土射津 昌久 (ツチイズ マサヒサ)

汎関数くりこみ群法による強相関電子系の研究 ―2次元電子系におけるネマティック秩序―
2020年05月, 固体物理, 55 (5), 195  203研究論文（学術雑誌）
Exploring new densityfunctional embedding techniques for strongly correlated electrons: From model to ab initio Hamiltonians
2018年08月19日, ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 256, web_of_scienceSiteoccupation embedding theory using Bethe ansatz local density approximations
Siteoccupation embedding theory (SOET) is an alternative formulation of density functional theory (DFT) for model Hamiltonians where the fully interacting Hubbard problem is mapped, in principle exactly, onto an impurityinteracting (rather than a noninteracting) one. It provides a rigorous framework for combining wavefunction (or Green function)based methods with DFT. In this work, exact expressions for the persite energy and double occupation of the uniform Hubbard model are derived in the context of SOET. As readily seen from these derivations, the socalled bath contribution to the persite correlation energy is, in addition to the latter, the key density functional quantity to model in SOET. Various approximations based on Bethe ansatz and perturbative solutions to the Hubbard and singleimpurity Anderson models are constructed and tested on a onedimensional ring. The selfconsistent calculation of the embedded impurity wave function has been performed with the densitymatrix renormalization group method. It has been shown that promising results are obtained in specific regimes of correlation and density. Possible further developments have been proposed in order to provide reliable embedding functionals and potentials., 2018年06月06日, Physical Review B, 97 (23), 235105, doi研究論文（学術雑誌）
Multistage electronic nematic transitions in cuprate superconductors: A functionalrenormalizationgroup analysis
Recently, complex rotational symmetrybreaking phenomena have been discovered experimentally in cuprate superconductors. To find the realized order parameters, we study various unconventional charge susceptibilities in an unbiased way by applying the functionalrenormalizationgroup method to the dp Hubbard model. Without assuming the wave vector of the order parameter, we reveal that the most dominant instability is the uniform (q=0) charge modulation on the px and py orbitals, which possesses d symmetry. This uniform nematic order triggers another nematic porbital density wave along the axial (CuCu) direction at Qa≈(π/2,0). It is predicted that uniform nematic order is driven by the spin fluctuations in the pseudogap region, and another nematic densitywave order at q=Qa is triggered by the uniform order. The predicted multistage nematic transitions are caused by AslamazovLarkintype fluctuationexchange processes., 2018年04月19日, Physical Review B, 97 (16), 165131, doi研究論文（学術雑誌）
An exotic band structure of a supramolecular honeycomb lattice formed by a pancake ππ interaction between triradical trianions of triptycene tribenzoquinone
A supramolecular honeycomb lattice was successfully formed by using triradical trianion species of triptycene tribenzoquinone (TT) which forms strong intermolecular ππ pancake bonds toward three directions. The crystal structure of Rb3TT·2H2O belongs to the hexagonal P6/m space group, the tightbinding band calculation of which reveals Dirac cones and flat bands., 2018年, Chem. Commun., 54 (31), 3815  3818, doi研究論文（学術雑誌）
Plain sWave Superconductivity near Magnetic Criticality: Enhancement of Attractive Electron–Boson Coupling Vertex Corrections
Recent experiments revealed that the plain swave state without any signreversal emerges in various metals near magnetic criticality. To understand this counterintuitive phenomenon, we study the gap equation for the multiorbital HubbardHolstein model, by analyzing the vertex correction (VC) due to the higherorder electroncorrelation effects. We find that the phononmediated orbital fluctuations are magnified by the VC for the susceptibility (chiVC). In addition, the chargechannel attractive interaction is enlarged by the VC for the couplingconstant (UVC), which is significant when the interaction has prominent qdependences; therefore the Migdal theorem fails. Due to both chiVC and UVC, the plain swave state is caused by the small electronphonon interaction near the magnetic criticality against the repulsive Coulomb interaction. We find that the direct Coulomb repulsion for the plain swave Cooper pair is strongly reduced by the " multiorbital screening effect"., 2017年07月, J. Phys. Soc. Jpn., 86 (7), 073703, doi;web_of_science研究論文（学術雑誌）
3D SpinLiquid State in an Organic Hyperkagome Lattice of Mott Dimers
We report the first 3D spin liquid state of isotropic organic spins. Structural analysis, and magnetic and heatcapacity measurements were carried out for a chiral organic radical salt, (TBA)(1.5) [()NDIDelta] (TBA denotes tetrabutylammonium and NDI denotes naphthalene diimide), in which ()NDIDelta forms a K4 structure due to its triangular molecular structure and an intermolecular pipi overlap between the NDI moieties. This lattice was identical to the hyperkagome lattice of S = 1/2 Mott dimers, and should exhibit 3D spin frustration. In fact, even though the hightemperature magnetic susceptibility followed the CurieWeiss law with a negative Weiss constant of theta = 15 K, the lowtemperature magnetic measurements revealed no longrange magnetic ordering down to 70 mK, and suggested the presence of a spin liquid state with a large residual paramagnetism chi(0) of 8.5 x 10(6) emu g(1) at the absolute zero temperature. This was supported by the N14 NMR measurements down to 0.38 K. Further, the lowtemperature heat capacities c(p) down to 68 mK clearly indicated the presence of c(p) for the spin liquid state, which can be fitted to the power law of T0.62 in the wide temperature range 0.074.5 K., 2017年07月, PHYSICAL REVIEW LETTERS, 119 (5), 057201, doi;web_of_science研究論文（学術雑誌）
Competing Unconventional ChargeDensityWave States in Cuprate Superconductors: SpinFluctuationDriven Mechanism
To understand the origin of unconventional chargedensitywave (CDW) states in cuprate superconductors, we establish the selfconsistent CDW equation, and analyze the CDW instabilities based on the realistic Hubbard model, without assuming any qdependence and the form factor. Many higherorder manybody processes, which are called the vertex corrections, are systematically generated by solving the CDW equation. When the spin fluctuations are strong, the uniform q = 0 nematic CDW with dform factor shows the leading instability. The axial nematic CDW instability at q = Q(a) = (delta, 0) (delta approximate to pi/2) is the second strongest, and its strength increases under the static uniform CDW order. The present theory predicts that uniform CDW transition emerges at a high temperature, and it stabilize the axial q = Qa CDW at T = TCDW. It is confirmed that the higherorder AslamazovLarkin processes cause the CDW orders at both q = 0 and Q(a)., 2017年06月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 86 (6), 063707, doi;web_of_science研究論文（学術雑誌）
Local density approximation in siteoccupation embedding theory
Siteoccupation embedding theory (SOET) is a density functional theory (DFT)based method which aims at modelling strongly correlated electrons. It is in principle exact and applicable to model and quantum chemical Hamiltonians. The theory is presented here for the Hubbard Hamiltonian. In contrast to conventional DFT approaches, the site (or orbital) occupations are deduced in SOET from a partially interacting system consisting of one (or more) impurity site(s) and noninteracting bath sites. The correlation energy of the bath is then treated implicitly by means of a siteoccupation functional. In this work, we propose a simple impurityoccupation functional approximation based on the twolevel (2L) Hubbard model which is referred to as twolevel impurity local density approximation (2LILDA). Results obtained on a prototypical uniform eightsite Hubbard ring are promising. The extension of the method to larger systems and more sophisticated model Hamiltonians is currently in progress. [GRAPHICS], 2017年, MOLECULAR PHYSICS, 115 (12), 48  62, doi;web_of_science研究論文（学術雑誌）
Threedimensional higherspin Dirac and Weyl dispersions in the strongly isotropic K4 crystal
We analyze the electronic structure in the threedimensional (3D) crystal formed by the sp(2) hybridized orbitals (K4 crystal), by the tightbinding approach based on the firstprinciples calculation. We discover that the bulk Diraccone dispersions are realized in the K4 crystal. In contrast to the graphene, the energy dispersions of the Dirac cones are isotropic in 3D and the pseudospin S = 1 Dirac cones emerge at the Gamma and H points of the bcc Brillouin zone, where three bands become degenerate and merge at a single point belonging to the T2 irreducible representation. In addition, the usual S = 1/2 Dirac cones emerge at the P point. By focusing the hoppings between the nearestneighbor sites, we show an analytic form of the tightbinding Hamiltonian with a 4 x 4 matrix, and we give an explicit derivation of the S = 1 and S = 1/2 Diraccone dispersions. We also analyze the effect of the spinorbit coupling to examine how the degeneracies at Dirac points are lifted. At the S = 1 Dirac points, the spinorbit coupling lifts the energy level with sixfold degeneracy into two energy levels with twodimensional (E) over bar (2) and fourdimensional (F) over bar representations. Remarkably, all the dispersions near the (F) over bar point show the linear dependence in the momentum with different velocities. We derive the effective Hamiltonian near the (F) over bar point and find that the band contact point is described by the S = 3/2 Weyl point., 2016年11月, PHYSICAL REVIEW B, 94 (19), 195426, doi;web_of_science研究論文（学術雑誌）
Phase Competition, Solitons, and Domain Walls in NeutralIonic Transition Systems
Phase competition and excitations in the onedimensional neutralionic transition systems are theoretically studied comprehensively. From the semiclassical treatment of the bosonized Hamiltonian, we examine the competition among the neutral (N), ferroelectricionic (Iferro), and paraelectricionic (Ipara) states. The phase transitions between them can become firstorder when the fluctuationinduced higherorder commensurability potential is considered. In particular, the description of the firstorder phase boundary between N and Iferro enables us to analyze NIferro domain walls. Soliton excitations in the three phases are described explicitly and their formation energies are evaluated across the phase boundaries. The characters of the soliton and domainwall excitations are classified in terms of the topological charge and spin. The relevance to the experimental observations in the molecular neutralionic transition systems is discussed. We ascribe the pressureinduced crossover in tetrathiafulvalenepchloranil (TTFCA) at a hightemperature region to that from the N to the Ipara state, and discuss its consequence., 2016年10月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 85 (10), 104705, doi;web_of_science研究論文（学術雑誌）
Functional renormalization group study of orbital fluctuation mediated superconductivity: Impact of the electronboson coupling vertex corrections
In various multiorbital systems, the emergence of the orbital fluctuations and their role on the pairing mechanism attract increasing attention. To achieve deep understanding on these issues, we perform a functional renormalization group (fRG) study for the twoorbital Hubbard model. The vertex corrections for the electronboson coupling (UVC), which are dropped in the MigdalEliashberg gap equation, are obtained by solving the RG equation. We reveal that the dressed electronboson coupling for the charge channel (U) over cap (c)(eff) becomes much larger than the bare Coulomb interaction (U) over cap (0) due to the UVC in the presence of moderate spin fluctuations. For this reason, the attractive pairing interaction due to the charge or orbital fluctuations is enlarged by the factor ((U) over cap (c)(eff) >> 1. In contrast, the spin fluctuation pairing interaction is suppressed by the spinchannel UVC, because of the relation (U) over cap (s)(eff) << (U) over cap (0). The present study demonstrates that the orbital or charge fluctuation pairing mechanism can be realized in various multiorbital systems thanks to the UVC, such as in Febased superconductors., 2016年09月, PHYSICAL REVIEW B, 94 (11), 115155, doi;web_of_science研究論文（学術雑誌）
Discovery of the K_4 Structure Formed by a Triangular π Radical Anion
The K4 structure was theoretically predicted for trivalent chemical species, such as sp(2) carbon. However, since attempts to synthesize the K4 carbon have not succeeded, this allotrope has been regarded as a crystal form that might not exist in nature. In the present work, we carried out electrochemical crystallization of the radical anion salts of a triangular molecule, naphthalene diimide (NDI)Delta, using various electrolytes. Xray crystal analysis of the obtained crystals revealed the K4 structure, which was formed by the unique intermolecular pi overlap directed toward three directions from the triangularshape NDIDelta radical anions: Electron paramagnetic resonance and static magnetic measurements confirmed the radical anion state of NDIDelta and indicated an antiferromagnetic intermolecular interaction With the Weiss constant of theta = 10 K. The band structure calculation suggested characteristic features of the present material, such as a metallic ground state, Dirac cones, and flat bands., 2015年06月, J. Am. Chem. Soc., 137 (24), 7612  7615, doi;web_of_science研究論文（学術雑誌）
Spintriplet superconductivity in Sr2RuO4 due to orbital and spin fluctuations: Analyses by twodimensional renormalization group theory and selfconsistent vertexcorrection method
We study the mechanism of the triplet superconductivity (TSC) in Sr2RuO4 based on the multiorbital Hubbard model. The electronic states are studied using the recently developed renormalization group method combined with the constrained randomphase approximation, called the RG + cRPA method. Thanks to the vertex correction (VC) for the susceptibility, which is dropped in the meanfieldlevel approximations, strong orbital and spin fluctuations at Q approximate to (2 pi/3,2 pi/3) emerge in the quasionedimensional Fermi surfaces (FSs) composed of d(xz) + d(yz) orbitals. Due to the cooperation of both fluctuations, we obtain the triplet superconductivity in the Eu representation, in which the superconducting gap is given by the linear combination of (Delta(x)(k), Delta y(k)) similar to (sin 3k(x), sin 3k(y)). Very similar results are obtained by applying the diagrammatic calculation called the selfconsistent VC method. Thus, the idea of "orbital + spin fluctuation mediated TSC" is confirmed by both the RG + cRPA method and the selfconsistent VC method. We also revealed that the large superconducting gap on the d(xy)orbital Fermi surface is induced from gaps on the quasionedimensional FSs, in consequence of the large orbital mixture due to the 4d spinorbit interaction., 2015年04月, PHYSICAL REVIEW B, 91 (15), 155103, doi;web_of_science研究論文（学術雑誌）
Composite pairing and superfluidity in a onedimensional resonant BoseFermi mixture
We study the groundstate properties of onedimensional mixtures of bosonic and fermionic atoms resonantly coupled to fermionic Feshbach molecules. When the particle densities of fermionic atoms and Feshbach molecules differ, the system undergoes various depletion transitions between binary and ternary mixtures as a function of the detuning parameter. However, when the particle densities of fermionic atoms and Feshbach molecules are identical, the molecular conversion and disassociation processes induce a gap in a sector of lowenergy excitations, and the remaining system can be described by a twocomponent TomonagaLuttinger liquid. Using a bosonization scheme, we derive the effective lowenergy Hamiltonian for the system, which has a similar form as that of the twochain problem of coupled TomonagaLuttinger liquids. With the help of improved perturbative renormalization group analysis of the latter problem, we determine the groundstate phase diagram and find that it contains a phase dominated by composite superfluid or pairing correlations between the open and closed resonant channels. © 2013 American Physical Society., 2013年10月16日, Physical Review A  Atomic, Molecular, and Optical Physics, 88 (4), 043620, doi研究論文（学術雑誌）
Orbital Nematic Instability in the TwoOrbital Hubbard Model: RenormalizationGroup plus Constrained RPA Analysis
Motivated by the nematic electronic fluid phase in Sr3Ru2O7, we develop a combined scheme of the renormalizationgroup method and the randomphaseapproximationtype method, and analyze orbital susceptibilities of the (d(xz), d(yz))orbital Hubbard model with high accuracy. It is confirmed that the present model exhibits a ferroorbital instability near the magnetic or superconducting quantum criticality, due to the AslamazovLarkintype vertex corrections. This mechanism of orbital nematic order presents a natural explanation for the nematic order in Sr3Ru2O7, and is expected to be realized in various multiorbital systems, such as Febased superconductors., 2013年07月, PHYSICAL REVIEW LETTERS, 111 (5), 057003, doi;web_of_science研究論文（学術雑誌）
Cation Dependence of the Electronic States in Molecular Triangular Lattice System β'X[Pd(dmit)_2]_2: A FirstPrinciples Study
The electronic structure of an isostructural series of molecular conductors, β'X[Pd(dmit)2]2, is systematically studied by a firstprinciples method based on the densityfunctional theory. The calculated band structures are fitted to the tightbinding model based on Pd(dmit)2 dimers on the triangular lattice. We find a systematic variation in the anisotropy of the transfer integrals along the three directions of the triangular lattice taking different values. The transfer integral along the facetoface stacking direction of Pd(dmit)2 dimers is always the largest. Around the quantum spin liquid, X = EtMe3Sb, the other two transfer integrals become comparable. We also report sensible differences in the distribution of wavefunctions near the Fermi level between the two dmit ligands of the Pd(dmit)2 molecule. © 2013 The Physical Society of Japan., 2013年03月, J. Phys. Soc. Jpn., 82 (3), 033709, doi;scopus;scopus_citedby研究論文（学術雑誌）
Spinfluctuationdriven orbital nematic order in Ruoxides: Selfconsistent vertex correction analysis for twoorbital model
To reveal the origin of the "nematic electronic fluid phase" in Sr3Ru2O7, we apply the selfconsistent vertex correction analysis to the (dxz, dyz)orbital Hubbard model. It is found that the AslamazovLarkin type vertex correction causes the strong coupling between spin and orbital fluctuations, which corresponds to the KugelKhomskii spinorbital coupling in the local picture. Owing to this mechanism, orbital nematic order with C2 symmetry is induced by the magnetic quantum criticality in multiorbital systems, whereas this mechanism is ignored in the randomphaseapproximation (RPA). This study provides a natural explanation for the close relationship between the magnetic quantum criticality and nematic state in Sr3Ru2O7 and Febased superconductors. © 2013 The Physical Society of Japan., 2013年01月, Journal of the Physical Society of Japan, 82 (1), 013707, doi研究論文（学術雑誌）
Ab initio derivation of multiorbital extended Hubbard model for molecular crystals
From configuration interaction (CI) ab initio calculations, we derive an effective twoorbital extended Hubbard model based on the gerade (g) and ungerade (u) molecular orbitals (MOs) of the chargetransfer molecular conductor (TTMTTP)I3 and the singlecomponent molecular conductor [Au(tmdt)(2)]. First, by focusing on the isolated molecule, we determine the parameters for the model Hamiltonian so as to reproduce the CI Hamiltonian matrix. Next, we extend the analysis to two neighboring molecule pairs in the crystal and we perform similar calculations to evaluate the intermolecular interactions. From the resulting tightbinding parameters, we analyze the band structure to confirm that two bands overlap and mix in together, supporting the multiband feature. Furthermore, using a fragment decomposition, we derive the effective model based on the fragment MOs and show that the staking TTMTTP molecules can be described by the zigzag twoleg ladder with the intermolecular transfer integral being larger than the intrafragment transfer integral within the molecule. The intersite interactions between the fragments follow a Coulomb law, supporting the fragment decomposition strategy. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3678307], 2012年01月, JOURNAL OF CHEMICAL PHYSICS, 136 (4), 044519, doi;web_of_science研究論文（学術雑誌）
Spectral functions in a twovelocities TomonagaLuttinger Liquid
We obtain the spectral functions of a twovelocities TomonagaLuttinger liquid at T = 0K in terms of the Appell hypergeometric functions F1 and F2. In the case of SU(N) spin symmetry, the spectral functions can be expressed with the Gauss hypergeometric functions. We discuss the singularities and thresholds of the spectral functions for both the SU(N) invariant case and the general case., 2012年, INTERNATIONAL CONFERENCE ON STRONGLY CORRELATED ELECTRON SYSTEMS (SCES 2011), 391, 012163, doi;web_of_science研究論文（国際会議プロシーディングス）
Spectral functions of twoband spinless fermion and singleband spin1/2 fermion models
We examine zerotemperature oneparticle spectral functions for the onedimensional twoband spinless fermions with different velocities and general forwardscattering interactions. By using the bosonization technique and diagonalizing the model to two TomonagaLuttinger liquid Hamiltonians, we obtain general expressions for the spectral functions, which are given in terms of the Appell hypergeometric functions. For the case of identical twoband fermions, corresponding to the SU(2) symmetric spin1/2 fermions with repulsive interactions, the spectral functions can be expressed in terms of the Gauss hypergeometric function and are shown to recover the doublepeak structure suggesting the wellknown "spincharge" separation. By tuning the difference in velocities for the twoband fermions, we clarify the crossover in spectral functions from the spincharge separation to the decoupled fermions. We discuss the relevance of our results to the spin1/2 Hubbard model under a magnetic field, which can be mapped onto twoband spinless fermions., 2011年10月, PHYSICAL REVIEW B, 84 (16), 165128, doi;web_of_science研究論文（学術雑誌）
MeanField Theory of IntraMolecular Charge Ordering in (TTMTTP)I3
We examine an intramolecular chargeordered (ICO) state in the multiorbital molecular compound (TTMTTP)I3 on the basis of an effective twoorbital model derived from ab initio calculations. Representing the model in terms of the fragment molecularorbital (MO) picture, the ICO state is described as the charge disproportionation on the left and right fragment MOs. By applying the meanfield theory, the phase diagram of the ground state is obtained as a function of the intermolecular Coulomb repulsion and the intramolecular transfer integral. The ICO state is stabilized by large interfragment Coulomb interactions, and the small intramolecular transfer energy between two fragment MOs. Furthermore, we examine the finitetemperature phase diagram. The relevance to the experimental observations in the molecular compound of (TTMTTP)I3 is also discussed., 2011年02月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 80 (2), 024707, doi;web_of_science研究論文（学術雑誌）
Multiorbital molecular compound (TTMTTP)I3: Effective model and fragment decomposition
The electronic structure of the molecular compound (TTMTTP)I3, which exhibits a peculiar intramolecular charge ordering, has been studied using multiconfiguration ab initio calculations. First we derive an effective Hubbardtype model based on the molecular orbitals (MOs) of TTMTTP we set up a twoorbital Hamiltonian for the two MOs near the Fermi energy and determine its full parameters: the transfer integrals, the Coulomb and exchange interactions. The tightbinding band structure obtained from these transfer integrals is consistent with the result of the direct band calculation based on density functional theory. Then, by decomposing the frontier MOs into two parts, i.e., fragments, we find that the stacked TTMTTP molecules can be described by a twoleg ladder model, while the interfragment Coulomb energies are scaled to the inverse of their distances. This result indicates that the fragment picture that we proposed earlier [M.L. Bonnet et al.: J. Chem. Phys. 132(2010) 214705] successfully describes the lowenergy properties of this compound. © 2011 The Physical Society of Japan., 2011年01月, Journal of the Physical Society of Japan, 80 (1), 013703, doi研究論文（学術雑誌）
FiniteTemperature Properties across the Charge Ordering Transition –Combined Bosonization, Renormalization Group, and Numerical Methods–
We theoretically describe the charge ordering (CO) metalinsulator transition based on a quasionedimensional extended Hubbard model, and investigate the finite temperature (T) properties across the transition temperature, TCO. In order to calculate T dependence of physical quantities such as the spin susceptibility and the electrical resistivity, both above and below TCO, a theoretical scheme is developed which combines analytical methods with numerical calculations. We take advantage of the renormalization group equations derived from the effective bosonized Hamiltonian, where Lanczos exact diagonalization data are chosen as initial parameters, while the CO order parameter at finiteT is determined by quantum Monte Carlo simulations. The results show that the spin susceptibility does not show a steep singularity at TCO, and it slightly increases compared to the case without CO because of the suppression of the spin velocity. In contrast, the resistivity exhibits a sudden increase at TCO, below which a characteristic T dependence is observed. We also compare our results with experiments on molecular conductors as well as transition metal oxides showing CO., 2010年09月, J. Phys. Soc. Jpn., 79 (9), 094714, doi;web_of_science研究論文（学術雑誌）
Intramolecular charge ordering in the multi molecular orbital system (TTMTTP)I3
Starting from the structure of the (TTMTTP)I3 molecularbased material, we examine the characteristics of frontier molecular orbitals using ab initio (CASSCF/CASPT2) configurations interaction calculations. It is shown that the singly occupied and secondhighestoccupied molecular orbitals are close to each other, i.e., this compound should be regarded as a twoorbital system. By dividing virtually the [TTMTTP] molecule into three fragments, an effective model is constructed to rationalize the origin of this picture. In order to investigate the lowtemperature, symmetry breaking experimentally observed in the crystal, the electronic distribution in a pair of [TTMTTP] molecules is analyzed from CASPT2 calculations. Our inspection supports and explains the speculated intramolecular charge ordering which is likely to give rise to lowenergy magnetic properties. (C) 2010 American Institute of Physics. [doi:10.1063/1.3432764], 2010年06月, JOURNAL OF CHEMICAL PHYSICS, 132 (21), 214705, doi;web_of_science研究論文（学術雑誌）
Competition of superfluidity and density waves in onedimensional BoseFermi mixtures
We study a mixture of onedimensional bosons and spinless fermions at incommensurate filling using phenomenological bosonization and Green's function techniques. We derive the relation between the parameters of the microscopic Hamiltonian and macroscopic observables. Galilean invariance results in extra constraints for the currentcurrent interactions. We obtain the exact exponents for the various response functions, and show that superfluid fluctuations are enhanced by the effective bosonfermion densitydensity interaction and suppressed by the effective bosonfermion currentcurrent interaction. In the case of a bosonized model with purely densitydensity interaction, when the effective bosonfermion densitydensity interaction is weak enough, the superfluid exponent of the bosons has a nonmonotonic variation with the ratio of the fermion velocity to the boson velocity. In contrast, the densitywave exponent and the exponent for fermionic superfluidity are monotonic functions of the velocity ratio. © 2010 The American Physical Society., 2010年05月24日, Physical Review A  Atomic, Molecular, and Optical Physics, 81 (5), doi研究論文（学術雑誌）
Competition of superfluidity and density waves in onedimensional BoseFermi mixtures
We study a mixture of onedimensional bosons and spinless fermions at incommensurate filling using phenomenological bosonization and Green's function techniques. We derive the relation between the parameters of the microscopic Hamiltonian and macroscopic observables. Galilean invariance results in extra constraints for the currentcurrent interactions. We obtain the exact exponents for the various response functions, and show that superfluid fluctuations are enhanced by the effective bosonfermion densitydensity interaction and suppressed by the effective bosonfermion currentcurrent interaction. In the case of a bosonized model with purely densitydensity interaction, when the effective bosonfermion densitydensity interaction is weak enough, the superfluid exponent of the bosons has a nonmonotonic variation with the ratio of the fermion velocity to the boson velocity. In contrast, the densitywave exponent and the exponent for fermionic superfluidity are monotonic functions of the velocity ratio., 2010年05月, PHYSICAL REVIEW A, 81 (5), 053626, doi;web_of_science研究論文（学術雑誌）
Correlation function for the onedimensional extended Hubbard model at quarter filling
We examine the densitydensity correlation function in the TomonagaLuttinger liquid state for the onedimensional extended Hubbard model with the onsite Coulomb repulsion U and the intersite repulsion V at quarter filling. By taking into account the effect of the marginally irrelevant umklapp scattering operator based on the bosonization and renormalizationgroup methods, we obtain the generalized analytical form of the correlation function. We show that, in the proximity to the gapped chargeordered phase, the correlation function exhibits anomalous crossover between the pure powerlaw behavior and the powerlaw behavior with logarithmic corrections, depending on the length scale. Such a crossover is also confirmed by the highly accurate numerical densitymatrix renormalizationgroup method., 2010年02月, PHYSICAL REVIEW B, 81 (8), 085116, doi;web_of_science研究論文（学術雑誌）
Mechanism for the Singlet to Triplet Superconductivity Crossover in QuasiOneDimensional Organic Conductors
Superconductivity of quasionedimensional organic conductors with a quarterfilled band is investigated using the twoloop renormalization group approach to the extended Hubbard model for which both the single electron hopping t(perpendicular to) and the repulsive interaction Vperpendicular to perpendicular to the chains are included. For a fourpatches Fermi surface with deviations to perfect nesting, we calculate the response functions for the dominant fluctuations and possible superconducting states. By increasing Vperpendicular to, it is shown that a dwave (singlet) to fwave (triplet) superconducting state crossover occurs, and is followed by a vanishing spin gap. Furthermore, we Study the influence of a magnetic field through the Zeeman coupling, from which a triplet superconducting state is found to emerge., 2009年10月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 78 (10), 104702, doi;web_of_science研究論文（学術雑誌）
Peierls ground state and excitations in the electronlattice correlated system (EDOTTF)(2)X
We investigate the exotic Peierls state in the onedimensional organic compound (EDOTTF)(2)X, wherein the Peierls transition is accompanied by the bending of molecules and also by a fourfold periodic array of charge disproportionation along the onedimensional chain. Such a Peierls state, wherein the interplay between the electron correlation and the electronphonon interaction takes an important role, is examined based on an extended PeierlsHoisteinHubbard model that includes the alternation of the elastic energies for both the lattice distortion and the molecular deformation. The model reproduces the experimentally observed pattern of the charge disproportionation and there exists a metastable state wherein the energy takes a local minimum with respect to the lattice distortion and/or molecular deformation. Furthermore, we investigate the excited states for both the Peierls ground state and the metastable state by considering the soliton formation of electrons. It is shown that the soliton excitation from the metastable state costs energy that is much smaller than that of the Peierls state, where the former is followed only by the charge degree of freedom and the latter is followed by that of spin and charge. Based on these results, we discuss the exotic photoinduced phase found in (EDOTTF)(2)PF(6)., 2008年05月, PHYSICAL REVIEW B, 77 (19), 195128, doi;web_of_science研究論文（学術雑誌）
Charge ordering and slidinginduced instability in thiazylradical organic crystal of (NT)3X
We examine exotic properties of the chargeordered (CO) state which has been found even at room temperature in the 3:1 organic compound (NT) 3GaCl4 (NT=naphtho[2,1d:6,5d']bis([1,2,3]dithiazole)). First we analyze the band structure by using the extended Hiickel method and reveal nontrivial bond modulations along the staking axis: a strong trimerization on one chain and a dimerization on another chain. Based on this lattice structure, we examine the origin of such a CO by using a twochain extended Hubbard model. Within the meanfield approximation, it is shown that the CO pattern observed in the experiment is well reproduced by taking into account the intermolecular Coulomb repulsions. Furthermore we analyze the variation of the energy by shifting the phases of densitywave order parameters and find that the characteristic energy for the melting of the CO state is quite small, ΔE = 20 meV. Based on these analyses, the anomalous nonlinear conductivity found in the compound is also discussed. © 2008 IOP Publishing Ltd., 2008年, Journal of Physics: Conference Series, 132, 012009, doi研究論文（学術雑誌）
Possible metastable state triggered by competition of Peierls state and charge ordered state
We examine a Peierls ground state and its competing metastable state in the onedimensional quarterfilled PeierlsHubbard model with the nearestneighbor repulsive interaction V and the electronphonon interaction (alpha 1/K with K being the elastic constant). From the meanfield approach, we obtain the phase diagram for the ground state on the plane of parameters V and K. The coexistent state of the spindensity wave and the charge ordering is realized for large V and K. With decreasing K, it exhibits a firstorder phase transition to the unconventional Peierls state which is described by the bondcentered chargedensitywave state. In the large region of the Peierls ground state in the phase diagram, there exists the metastable state where the energy takes a local minimum with respect to the lattice distortion. On the basis of the present calculation, we discuss the photoinduced phase observed in the (EDOTTF)(2)PF6 compound., 2007年11月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 76 (11), 114709, doi;web_of_science研究論文（学術雑誌）
Chargeordered state versus DimerMott insulator at finite temperatures
We theoretically investigate the competition between chargeordered state and Mott insulating state at finite temperatures in quarterfilled quasionedimensional electron systems, by studying dimerized extended Hubbard chains with interchain Coulomb interactions. In order to take into account onedimensional fluctuations properly, we apply the bosonization method to an effective model obtained by the interchain meanfield approximation. The results show that lattice dimerization, especially in the critical region, and frustration in the interchain Coulomb interactions reduce the chargeordering phase transition temperature and enlarge the dimerMott insulating phase. We also derive a general formula of the Knight shift in the chargeordered phase and its implication to experiments is discussed., 2007年10月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 76 (10), 103701, doi;web_of_science研究論文（学術雑誌）
Interchainfrustrationinduced metallic state in quasionedimensional Mott insulators
The mechanism that drives a metalinsulator transition in an undoped quasionedimensional Mott insulator is examined in the framework of the Hubbard model with two different hoppings t(perpendicular to 1) and t(perpendicular to 2) between nearestneighbor chains. By applying an Nperpendicular tochain renormalization group method at the twoloop level, we show how a metallic state emerges when both t(perpendicular to 1) and t(perpendicular to 2) exceed critical values. In the metallic phase, the quasiparticle weight becomes finite and develops a strong momentum dependence. We discuss the temperature dependence of the resistivity and the impact of our theory in the understanding of recent experiments on halffilled molecular conductors., 2007年09月, PHYSICAL REVIEW LETTERS, 99 (12), 126404, doi;web_of_science研究論文（学術雑誌）
Effect of interchain frustration in quasionedimensional conductors at halffilling
We examine the effect of frustrated interchain hoppings t(perpendicular to 1) and t(perpendicular to 2) on one dimensional Mott insulators. By applying an Nperpendicular to chain twoloop renormalization group method to the half filled quasi one dimensional Hubbard model, we show that the system remains insulating even for the large t(perpendicular to 1) as far as t(perpendicular to 2) = 0 and vice versa, whereas a metallic state emerges by increasing both interchain hoppings. We also discuss the metallic behaviour suggested in the quasi one dimensional organic compound ( TTM TTP) I3 under high pressure., 2007年04月, JOURNAL OF PHYSICSCONDENSED MATTER, 19 (14), 145228, doi;web_of_science研究論文（学術雑誌）
Role of fluctuations in density wave and superconductivity of quasionedimensional conductors
The response functions, chi, for the spin density wave (SDW) and the superconductivity in the quasionedimensional electron systems at quarterfilling are calculated by the newly developed Nchain renormalization group method. It is shown that the interchain hopping, t(perpendicular to), enhances chi of SDW having the dependence on temperature with the powerlaw even below the crossover temperature Tx similar to t(perpendicular to). Further, chi of the dwave singlet superconductivity (dSS) is enhanced rapidly below Tx by t(perpendicular to), while there is no dominant behavior for dSS in the onedimensional case (t(perpendicular to) = 0). The results suggest that t(perpendicular to) (or the pressure) rises the transition temperature of dSS even after the SDW is destroyed by the pressure. (c) 2006 Published by Elsevier B.V., 2007年03月, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 310 (2), 1128  1129, doi;web_of_science研究論文（学術雑誌）
Chargeordered insulating state in βNa0.33V2O5
We discuss a possible scenario for the chargeordered insulating state observed in the quasionedimensional vanadium bronze betaNa0.33V2O5 at ambient pressure, based on the picture of weaklycoupled twoleg ladders, proposed by Doublet and Lepetit [Phys. Rev. B 71 ( 2005) 075119]. The resultant chargeordering pattern can have a 3fold and/or 6fold periodicity along the onedimensional axis which is consistent with the Xray experiments. This scenario is also consistent with the photoemission spectroscopy measurement indicating that the Fermi momentum takes a value k(b) approximate to +/pi/4b. Moreover the resultant chargeordering pattern reproduces well the results of the recent detailed NMR measurements. (c) 2006 Elsevier B.V. All rights reserved., 2007年03月, J. Mag. Mag. Mater., 310 (2), E200  E202, doi;web_of_science研究論文（学術雑誌）
Role of interchain hopping in the magnetic susceptibility of quasionedimensional electron systems
A role of interchain hopping in quasionedimensional (Q1D) electron systems is investigated by extending the KadanoffWilson renormalization group of onedimensional (1D) systems to Q1D systems. This scheme is applied to the extended Hubbard model to calculate the temperature (T) dependence of the magnetic susceptibility, chi(T). The calculation is performed by taking into account not only the logarithmic Cooper and Peierls channels, but also the nonlogarithmic Landau and finite momentum Cooper channels, which give relevant contributions to the uniform response at finite temperatures. It is shown that the interchain hopping, t(perpendicular to), reduces chi(T) at low temperatures, while it enhances chi(T) at high temperatures. This notable t(perpendicular to) dependence is ascribed to the fact that t(perpendicular to) enhances the anti ferromagnetic spin fluctuation at low temperatures, while it suppresses the 1D fluctuation at high temperatures. The result is at variance with the randomphaseapproximation approach, which predicts an enhancement of chi(T) by t(perpendicular to) over the whole temperature range. The influence of both the longrange repulsion and the nesting deviations on chi(T) is further investigated. We discuss the present results in connection with the data of chi(T) in the (TMTTF)(2)X and (TMTSF)(2)X series of Q1D organic conductors, and propose a theoretical prediction for the effect of pressure on magnetic susceptibility., 2007年01月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 76 (1), 014709, doi;web_of_science研究論文（学術雑誌）
Twoloop renormalization group theory for the quasionedimensional Hubbard model at half filling
We derive twoloop renormalization group equations for the half filled onedimensional Hubbard chains coupled by the interchain hopping. Our renormalization group scheme for the quasionedimensional electron system is a natural extension of that for the purely onedimensional systems in the sense that transversemomentum dependences are introduced in the gological coupling constants and we regard the transverse momentum as a patch index. We develop symmetry arguments for the particlehole symmetric half filled Hubbard model and obtain constraints on the gological coupling constants by which resultant renormalization equations are given in a compact form. By solving the renormalization group equations numerically, we estimate the magnitude of excitation gaps and clarify that the charge gap is suppressed due to the interchain hopping but is always finite even for the relevant interchain hopping. To show the validity of the present analysis, we also apply this to the twoleg ladder system. By utilizing the fieldtheoretical Bosonization and Fermionization method, we derive lowenergy effective theory and analyze the magnitude of all the excitation gaps in detail. It is shown that the lowenergy excitations in the twoleg Hubbard ladder have SO(3)xSO(3)xU(1) symmetry when the interchain hopping exceeds the magnitude of the charge gap., 2006年10月, PHYSICAL REVIEW B, 74 (15), 155109, doi;web_of_science研究論文（学術雑誌）
Finitetemperature chargeordering transition and fluctuation effects in quasi onedimensional electron systems at quarter filling
Finitetemperature chargeordering phase transition in quasi onedimensional (ID) molecular conductors is investigated theoretically, based on a quasi I D extended Hubbard model at quarter filling with interchain Coulomb repulsion Vperpendicular to. The interchain term is treated within a meanfield approximation, and the ID fluctuations in the chains are fully taken into account by the bosonization theory. Three regions are found depending on the appearance of the chargeordered state at a finite temperature when Vperpendicular to is introduced: (i) a weakcoupling region where the system transforms from a metal to a chargeordered insulator with a finite transition temperature at a finite critical value of Vperpendicular to, (ii) an intermediate region where this transition occurs by infinitesimal Vperpendicular to due to the stability of the inherent ID fluctuation, and (iii) a strongcoupling region where the charge ordered state is already realized in a purely 1D case, of which the transition temperature becomes finite with infinitesimal Vperpendicular to. An analytical formula for the Vperpendicular to dependence of the transition temperature is derived for each region., 2006年06月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 75 (6), 063706, doi;web_of_science研究論文（学術雑誌）
Dimensionality effect in quasionedimensional mott insulators
We examine the effect of interchain hopping on the quasionedimensional Hubbard model at half filling. Treating interactions within the weakcoupling perturbation theory, we derive twoloop renormalizationgroup equations for vertices and self energies. It is shown that the Mott gap is suppressed for weak interchain hopping, while it is enhanced for strong interchain hopping., 2006年02月, JOURNAL OF LOW TEMPERATURE PHYSICS, 142 (34), 651  654, doi;web_of_science研究論文（学術雑誌）
Magnetic susceptibility in normal states of quasionedimensional superconductors
The magnetic susceptibility chi(T) in normal states of the quasionedimensional (Q1D) superconductor is calculated by using the Nchain renormalization group technique developed newly. For temperatures above Tcross being of the order of the interchain hopping t(perpendicular to), chi(T) of Q1D system agrees, with that qf onedimensional systems. For temperatures below Tcross where the Fermi liquid state is expected, it is found that d chi(T)/dT > 0 in contrast to the case of the normal Fermi liquid, i.e., d chi(T)/dT = 0. The origin qf such a "pseudogap like" behavior is discussed in terms of antiferromagnetic spin fluctuations., 2006年02月, JOURNAL OF LOW TEMPERATURE PHYSICS, 142 (34), 507  510, doi;web_of_science研究論文（学術雑誌）
Effect of intersite repulsions on magnetic susceptibility of onedimensional electron systems at quarter filling
The temperature dependence of magnetic susceptibility, chi(T), is investigated for onedimensional interacting electron systems at quarter filling within the KadanoffWilson renorrualizationgroup method. The forward scattering on the same branch (the g(4)process) is examined together with the backward (g(1)) and forward (g(2)) scattering amplitudes on opposite branches. In connection with lattice models, we show that chi(T) is strongly enhanced by the nearestneighbor interaction, an enhancement which surpasses that of the nextnearestneighbor interaction. A connection between our predictions for chi(T) and experimental results for chi(T) in quasionedimensional organic conductors is presented., 2005年12月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 74 (12), 3159  3162, doi;web_of_science研究論文（学術雑誌）
Magnetic response and quantum critical behavior in the doped twoleg extended Hubbard ladder
We have investigated quantum critical behavior in the doped twoleg extended Hubbard ladder, by using a weakcoupling bosonization method. In the ground state, the dominant fluctuation changes from the conventional dwavelike superconducting (SCd) state into densitywave states, with increasing nearestneighbor repulsions and/or decreasing doping rate. The competition between the SCd state and the chargedensitywave state coexisting with the pdensitywave state becomes noticeable on the critical point, at which the gap for magnetic excitations vanishes. Based on the Majoranafermion description of the effective theory, we calculate the temperature dependence of the magnetic response such as the spin susceptibility and the NMR relaxation rate, which exhibit unusual properties due to two kinds of spin excitation modes. On the quantum critical point, the spin susceptibility shows paramagnetic behavior with logarithmic corrections and the NMR relaxation rate also exhibits anomalous powerlaw behavior. We discuss the commensurability effect due to the umklapp scattering and relevance to the twoleg ladder compounds Sr14xCaxCu24O41., 2005年08月, PHYSICAL REVIEW B, 72 (7), 075121, doi;web_of_science研究論文（学術雑誌）
Quantum phase transition between the spinPeierls state and the antiferromagnetic state in the TMTTF organic compounds
A competition between the spinPeierls (SP) state and the anti ferromagnetic (AF) state has been examined for the halffilled quasi onedimensional electron system coupled with dynamical phonon. By considering the antiadiabatic limit of phonon, an effective onedimensional Hamiltonian is derived which includes fluctuations of both the SP state and the AF state. We have shown, by using the renormalizationgroup method, that a quantum critical point between the SP state and the AF state emerges due to an interference effect of these fluctuations. We discuss the quantum critical behavior between the SP state and the AF state which is predicted in quasionedimensional conductors, TMTTF compounds. (c) 2005 Elsevier B.V. All rights reserved., 2005年04月, PHYSICA BCONDENSED MATTER, 358 (14), 42  49, doi;web_of_science研究論文（学術雑誌）
SpinPeierls transition temperature in quarterfilled organic conductors
We examine the transition temperature of spinPeierls state, TSP, in the onedimensional quarterfilled PeierlsHubbard model with lattice dimerization and intersite repulsive interaction, V, by applying the bosonization and the renormalizationgroup method. It is shown that for large V the transition temperature TSP is suppressed due to the appearance of the charge ordering. In the spinPeierls state, the location of the spinsinglet state is shifted continuously from the bond of the lattice to the onsite by the increase of the charge ordering. The result is compatible with our previous results of the ground state. We discuss the pressure effect on TSP, which has been observed in the NMR measurement., 2005年03月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 74 (3), 983  987, doi;web_of_science研究論文（学術雑誌）
Interplay of lattice dimerization and charge ordering in onedimensional quarterfilled electron systems
The effect of charge ordering on lattice dimerization has been investigated in the onedimensional extended PeierlsHubbard model at quarter filling. We estimate the strength of the lattice dimerization by applying the bosonization method and the perturbative renormalizationgroup method. In the ground state, it is shown that a firstorder phase transition takes place between the dimerized insulator and the chargeordered insulator. The lattice dimerization disappears in the chargeordered state, while the lattice dimerization is appreciably suppressed even in the dimerized insulating state due to charge fluctuations. The phase diagram at finite temperature is also discussed., 2004年09月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 73 (9), 2487  2493, doi;web_of_science研究論文（学術雑誌）
Chargedensitywave formation in the doped twoleg extended hubbard ladder
We investigate the electronic properties of a doped twoleg Hubbard ladder with both the onsite and nearestneighbor Coulomb repulsions by using the weakcoupling renormalizationgroup method. It is shown that, for strong nearestneighbor repulsions, the chargedensitywave state coexisting with the pdensitywave state becomes a dominant fluctuation where spins form intrachain singlets. By increasing doping rate, we have also shown that the effects of the nearestneighbor repulsions are reduced and the system exhibits a quantum phase transition into the dwavelike (or rungsinglet) superconducting state. We derive the effective fermion theory which describes the critical properties of the transition point with the gapless excitation of magnon. The phase diagram of the twoleg ladder compound, Sr14xCaxCu24O41, is discussed., 2004年04月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 73 (4), 804  807, doi;web_of_science研究論文（学術雑誌）
Groundstate phase diagram of the onedimensional halffilled extended Hubbard model
We revisit the groundstate phase diagram of the onedimensional halffilled extended Hubbard model with onsite (U) and nearestneighbor (V) repulsive interactions. In the first half of the paper, using the weakcoupling renormalizationgroup approach (gology) including secondorder corrections to the coupling constants, we show that bondchargedensitywave (BCDW) phase exists for Uapproximate to2V in between chargedensitywave (CDW) and spindensitywave (SDW) phases. We find that the umklapp scattering of parallelspin electrons disfavors the BCDW state and leads to a bicritical point where the CDWBCDW and SDWBCDW continuoustransition lines merge into the CDWSDW firstorder transition line. In the second half of the paper, we investigate the phase diagram of the extended Hubbard model with either additional staggered site potential Delta or bond alternation delta. Although the alternating site potential Delta strongly favors the CDW state (that is, a band insulator), the BCDW state is not destroyed completely and occupies a finite region in the phase diagram. Our result is a natural generalization of the work by Fabrizio, Gogolin, and Nersesyan [Phys. Rev. Lett. 83, 2014 (1999)], who predicted the existence of a spontaneously dimerized insulating state between a band insulator and a Mott insulator in the phase diagram of the ionic Hubbard model. The bond alternation delta destroys the SDW state and changes it into the BCDW state (or Peierls insulating state). As a result the phase diagram of the model with delta contains only a single critical line separating the Peierls insulator phase and the CDW phase. The addition of Delta or delta changes the universality class of the CDWBCDW transition from the Gaussian transition into the Ising transition., 2004年01月, PHYSICAL REVIEW B, 69 (3), 035103, doi;web_of_science研究論文（学術雑誌）
Staggered flux state in the twoleg Hubbard ladder at half filling
We investigate the staggered flux state in the halffilled extended Hubbard ladder using the strongcoupling perturbation theory. The staggered flux state has longrange order of currents on rungs flowing in the alternating direction. We derive a lowenergy effective Hamiltonian and study an Isingtype quantum phase transition between the staggered flux state and the dwavepairing Mott insulating state. (C) 2003 Elsevier Science B.V. All rights reserved., 2003年05月, PHYSICA BCONDENSED MATTER, 329, 981  982, doi;web_of_science研究論文（学術雑誌）
Commensurateincommensurate transition in a onedimensional dimerized electron system close to quarterfilling
We investigate electronic states upon doping (i.e., by varying the chemical potential) for a quarterfilled onedimensional electron system with a dimerization, onsite and nearestneighbor repulsive interaction. By applying the renormalization group method to the bosonized Hamiltonian, the metalinsulator transition followed by commensurateincommensurate transition is obtained for Mott insulating region as well as charge ordered insulating regime. We discuss the phase diagram on the plane of the chemical potential and the interaction. (C) 2002 Elsevier Science B.V. All rights reserved., 2003年03月, SYNTHETIC METALS, 133, 49  50, doi;web_of_science研究論文（学術雑誌）
Generalized twoleg Hubbard ladder at half filling: Phase diagram and quantum criticalities
The groundstate phase diagram of the halffilled twoleg Hubbard ladder with intersite Coulomb repulsions and exchange coupling is studied by using the strongcoupling perturbation theory and the weakcoupling bosonization method. Considered here as possible ground states of the ladder model are four types of densitywave states with different angular momentum (sdensitywave state, pdensitywave state, ddensitywave state, and fdensitywave state) and four types of quantum disordered states, i.e., Mott insulating states (SMott, DMott, S'Mott, and D'Mott states, where S and D stand for s and dwave symmetry). The sdensitywave state, the ddensitywave state, and the DMott state are also known as the chargedensitywave state, the staggeredflux state, and the rungsinglet state, respectively. Strongcoupling approach naturally leads to the Ising model in a transverse field as an effective theory for the quantum phase transitions between the staggeredflux state and the DMott state and between the chargedensitywave state and the SMott state, where the Ising ordered states correspond to doubly degenerate ground states in the staggeredflux or the chargedensitywave state. From the weakcoupling bosonization approach it is shown that there are three cases in the quantum phase transitions between a densitywave state and a Mott state: the Ising (Z(2)) criticality, the SU(2)(2) criticality, and a firstorder transition. The quantum phase transitions between Mott states and between densitywave states are found to be the U(1) Gaussian criticality. The groundstate phase diagram is determined by integrating perturbative renormalizationgroup equations. It is shown that the SMott state and the staggeredflux state exist in the region sandwiched by the chargedensitywave phase and the DMott phase. The pdensitywave state, the S'Mott state, and the D'Mott state also appear in the phase diagram when the nextnearestneighbor repulsion is included. The correspondence between Mott states in extended Hubbard ladders and spinliquid states in spin ladders is also discussed., 2002年12月, PHYSICAL REVIEW B, 66 (24), 245106, doi;web_of_science研究論文（学術雑誌）
Ising transition in a onedimensional quarterfilled electron system with dimerization
We examine critical properties of the quarterfilled onedimensional Hubbard model with dimerization and with the onsite and nearestneighbor Coulomb repulsion U and V. By utilizing the bosonization method, it is shown that the system exhibits an Ising quantum phase transition from the Mott insulating state to the chargeordered insulating state. It is also shown that the dielectric permittivity exhibits a strong enhancement as decreasing temperature with powerlaw dependence at the Ising critical point. (C) 2002 Elsevier Science Ltd. All rights reserved., 2002年06月, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 63 (68), 1459  1463, doi;web_of_science研究論文（学術雑誌）
Phase diagram of the onedimensional extended Hubbard model at half filling
We reexamine the groundstate phase diagram of the onedimensional halffilled Hubbard model with onsite and nearestneighbor repulsive interactions. We calculate secondorder corrections to coupling constants in the weakcoupling renormalizationgroup approach (gology) to show that the bondchargedensitywave (BCDW) phase exists for weak couplings in between the chargedensitywave (CDW) and spindensitywave (SDW) phases. We find that the umklapp scattering of parallelspin electrons destabilizes the BCDW state and gives rise to a bicritical point where the CDWBCDW and SDWBCDW continuoustransition lines merge into the CDWSDW firstorder transition line., 2002年02月, PHYSICAL REVIEW LETTERS, 88 (5), 056402, doi;web_of_science研究論文（学術雑誌）
Crossover from quarterfilling to halffilling in a onedimensional electron system with a dimerized and quarterfilled band
The interplay between quarterfilled and halftilled umklapp scattering has been examined by applying the renormalization group method to a onedimensional quarterfilled electron system with dimerization, onsite (U) and nearestneighbor (V) repulsive interactions. The phase diagram on the UV plane is obtained at absolute zero temperature where the Mott, insulator (the charge ordered insulator) is found for smaller (larger) V. By choosing the moderate parameter in the region of Mott insulator, it is shown that the resistivity exhibits a crossover from behavior of quarterfilling to that of halffilling with decreasing temperature., 2001年06月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 70 (6), 1460  1463, doi;web_of_science研究論文（学術雑誌）
Effects of nextnearestneighbor repulsion on onedimensional quarterfilled electron systems
We examine effects of the nextnearestneighbor repulsion on electronic states of a onedimensional interacting electron system which consists of quarterfilled band and interactions of onsite and nearestneighbor repulsion. We derive the effective Hamiltonian for the electrons around wave number +/k(F) (KF: Fermi wave number) and apply the renormalization group method to the bosonized Hamiltonian. It is shown that the nextnearestneighbor repulsion makes 4k(F)charge ordering unstable and suppresses the spin fluctuation. Further the excitation gaps and spin susceptibility are also evaluated., 2001年03月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 70 (3), 762  773, doi;web_of_science研究論文（学術雑誌）
Spinless fermion ladders at half filling
We study a halffilled ladder of spinless fermions. We show that contrary to a single chain, the ladder becomes a Mott insulator for arbitrarily small repulsive interactions. We obtain the full phase diagram and physical quantities such as the charge gap. We show that there is only a single insulating phase for repulsive interactions, regardless of the strength of the interchain hopping and single chain Mott gap. There is thus no confinementdeconfinement transition in this system but a simple crossover. Using bosonization with the renormalization group we show that upon doping the system becomes a Luttinger liquid with a universal parameter K = 1/2 different from the one of the single chain (K = 1/4)., 2001年01月, PHYSICAL REVIEW B, 63 (4), 045121, doi;web_of_science研究論文（学術雑誌）
Commensurateincommensurate transition in twocoupled chains of nearly halffilled electrons
We investigate the physical properties of two coupled chains of electrons, with a nearly halffilled band, as a function of the interchain hopping t(perpendicular to) and the doping. Mie show that upon doping, the system undergoes a metalinsulator transition well described by a commensurateincommensurate transition. By using bosonization and renormalization we determine the full phase diagram of the system, and the physical quantities such as the charge gap. In the commensurate phase two different regions, for which the interchain hopping is relevant and irrelevant exist, leading to a confinementdeconfinement crossover in this phase. A minimum of the charge gap is observed for values of ti close to this crossover. At large t(perpendicular to) the region of the commensurate phase is enhanced, compared to a single chain. At the metalinsulator transition the Luttinger parameter takes the universal value Krho* = 1, in agreement with previous results on special limits of this model., 2001年01月, EUROPEAN PHYSICAL JOURNAL B, 19 (2), 185  193, doi;web_of_science研究論文（学術雑誌）
Electronic states in correlated threecoupled chains
Threecoupled chains of a quarterfilled Hubbard model with dimerization have been studied for periodic boundary conditions. By the use of a renormalization group method, it is found that the interchain hopping is renormalized to zero leading to confinement when the charge gap becomes larger than the effective interchain hopping energy. Such a result of confinement is compared with that of twocoupled chains. (C) 2000 Elsevier Science Ltd. All rights reserved., 2001年01月, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 62 (12), 427  430, doi;web_of_science研究論文（学術雑誌）
Commensurability in onedimensional electron systems at quarterfilling
We examine the role of commensurability in electronic states and excitations for a onedimensional interacting electron system at quarterfilling with onsite (U), nearestneighbor (V1) and nextnearestneighbor (V2) repulsive interactions, By applying the renormalization group method to the bosonized model, it is shown that the nextnearestneighbor repulsion stabilizes 2k(F) density waves and induces the excitation gap in spin fluctuation for V2 > U/2, (C) 2000 Elsevier Science Ltd. All rights reserved., 2001年01月, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 62 (12), 419  422, doi;web_of_science研究論文（学術雑誌）
Mechanism of confinement in lowdimensional organic conductors
Confinementdeconfinement transition in quarterfilled twocoupled chains comprising dimerization, repulsive interactions and interchain hopping has been demonstrated by applying the renormalization group method to the bosonized Hamiltonian. The confinement given by the irrelevant interchain hopping occurs with increasing Umklapp scattering which is induced by the dimerization leading effective to halffilling. It is shown that the transition originates from a competition between the charge gap and the renormalized interchain hopping. (C) 2000 Elsevier Science Ltd. All rights reserved., 2001年01月, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 62 (12), 93  97, doi;web_of_science研究論文（学術雑誌）
Spinless fermion ladders at half filling
We study a halffilled ladder of spinless fermions. We show that contrary to a single chain, the ladder becomes a Mott insulator for arbitrarily small repulsive interactions. We obtain the full phase diagram and physical quantities such as the charge gap. We show that there is only a single insulating phase for repulsive interactions, regardless of the strength of the interchain hopping and single chain Mott gap. There is thus no confinementdeconfinement transition in this system but a simple crossover. Using bosonization with the renormalization group we show that upon doping the system becomes a Luttinger liquid with a universal parameter K = 1/2 different from the one of the single chain (K = 1/4)., 2001年, Physical Review B  Condensed Matter and Materials Physics, 63 (4), 451211  451218, doi研究論文（学術雑誌）
Correlation gap in onedimensional quarterfilled electron system
Umklapp scattering in an onedimensional quarterfilled electron system with both onsite U(> 0) and nearestneighbor V(> 0) interactions is examined by applying the renormalization group method to the bosonized Hamiltonian. We calculate a correlation gap, which appears for V larger than a critical value. In such a case, the spin density wave (SDW) coexists with the charge disproportion with twofold periodicity. (C) 2000 Elsevier Science B.V. All rights reserved., 2000年07月, PHYSICA B, 284, 1547  1548, doi;web_of_science研究論文（学術雑誌）
Confinement by charge gap in organic conductor Bechgaard salts
In Bechgaard salts, which are effectively halffilling due to dimerization, a confinement has been found for the large charge gap in the optical experiments. In order to understand the crucial role of the charge gap, the irrelevance of interchain hopping corresponding to the confinement is calculated for twocoupled chains of quarterfilled Hubbard model with dimerization. It is demonstrated that a transition from deconfinement to confinement occurs when the charge gap induced by the dimerization becomes larger than the inerchain hopping. (C) 2000 Elsevier Science B.V. All rights reserved., 2000年06月, PHYSICA B, 281, 684  685, doi;web_of_science研究論文（学術雑誌）
Correlation effects in a onedimensional quarterfilled electron system with repulsive interactions
A onedimensional electron system at quarterfilling has been examined by applying the renormalization group method to a bosonized model with onsite (U) and nearestneighbor (V) repulsive interactions. By evaluating both normal scattering and Umklapp scattering perturbatively, we obtain a phase diagram in which a metallic state with a 2k(F) spin density wave (k(F) is the Fermi wave number) moles into an insulating state with charge disproportionation of a 4k(F) charge density wave with an increase in both U and V. The effect of the nextnearestneighbor repulsion is also discussed., 2000年03月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 69 (3), 651  654, doi;web_of_scienceElectronic states in halffilled correlated system with alternating potential
The effect of an alternating potential on a onedimensional halftilled Hubbard model with repulsive interaction has been examined hy applying the renormalization group method to the bosonized Hamiltonian. The electronic slate, which is determined by the competition between alternating potential and umklapp scattering, is calculated where the relevance and the irrelevance of the alternating potential lead to the band insulator and the Mott insulator respectively The excitation gaps for charge and spin fluctuations are calculated for both states., 1999年12月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 68 (12), 3966  3974, doi;web_of_science研究論文（学術雑誌）
Theoretical study on the charge gap of organic conductor  Bechgaard salts
The charge gap in a quarterfilled organic conductor, which is induced hy dimerization and/or alternating potential is studied by applying the renormalization group method to the bosonized onedimensional Kamiltonian. The interplay of dimerization and alternating potential gives rise to the unconventional enhancement of the charge gap in addition to the onsite repulsive interaction. The experimental results of resistivity and optical conductivity in Bechgaard salts are examined based on the present calculation., 1999年06月, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 68 (6), 1809  1812, doi;web_of_scienceCompetition between SDW and SC states in twoHubbard chains
Two coupled chains of Hubbard model with spin anisotropic backward scattering have been studied by applying the renormalization group method to the bosonized Hamiltonian. From two kinds of fixed points for transverse spin fluctuations, which exist in the presence of anisotropic interactions, it has been shown that the dominant state moves from a singlet superconducting (SC) state with interchain pairing into a longitudinal spin density wave (SDW) state with intrachain pairing when the repulsive interaction increases. Phase diagram at finite temperatures is also obtained., 1999年06月, SYNTHETIC METALS, 103 (13), 2189  2190, doi;web_of_science研究論文（学術雑誌）
Confinement by umklapp scattering in two coupled chains
The effect of umklapp scattering on two coupled chains with both halffilled and doped band has been examined by renormalization group method. It has been found that electrons are confined to a single chain when the doping rate becomes small and when the correlation gap induced by umklapp scattering becomes larger than the interchain hopping energy. A crossover from an insulating state into a metallic state is discussed for several Bechgaard salts at temperatures above spin density wave state., 1999年06月, SYNTHETIC METALS, 103 (13), 2191  2192, doi;web_of_science研究論文（学術雑誌）
Confinementdeconfinement transition in two coupled chains with umklapp scattering
A role of umklapp scattering has been examined for twocoupled chains with both forward and backward scattering by applying the renormalization group method to the bosonized Hamiltonian. It has been found that a state with relevant interchain hopping changes into a state with irrelevant (confined) one when the magnitude of umklapp scattering becomes larger than that of interchain hopping. The critical value of umklapp scattering for such a confinementdeconfinement transition is calculated as the function of interchain hopping and intrachain interactions. A crossover from onedimensional regime into that of coupled chains is also shown with decreasing temperature. [S01631829(99)110191]., 1999年05月, PHYSICAL REVIEW B, 59 (19), 12326  12337, doi;web_of_science研究論文（学術雑誌）
Renormalized interchain hopping vs charge gap in two coupled chains
The role of intrachain interactions, which renormalize interchain hopping, has been examined for two coupled chains with umklapp scattering by use of the renormalization group method. It is clarified that the confinementdeconfinement transition occurs when the charge gap becomes larger than the renormalized interchain hopping., 1999年03月, PROGRESS OF THEORETICAL PHYSICS, 101 (3), 763  768, doi;web_of_science研究論文（学術雑誌）
Twocoupled chains with spinanisotropic backward scattering
By applying renormalization group method to the bosonized Hamiltonian of twocoupled chains with repulsive intrachain interaction, we have examined a role of backward scattering with a spinanisotropy which competes with interchain hopping. From calculation of a dominant state in the limit of low energy, it is found that superconducting state moves into spin density wave state when the anisotropy becomes larger than a critical value. Further phase diagram is shown on the plane of gology. (C) 1998 Elsevier Science B.V. All rights reserved., 1998年07月, PHYSICA C, 303 (34), 246  256, doi;web_of_science研究論文（学術雑誌）
Confinement of interchain hopping by umklapp scattering in two coupled chains
The effect of umklapp scattering on interchain hopping has been investigated for two coupled chains of interacting electrons with a halffilled band. By analyzing in terms of the renormalizationgroup method, we have found that interchain hopping is renormalized to zero and is confined when a gap induced by umklapp scattering becomes larger than a critical value. From a phase diagram calculated on a plane of the interchain hopping and the gap, we discuss a role of the correlation gap that has been studied in the metallic state at temperatures above the spindensitywave state in organic conductors., 1998年06月, PHYSICAL REVIEW B, 57 (24), 15040  15043, doi;web_of_science研究論文（学術雑誌）
Crossover between high and low energystates in twocoupled chains of Tomonaga model
By applying the renormalization group method to twocoupled chains in the Tomonaga model, the role of interchain hopping has been studied in the entire energy region. The energy for a crossover from the perturbational regime to the relevant regime becomes smaller than that of the interchain hopping due to onedimensional fluctuations of the mutual interaction. From the calculation of response functions for charge density waves and superconducting states, the phase diagram of dominant and subdominant states has been obtained in the plane of mutual interactions with fixed energy., 1997年11月, PROGRESS OF THEORETICAL PHYSICS, 98 (5), 1045  1062, doi;web_of_science研究論文（学術雑誌）
汎関数くりこみ群を用いた量子液晶状態の研究
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奈良女子大学「2017年度 数学・物理学・情報科学シンポジウム 〜 理のクロスオーバー 〜」, 2017年FunctionalRenormalizationGroup Analysis on Electron Nematic State and ChargeDensityWave State in Cuprate Superconductors
International Conference on Quantum Physics in Complex Matter: Superconductivity, Magnetism and Ferroelectricity (SUPERSTRIPES 2017), Italiy, 2017年汎関数繰り込み群法による多軌道強相関電子系の解析
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ディラック線ノードをもつジャイロイドにおける輸送特性 II
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