- or interorbital electron-electron attractions as ∼-0.4 eV and exchange or pair-hopping terms as ∼-0.02 eV. We analyze the model with the derived interactions together with the Coulomb repulsions within the random phase approximation. We find that the enhancement of the orbital fluctuations due to the electron-phonon interactions is small, and that the spin fluctuations enhanced by the Coulomb repulsions dominate. It leads to the superconducting state with the sign reversal in the gap functions (s± wave). © 2014 American Physical Society.

DOI： 10.1103/PhysRevLett.112.027002

Kyutacar

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記述言語：日本語   掲載種別：研究論文（研究会，シンポジウム資料等）

DOI： 10.11316/jpsgaiyo.69.2.3.0_393_4

CiNii Article

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記述言語：日本語   掲載種別：研究論文（研究会，シンポジウム資料等）

DOI： 10.11316/jpsgaiyo.69.1.2.0_298_1

CiNii Article

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

We present an ab initio GW calculation to study dynamical effects on an organic compound (TMTSF)2PF6. Calculated polarized reflectivities reproduce experimental plasma edges at around 0.2 eV for Eb ′ and 1.0 eV for Ea. The low-energy plasmons come out from the low-energy narrow bands energetically isolated from other higher-energy bands, and affect the low-energy electronic structure via the GW-type self-energy. Because of the quasi-one-dimensional band structure, a distinct plasmaron state is observed along the Y-Γ line and a large plasmon-induced electron scattering is found in the low-energy occupied states along the X-M line. © 2013 American Physical Society.

DOI： 10.1103/PhysRevB.88.125128

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記述言語：日本語   掲載種別：研究論文（研究会，シンポジウム資料等）

DOI： 10.11316/jpsgaiyo.68.2.4.0_774_3

CiNii Article

その他リンク： https://ci.nii.ac.jp/naid/110009756148

記述言語：日本語   掲載種別：研究論文（研究会，シンポジウム資料等）

DOI： 10.11316/jpsgaiyo.68.1.2.0_320_1

CiNii Article

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

We present ab initio two-dimensional extended Hubbard-type multiband models for EtMe 3Sb[Pd(dmit) 2] 2 (where dmit is 1,3-dithiole-2-thione-4,5-dithiolate) and κ-(BEDT-TTF) 2Cu(NCS) 2 [where BEDT-TTF is bis(ethylenedithio)-tetrathiafulvalene] after a downfolding scheme based on the constrained random-phase approximation (cRPA) and maximally localized Wannier orbitals, together with the dimensional downfolding. In the Pd(dmit) 2 salt, the antibonding state of the highest occupied molecular orbital (HOMO) and the bonding/antibonding states of the lowest unoccupied molecular orbital (LUMO) are considered to be the orbital degrees of freedom, while, in the κ-BEDT-TTF salt, the HOMO-antibonding/bonding states are considered. Accordingly, a three-band model for the Pd(dmit) 2 salt and a two-band model for the κ-(BEDT-TTF) salt are derived. We derive single-band models for the HOMO-antibonding state for both of the compounds as well. The HOMO antibonding band of the Pd(dmit) 2 salt has a triangular structure of the transfers with a one-dimensional anisotropy, in contrast to the nearly equilateral triangular structure predicted in the extended Hückel results. The ratio of the larger interchain transfer t b to the intrachain transfer t a is around t b/t a∼0.82. Our calculated screened onsite interaction U and the largest offsite interaction V are ∼0.7 and ∼0.23 eV, respectively, for EtMe 3Sb[Pd(dmit) 2] 2 and ∼0.8 and ∼0.2 eV for κ-(BEDT-TTF) 2Cu(NCS) 2. These values are large enough compared to transfers t as ∼55 meV for the Pd(dmit) 2 salt and ∼65 meV for the κ-BEDT-TTF one, and the resulting large correlation strength (U-V)/t∼10 indicates that the present compounds are classified as the strongly correlated electron systems. In addition, the validity whether the present multiband model can be reduced to the single-band model for the HOMO-antibonding state, widely accepted in the literature, is discussed. For this purpose, we estimated the order of vertex corrections ignored in the cRPA downfolding to the single-band model, which is given by W′/D, where W′ is a full-screened-interaction matrix element between the HOMO-antibonding and other bands away from the Fermi level (namely, HOMO-bonding or LUMO-bonding/antibonding bands), whereas D is the energy distance between the Fermi level and the bands away from the Fermi level. In the present materials, W′/D estimated as 0.3-0.5 signals a substantial correction and thus the exchange process between the low-energy HOMO-antibonding and other bands away from the Fermi level may play a key role to the low-energy ground state. This supports that the minimal models to describe the low-energy phenomena of the organic compounds are the multiband models and may not be reduced to the single-band model. © 2012 American Physical Society.

DOI： 10.1103/PhysRevB.86.205117

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We present an ab initio analysis with density functional theory for superconductors (SCDFT) to understand the superconducting mechanism of doped layered nitrides β-Li xMNCl (M=Ti, Zr, and Hf). The current version of SCDFT is based on the Migdal-Eliashberg theory and has been shown to reproduce accurately experimental superconducting-transition temperatures T c of a wide range of phonon-mediated superconductors. In the present case, however, our calculated T c≤4.3 K (M=Zr) and ≤10.5 K (M=Hf) are found to be less than half of the experimental T c. In addition, T c obtained in the present calculation increases with increasing doping concentration x, opposite to that observed in the experiment. Our results indicate that we need to consider some factors missing in the current SCDFT based on the Migdal-Eliashberg theory. © 2012 American Physical Society.

DOI： 10.1103/PhysRevB.86.054513

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記述言語：英語   掲載種別：研究論文（学術雑誌）

A scheme to incorporate nonlocal polarizations into the dynamical mean-field theory (DMFT) and a tailor-made way to determine the effective interaction for DMFT are systematically investigated. Applying it to the two-dimensional Hubbard model, we find that nonlocal polarizations induce a nontrivial filling-dependent antiscreening effect for the effective interaction. The present scheme combined with density functional theory offers an ab initio way to derive effective on-site interactions for the impurity problem in DMFT. We apply it to SrVO3 and find that the antiscreening competes with the screening caused by the off-site interaction. © 2012 American Physical Society.

DOI： 10.1103/PhysRevB.86.085117

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We predict that iron-based superconductors discovered near d6 configuration (5 Fe 3d orbitals filled by 6 electrons) is located on the foot of an unexpectedly large dome of correlated electron matter centered at the Mott insulator at d5 (namely, half filling). This is based on the many-variable variational Monte Carlo results for abinitio low-energy models derived by the downfolding. The d5 Mott proximity extends to subsequent emergence of incoherent metals, orbital differentiations due to the Mott physics, and Hund's rule coupling, followed by antiferromagnetic quantum criticality, in quantitative accordance with available experiments. © 2012 American Physical Society.

DOI： 10.1103/PhysRevLett.108.177007

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We present a systematic study for understanding the relation between electronic correlation and superconductivity in C 60 and aromatic compounds. We derived, from first principles, extended Hubbard models for twelve compounds: fcc K 3C 60, Rb 3C 60, Cs 3C 60 (with three different lattice constants), A15 Cs 3C 60 (with four different lattice constants), doped solid picene, coronene, and phenanthrene. We show that these compounds are strongly correlated and have similar energy scales of their bandwidths and interaction parameters. However, they have a different trend in the relation between the strength of the electronic correlation and superconducting- transition temperature. While the C 60 compounds have a positive correlation, the aromatic compounds exhibit a negative correlation. © 2012 American Physical Society.

DOI： 10.1103/PhysRevB.85.155452

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We study how transition-metal substitution changes the electronic structure of the iron-based superconductor LaFeAsO in real and momentum space. We first perform ab initio density functional calculation for various sizes of supercells with one transition-metal impurity. For various substitutes (Mn, Co, Ni, Zn, and Ru), we derive effective tight-binding models by constructing the maximally localized Wannier functions from the d bands around the Fermi level. The local electronic structure around the impurity site is quantitatively characterized by their onsite potential and transfer hoppings to neighboring sites. We find that the impurities are classified into three groups according to the derived parameters. For Mn, Co, and Ni, their impurity 3d levels measured from the Fe 3d level are ∼0.3, -0.3, and -0.8 eV, respectively, while, for the Zn case, its d level is extremely deep as ∼8 eV. For the Ru case, although the onsite-level difference is much smaller (∼O(0.1) eV), the transfer integrals around the impurity ion are larger than those of the pure system by 20% ∼ 30%, due to the large spatial spread of the Ru 4d orbitals. We also show that the charge distribution of the extra d electrons is confined around the impurity ion. We then unfold the first Brillouin zone (BZ) for the supercell to calculate the spectral function in the BZ for the normal cell for the case of Co and Ni doping. While the charge distribution seems to suggest that Co and Ni impurities do not change the amount of mobile carriers in the system, the momentum-space analysis clearly shows that the Fermi-surface volume indeed expands by Co and Ni substitutions, which can be well described by the rigid-band shift approximation. © 2012 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ssc.2011.12.048

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We present an ab initio analysis for the ground-state properties of a correlated organic compound κ-(BEDT-TTF) 2Cu(NCS) 2. First, we derive an effective two-dimensional low-energy model from first principles, having shortranged transfers and short-ranged Coulomb and exchange interactions. Then, we perform many-variable variational Monte Carlo calculations for this model and draw a ground-state phase diagram as functions of scaling parameters for the onsite and off-site interactions. The phase diagram consists of three phases; a paramagnetic metallic phase, an antiferromagnetic (Mott) insulating phase, and a charge-ordered phase. In the phase diagram, the parameters for the real compound are close to the first-order Mott transition, being consistent with experiments. We show that the off-site Coulomb and exchange interactions affect the phase boundary; (i) they appreciably stabilize the metallic state against the Mott insulating phase and (ii) enhance charge fluctuations in a wide parameter region in the metallic phase. We observe arc-like structure in Fermi surface around the region where the charge fluctuations are enhanced. Possible relevance of the charge fluctuations to the experimentally observed dielectric anomaly in the κ-BEDT-TTF family compounds is also pointed out. © 2012 The Physical Society of Japan.

DOI： 10.1143/JPSJ.81.034701

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We study how Co- and Ni-substitution affect the electronic structure of the iron-based superconductor, LaFeAsO. We perform ab initio supercell calculations and unfold the first Brillouin zone (BZ) to calculate the spectral function in the BZ for the normal cell. The charge density distribution in real space shows that doped extra electrons are trapped around Co (Ni) atom. This seems to suggest that carriers are not doped by Co(Ni)-substitution. However, the present momentum-space analysis indicates that the Fermi-surface volume indeed expands by substitutions, which can be well described by the rigid-band shift approximation. By taking into account this effective doping, we discuss whether the sign-reversing s-wave (s±-wave) scenario is compatible with experiments. © 2011 The Physical Society of Japan.

DOI： 10.1143/JPSJ.80.123701

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We derive an effective low-energy Hamiltonian for potassium loaded zeolite A, a unique ferromagnet from nonmagnetic elements. We perform ab initio density functional calculations and construct maximally localized Wannier functions for low-energy states made from potassium s electrons. The resulting Wannier orbitals, spreading widely in the alminosilicate cage, are found to be the superatomic s and p orbitals in the confining potential formed by the host cage. We then make a tight-binding model for these superatomic orbitals and introduce interaction parameters such as the Hubbard U. After mean-field calculations for the effective model, we find that ab initio spin density functional results are well reproduced by choosing appropriate sets of the interaction parameters. The interaction parameters turn out to be as large as the band width, ~0.5 eV, indicating the importance of electron correlation, and that the present system is an interesting analog of correlated multi-orbital transition metal oxides. © 2011 The Physical Society of Japan.

DOI： 10.1143/JPSJ.80.124705

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We present a systematic study that clarifies the validity and limitation of current hybrid functionals in density functional theory for structural and electronic properties of various semiconductors and insulators. The three hybrid functionals, PBE0 by Perdew, Ernzerhof, and Burke, HSE by Heyd, Scuseria, and Ernzerhof, and a long-range corrected (LC) functional, are implemented in a well-established plane-wave-basis-set scheme combined with norm-conserving pseudopotentials, thus enabling us to assess the applicability of each functional on an equal footing to the properties of the materials. The materials we have examined range from covalent to ionic materials as well as a rare-gas solid whose energy gaps determined by experiments are in the range of 0.6-14.2 eV, i.e., Ge, Si, BaTiO3, β-GaN, diamond, MgO, NaCl, LiCl, Kr, and LiF. We find that the calculated bulk moduli by the hybrid functionals show better agreement with the experiments than that provided by the generalized-gradient approximation (GGA), whereas the calculated lattice constants by the hybrid functionals and the GGA show comparable accuracy. The calculated energy band gaps and the valence-band widths for the ten prototype materials show substantial improvement using the hybrid functional compared with the GGA. In particular, it is found that the band gaps of the ionic materials as well as the rare-gas solid are well reproduced by the LC-hybrid functional, whereas those of covalent materials are well described by the HSE functional. We also examine exchange effects due to short-range and long-range components of the Coulomb interaction, and we propose an optimum recipe to the short-range and long-range separation in treating the exchange energy. © 2011 American Physical Society.

DOI： 10.1103/PhysRevB.84.075205

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

We present a systematic ab initio study based on density functional calculations to understand impurity effects in iron-based superconductors. Effective tight-binding Hamiltonians for the d bands of LaFeAsO with various transition-metal impurities such as Mn, Co, Ni, Zn, and Ru are constructed using maximally localized Wannier orbitals. Local electronic structures around the impurity are quantitatively characterized by their onsite potential and transfer hoppings to neighboring sites. We found that the impurities are classified into three groups according to the derived parameters: For Mn, Co, and Ni, their impurity 3d levels measured from the Fe 3d level are nearly, 0.3-0.3, and -0.8 eV, respectively, while, for the Zn case, the d level is considerably deep as -8 eV. For the Ru case, although the onsite-level difference is much smaller as O(0.1) eV, the transfer integrals around the impurity site are larger than those of the pure system by 20% ~ 30%, due to the large spatial spread of the Ru 4d orbitals. We also show that, while excess carriers are tightly trapped around the impurity site (due to the Friedel sum rule), there is a rigid shift of band structure near the Fermi level, which has the same effect as carrier doping. © 2011 American Physical Society.

DOI： 10.1103/PhysRevB.83.144512

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記述言語：英語   掲載種別：研究論文（学術雑誌）

By using a variational Monte Carlo method, we examine an effective low-energy model for LaFeAsO derived from an ab initio downfolding scheme. We show that quantum and many-body fluctuations near the antiferromagnetic (AF) quantum critical point largely reduce the antiferromagnetic ordered moment. Our derived model not only quantitatively reproduces the small ordered moment in LaFeAsO, but also accounts for the diversity from LaFePO, BaFe 2As2 to FeTe. Electron correlation is found to determine the observed material dependence. We also find that LaFeAsO is subject to large orbital fluctuations, sandwiched by the AF Mott insulator and weakly correlated metals. The orbital fluctuations and Dirac-cone dispersion hold keys for the diverse magnetic properties. © 2011 The Physical Society of Japan.

DOI： 10.1143/JPSJ.80.023704

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

An ab initio downfolding method is formulated to construct low-dimensional models for correlated electrons. In addition to band downfolding by a constrained random phase approximation formulated for 3D models, screening away from the target layer (chain) is further involved. Eliminating the off-target degrees of freedom, namely, dimensional downfolding, yields ab initio low-dimensional models. The method is applied to derive a 2D model for the layered superconductor LaFeAsO, where interlayer screening crucially makes the effective interaction short-ranged and reduces the onsite Coulomb interactions by 10-20% compared with the 3D model for the five iron-3d orbitals. © 2010 The Physical Society of Japan.

DOI： 10.1143/JPSJ.79.123708

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The magnetic correlation between magnetic impurities in semiconductors is investigated by performing the quantum Monte Carlo (QMC) simulation. The Anderson Hamiltonian with the realistic parameters obtained by the local density approximation (LDA) calculation is employed. The LDA calculation gives a dispersion of the host (GaAs) electron and the mixing energy between host and magnetic impurity (Mn). The mixing between host and impurity electrons generates the impurity bound state in the energy gap of semiconductors. The long range ferromagnetic coupling is observed when the Fermi energy locates between the band edge and the impurity bound state. The ferromagnetic coupling is enhanced by decreasing temperature. © 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2009.06.037

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The isovalent substitution of Te for Se in the superconducting α-FeSe raises TCwhere the average chalcogen-Fe bond angle decreases and the chalcogen-Fe distance increases. Locally, however, the Se and Te ions do not share the same site and have two distinct z coordinates, in contrast to what is presumed in the P4/nmm symmetry. The local bond angle between the chalcogens and Fe increases with the substitution, consistent with the rise in TC, the Fe-Te bonds become shorter than in the binary FeTe, while the Fe-Se bonds stay the same as in the binary. Ab initio calculations based on spin density functional theory yielded an optimized structure with distinct z coordinates for Se and Te, in addition to a stronger hybridization of Te with Fe. © 2010 The American Physical Society.

DOI： 10.1103/PhysRevB.81.134524

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Effective low-energy Hamiltonians for several different families of iron-based superconductors are compared after deriving them from the downfolding scheme based on first-principles calculations. Systematic dependences of the derived model parameters on the families are elucidated, many of which are understood from the systematic variation of the covalency between Fe-3d and pnictogen-/chalcogenp orbitals. First, LaFePO, LaFeAsO (1111), BaFe 2As2 (122), LiFeAs (111), FeSe, and FeTe (11) have overall similar band structures near the Fermi level, where the total widths of 10-fold Fe-3d bands are mostly around 4.5 eV. However, the derived effective models of the 10-fold Fe-3d bands (d model) for FeSe and FeTe have substantially larger effective onsite Coulomb interactions U ̃ 4:2 and 3.4 eV, respectively, after the screening by electrons on other bands and after averaging over orbitals, as compared to ̃2:5 eV for LaFeAsO. The difference is similar in the effective models containing p orbitals of As, Se or Te (dp or dpp model), where U ranges from ̃4 eV for the 1111 family to ̃7 eV for the 11 family. The exchange interaction J has a similar tendency. The family dependence of models indicates a wide variation ranging from weak correlation regime (LaFePO) to substantially strong correlation regime (FeSe). The origin of the larger effective interaction in the 11 family is ascribed to smaller spread of the Wannier orbitals generating larger bare interaction, and to fewer screening channels by the other bands. This variation is primarily derived from the distance h between the pnictogen/chalcogen position and the Fe layer: The longer h for the 11 family generates more ionic character of the bonding between iron and anion atoms, while the shorter h for the 1111 family leads to more covalent-bonding character, the larger spread of the Wannier orbitals, and more efficient screening by the anion p orbitals. The screened interaction of the d model is strongly orbital dependent, which is also understood from the Wannier spread. The dp and dpp models show much weaker orbital dependence. The larger h also explains why the 10-fold 3d bands for the 11 family are more entangled with the smearing of the "pseudogap" structure above the Fermi level seen in the 1111 family. While the family-dependent semimetallic splitting of the bands primarily consists of dyz/dzx and dx 2-y2 orbitals, the size of the pseudogap structure is controlled by the hybridization between these orbitals and dxy/ d3z2-r2 : A large hybridization in the 1111 family generates a large "band-insulating"-like pseudogap (hybridization gap), whereas a large h in the 11 family weakens them, resulting in a "half-filled" like bands of orbitals. This may enhance strong correlation effects in analogy with Mott physics and causes the orbital selective crossover in the three orbitals. On the other hand, the geometrical frustration t'/t, inferred from the ratio of the next-nearest transfer t0 to the nearest one t of the d model is relatively larger for the 1111 family than the 11 one. The models comprehensively derived here may serve as a firm starting basis of understanding both common and diverse properties of the iron-based superconductors including magnetism and superconductivity. © 2010 The Physical Society of Japan.

DOI： 10.1143/JPSJ.79.044705

Kyutacar

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記述言語：英語   掲載種別：研究論文（学術雑誌）

In order to clarify the mechanism of spin polarization in potassium-loaded zeolite A, we perform ab initio density functional calculations. We find that (i) the system comprising only nonmagnetic elements (Al, Si, O, and K) can indeed exhibit ferromagnetism, (ii) the host cage makes a confining quantum-well potential in which superatom s - and p -like states are formed, the latter p states are responsible for the spin polarization, and (iii) the size of the magnetic moment depends sensitively on atomic positions of potassium clusters. We show that the spin polarization can be described systematically in terms of the confining potential and the crystal-field splitting in the superatom p states. © 2009 The American Physical Society.

DOI： 10.1103/PhysRevB.80.220410

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記述言語：英語   掲載種別：研究論文（学術雑誌）

An effective low-energy model describing magnetic properties of alkali-cluster-loaded sodalites is derived by ab initio downfolding. We start with constructing an extended Hubbard model for maximally localized Wannier functions. Ab initio screened Coulomb and exchange interactions are calculated by constrained random-phase approximation. We find that the system resides in the strong-coupling regime and thus the Heisenberg model is derived as a low-energy model of the extended Hubbard model. We obtain antiferromagnetic couplings ∼O (10 K), being consistent with the experimental temperature dependence of the spin susceptibility. Importance of considering the screening effect in the derivation of the extended Hubbard model is discussed. © 2009 The American Physical Society.

DOI： 10.1103/PhysRevB.80.174420

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

We derive effective Hubbard-type Hamiltonians of κ-(BEDT-TTF)2X, using an ab initio downfolding technique, for the first time for organic conductors. They contain dispersions of the highest occupied Wannier-type molecular orbitals with the nearest neighbor transfer t ∼ 0:067 eV for a metal X - Cu(NCS)2and 0.055 eV for a Mott insulator X - Cu2(CN)3, as well as screened Coulomb interactions. It shows unexpected differences from the conventional extended Hückel results, especially much stronger onsite interaction U ∼ 0:8eV (U/t ∼ 12{15) than the Hückel estimates (U/t ∼ 7{8) as well as an appreciable longer-ranged interaction. Reexamination on physics of this family of materials is required from this realistic basis. © 2009 The Physical Society of Japan.

DOI： 10.1143/JPSJ.78.083710

Kyutacar

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We present a realistic study for electronic and magnetic properties in dilute magnetic semiconductor (Ga,Mn)As. A multi-orbital Haldane-Anderson model parameterized by density-functional calculations is presented and solved with the Hirsch-Fye quantum Monte Carlo algorithm. Results well reproduce experimental results in the dilute limit. When the chemical potential is located between the top of the valence band and an impurity bound state, a long-range ferromagnetic correlations between the impurities, mediated by antiferromagnetic impurity-host couplings, are drastically developed. We observe an anisotropic character in local density of states at the impurity-bound-state energy, which is consistent with the STM measurements. The presented combined approach thus offers a firm starting point for realistic calculations of the various family of dilute magnetic semiconductors. © 2009 The Physical Society of Japan.

DOI： 10.1143/JPSJ.78.083703

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

Effective Hamiltonians for LaFeAsO and LaFePO are derived from the downfolding scheme based on first-principles calculations and provide insights for newly discovered superconductors in the family of LnFeAsO1-xFx, Ln = La, Ce, Pr, Nd, Sm, and Gd. Extended Hubbard Hamiltonians for five maximally localized Wannier orbitals per Fe are constructed dominantly from five-fold degenerate iron-3d bands. They contain parameters for effective Coulomb and exchange interactions screened by the polarization of other electrons away from the Fermi level. The onsite Coulomb interaction estimated as 2.2 - 3.3 eV is compared with the transfer integrals between the nearest-neighbor Fe-3d Wannier orbitals, 0.2 -0.3 eV, indicating moderately strong electron correlation. The Hund's rule coupling is found to be 0.3- 0.6 eV. The derived model offers a firm basis for further studies on physics of this family of materials. The effective models for As and P compounds turn out to have very similar screened interactions with slightly narrower bandwidth for the As compound. ©2008 The Physical Society of Japan.

DOI： 10.1143/JPSJ.77.093711

Kyutacar

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We examine whether the essence and quantitative aspects of electronic excitation spectra are correctly captured by an effective low-energy model constructed from an ab initio downfolding scheme. A global electronic structure is first calculated by ab initio density-functional calculations with the generalized gradient approximation. With the help of constrained density-functional theory, the low-energy effective Hamiltonian for bands near the Fermi level is constructed by the downfolding procedure in the basis of maximally localized Wannier functions. The excited states of this low-energy effective Hamiltonian ascribed to an extended Hubbard model are calculated by using a low-energy solver. As the solver, we employ the Hartree-Fock approximation supplemented by the single-excitation configuration-interaction method considering electron-hole interactions. The present three-stage method is applied to GaAs, where eight bands are retained in the effective model after the downfolding. The resulting spectra well reproduce the experimental results, which indicate that our downfolding scheme offers a satisfactory framework of the electronic-structure calculation, particularly for the excitations and dynamics as well as for the ground state. © 2008 The American Physical Society.

DOI： 10.1103/PhysRevB.77.195126

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We present an ab initio method for calculating effective onsite Coulomb interactions of solid. The method is based on constrained local density functional theory formulated in terms of maximally localized Wannier functions. This scheme can be implemented with any basis, and thus allows us to perform the constrained calculation with plane-wave-based electronic-structure codes. We apply the developed method to the evaluation of the onsite interaction of 3 d transition-metal series. The results are discussed using a heuristic formula for screened Coulomb interactions. © 2006 The American Physical Society.

DOI： 10.1103/PhysRevB.74.235113

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記述言語：英語   掲載種別：研究論文（学術雑誌）

In this paper we present theoretical analyses on an infrared (IR) spectrum of amorphous selenium. The system is described by a 216-atom-chain model, and a set of molecular-dynamics simulations is performed to generate vitreous structures and vibrational modes. To describe an electronic structure of the system we employ a complete neglect of differential overlap model parametrized by ab initio cluster calculations. An IR intensity is evaluated with the Berry-phase formula for an electronic polarization. The effect of the through-space electron transfer on the IR spectrum is studied by artificially changing the magnitude of matrix elements associated with the electron transfer between nonbonded atoms in the chain. We find that the through-space electron transfer leads to (i) the enhancement of the bending IR peak at 135 cm-1 and (ii) the appearance of a new low-frequency peak around 50 cm-1, thus resulting in a good agreement with the experiment. The mechanism is discussed by a simple dipole model. © 2005 American Institute of Physics.

DOI： 10.1063/1.1898217

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記述言語：英語   掲載種別：研究論文（学術雑誌）

This paper presents development and motion analysis of a cleaner robot for vertical type air conditioning duct. The air conditioning duct is used for ventilation and air conditioning. After years use of the duct without cleaning, much dust is accumulated inside the duct. That leads scattering the dust into rooms. This is not desirable in the view point of sanitation and health. Thus, periodic cleaning of the duct is requested. For the cleaning work, an air hose type cleaner robot is developed. An air nozzle is attached with the tip of the hose type robot. Compressed air blows off the dust from the air nozzle, then the dust inside the duct is collected by a dust collector with a filter. The part of nozzle is manipulated by wires' constraint using two DC motors. By watching TV monitor which shows inside of the duct, the cleaning work is done remotely using a control stick operation. To control the nozzle of the hose robot, the paper discusses a relation between position of nozzle (tip of hose robot) and wire's length. Discussions on wire force for the position is also given. An experiment of the motion for the hose robot is presented, then theoretical solutions and experimental results are compared.

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

In this paper we present a theoretical study on a glassy structure and infrared (IR)/Raman spectra of amorphous selenium (a-Se). The vibrational spectra are calculated for two-hundred independent amorphous samples obtained by performing molecular-dynamics (MD) simulations for one linear chain of 216 Se atoms. The interaction potential used in the MD simulation is constructed on the basis of ab initio molecular-orbital calculations (with the Hartree-Fock and second-order Møller-Placed methods) for various Se clusters. The parameters in the bond-current and bond-polarizability models needed for the spectral calculations are also determined by the same ab initio approach. The calculated static structure factor, vibrational density of states, and IR/Raman spectra well reproduce experimental results. It is found that there exists a remarkable medium-range order in the disordered Se chain; i.e., a chain segment having four consecutive dihedral angles with alternate signs [i.e., (+, -, +, -) or (-, +, -, +)] tends to be excluded from the chain structure, which is attributed to the steric hindrance effect existing in those configurations. By comparing the polarized Raman spectra for two different random chains with and without the above steric hindrance effect, we find that only the former spectrum exhibits a distinct peak at 80 cm-1which is observed also in the experiment. Therefore the appearance of this peak provides useful information on the medium-range order existing in the disordered Se chain.

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記述言語：英語   掲載種別：研究論文（学術雑誌）

In this paper we calculated the phonon dispersion curves and the infrared (IR)/Raman spectra of an infinite regular helical selenium (Se) chain. The ingredients needed for their calculations, i.e., the force constant matrix, dynamical-charge tensor (DCT), and polarizability-derivative tensor (PDT), were obtained from ab initio molecular orbital calculations with the second-order Møller-Plesset perturbation theory for a Se chain with finite length. Assignments for the IR and Raman spectra were performed in terms of a rotational angle τ of the helix; i.e., the phonon modes with wave number Q=0 or τ are IR active, while the phonon modes with Q=0, τ, or 2τ are Raman active. Therefore, IR and Raman spectroscopy are useful, not only for identifying the static structure but also for deriving the phonon dispersions of the Se chain. From analyses based on a valence force field model, we found that the ab initio phonon dispersion curve of the stretching band strongly depends on the off-diagonal couplings in the force constant matrix and on the chain geometry, especially the bond angle. The ab initio DCT and PDT were also analyzed with the so-called bond-current and bond-polarizability models, respectively. We found that these simple models reproduce the ab initio IR/Raman intensities quite accurately.

DOI： 10.1103/PhysRevB.66.024306

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記述言語：英語   掲載種別：研究論文（学術雑誌）

A chain model based on ab initio molecular orbital (MO) calculations was used to study the vibrational spectra of amorphous selenium (Se). The Infrared (IR) and Raman spectra of amorphous selenium were calculated for atomic structures obtained by classical molecular dynamics simulations. The phonon dispersion of a regular helical Se chain was used to explain the relative position between the main IR and polarized Raman peaks in the stretching band.

DOI： 10.1016/S0022-3093(02)01680-0

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The infrared (IR) absorption spectrum of amorphous selenium (a-Se) are calculated for the atomic structure constructed by a molecular dynamics with recently proposed chain model using an semiempirical electronic structure calculation. Its microscopic mechanism is investigated with an IR formula for chains in terms of bond and induced current correlations. The stretching band in the IR spectrum has two peaks, which are related to the intermediate range order that dihedral angle sequences with the signs of (+,-,+,-) and (-,+,-,+) hardly occur due to steric hindrance. The result urges us to modify the conventional picture that a-Se consists of the polymer chains with completely random sequences of the signs of the dihedral angles. © 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0010-4655(01)00364-2

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Atomic and electronic structures of semiconducting liquid and amorphous selenium are studied with a chain model including an anisotropic interaction between the Se atoms due to the p-lone pair on each atom.

DOI： 10.1143/PTPS.138.266

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講演種別：招待講演  

講演種別：招待講演  

講演種別：招待講演   開催地：東京大学物性研究所  

講演種別：招待講演   開催地：東京大学物性研究所  

講演種別：招待講演   開催地：東京大学柏フューチャーセンター  

講演種別：招待講演   開催地：早稲田大学  

講演種別：招待講演   開催地：東京大学理学部  

講演種別：招待講演   開催地：The University of Tokyo  

開催期間： 2015年01月   発表言語：日本語   講演種別：特別講演  

CiNii Article

講演種別：招待講演   開催地：The University of Tokyo  

講演種別：招待講演   開催地：東京大学理学部  

講演種別：招待講演   開催地：山形大学理学部  

講演種別：招待講演   開催地：勝浦観光ホテル  

講演種別：招待講演   開催地：Juelich FORSCHUNGSZENTRUM, Germany  

講演種別：招待講演   開催地：Ecole Polytechnique, France  

講演種別：招待講演   開催地：Seč-Ústupky, Czech Republic  

講演種別：招待講演   開催地：The international House of Japan, Minato-ku Tokyo  

高温超伝導体のさらなる転移温度向上を目指した物質設計ユニットを本学の教員8名より形成し，学内プロジェクトとして研究を行っている。

2013年5月より九州工業大学物性グループセミナーを組織して定期的にセミナーを開催している。本学の18名の教員が参加している。2018年3月までに38回のセミナーを開催している。これらのセミナーを通した本学教員の交流により、共同研究、科研費の共同提出、学内プロジェクトへの共同提出（三件採択）に繋がっている。