## Tsuchiizu MasahisaFaculty Division of Natural Sciences Research Group of Physics Associate Professor |

Last Updated :2021/07/07

- Natural sciences, Magnetism, superconductivity, and strongly correlated systems
- Natural sciences, Semiconductors, optical and atomic physics

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―

May 2020, 固体物理, 55 (5), 195 - 203Scientific journal

Exploring new density-functional embedding techniques for strongly correlated electrons: From model to ab initio Hamiltonians

19 Aug. 2018, ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 256, web_of_scienceSite-occupation embedding theory using Bethe ansatz local density approximations

Site-occupation 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 impurity-interacting (rather than a noninteracting) one. It provides a rigorous framework for combining wave-function (or Green function)-based methods with DFT. In this work, exact expressions for the per-site energy and double occupation of the uniform Hubbard model are derived in the context of SOET. As readily seen from these derivations, the so-called bath contribution to the per-site 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 single-impurity Anderson models are constructed and tested on a one-dimensional ring. The self-consistent calculation of the embedded impurity wave function has been performed with the density-matrix 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., American Physical Society, 06 Jun. 2018, Physical Review B, 97 (23), 235105, doiScientific journal

Multistage electronic nematic transitions in cuprate superconductors: A functional-renormalization-group analysis

Recently, complex rotational symmetry-breaking 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 functional-renormalization-group method to the d-p 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 p-orbital density wave along the axial (Cu-Cu) 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 density-wave order at q=Qa is triggered by the uniform order. The predicted multistage nematic transitions are caused by Aslamazov-Larkin-type fluctuation-exchange processes., American Physical Society, 19 Apr. 2018, Physical Review B, 97 (16), 165131, doiScientific journal

An exotic band structure of a supramolecular honeycomb lattice formed by a pancake π-π 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 π-π pancake bonds toward three directions. The crystal structure of Rb3TT·2H2O belongs to the hexagonal P6/m space group, the tight-binding band calculation of which reveals Dirac cones and flat bands., Royal Society of Chemistry, 2018, Chemical Communications, 54 (31), 3815 - 3818, doiScientific journal

Plain s-Wave Superconductivity near Magnetic Criticality: Enhancement of Attractive Electron-Boson Coupling Vertex Corrections

Recent experiments revealed that the plain s-wave state without any sign-reversal emerges in various metals near magnetic criticality. To understand this counter-intuitive phenomenon, we study the gap equation for the multiorbital Hubbard-Holstein model, by analyzing the vertex correction (VC) due to the higher-order electron-correlation effects. We find that the phonon-mediated orbital fluctuations are magnified by the VC for the susceptibility (chi-VC). In addition, the charge-channel attractive interaction is enlarged by the VC for the coupling-constant (U-VC), which is significant when the interaction has prominent q-dependences; therefore the Migdal theorem fails. Due to both chi-VC and U-VC, the plain s-wave state is caused by the small electron-phonon interaction near the magnetic criticality against the repulsive Coulomb interaction. We find that the direct Coulomb repulsion for the plain s-wave Cooper pair is strongly reduced by the " multiorbital screening effect"., PHYSICAL SOC JAPAN, Jul. 2017, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 86 (7), 073703, doi;web_of_scienceScientific journal

3D Spin-Liquid 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 heat-capacity measurements were carried out for a chiral organic radical salt, (TBA)(1.5) [(-)-NDI-Delta] (TBA denotes tetrabutylammonium and NDI denotes naphthalene diimide), in which (-)-NDI-Delta forms a K-4 structure due to its triangular molecular structure and an intermolecular pi-pi 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 high-temperature magnetic susceptibility followed the Curie-Weiss law with a negative Weiss constant of theta = -15 K, the low-temperature magnetic measurements revealed no long-range 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 N-14 NMR measurements down to 0.38 K. Further, the low-temperature 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 T-0.62 in the wide temperature range 0.07-4.5 K., AMER PHYSICAL SOC, Jul. 2017, PHYSICAL REVIEW LETTERS, 119 (5), 057201, doi;web_of_scienceScientific journal

Competing Unconventional Charge-Density-Wave States in Cuprate Superconductors: Spin-Fluctuation-Driven Mechanism

To understand the origin of unconventional charge-density-wave (CDW) states in cuprate superconductors, we establish the self-consistent CDW equation, and analyze the CDW instabilities based on the realistic Hubbard model, without assuming any q-dependence and the form factor. Many higher-order many-body 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 d-form 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 = T-CDW. It is confirmed that the higher-order Aslamazov-Larkin processes cause the CDW orders at both q = 0 and Q(a)., PHYSICAL SOC JAPAN, Jun. 2017, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 86 (6), 063707, doi;web_of_scienceScientific journal

Local density approximation in site-occupation embedding theory

Site-occupation 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 non-interacting bath sites. The correlation energy of the bath is then treated implicitly by means of a site-occupation functional. In this work, we propose a simple impurity-occupation functional approximation based on the two-level (2L) Hubbard model which is referred to as two-level impurity local density approximation (2L-ILDA). Results obtained on a prototypical uniform eight-site Hubbard ring are promising. The extension of the method to larger systems and more sophisticated model Hamiltonians is currently in progress. [GRAPHICS], TAYLOR & FRANCIS LTD, 2017, MOLECULAR PHYSICS, 115 (1-2), 48 - 62, doi;web_of_scienceScientific journal

Three-dimensional higher-spin Dirac and Weyl dispersions in the strongly isotropic K-4 crystal

We analyze the electronic structure in the three-dimensional (3D) crystal formed by the sp(2) hybridized orbitals (K-4 crystal), by the tight-binding approach based on the first-principles calculation. We discover that the bulk Dirac-cone 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 T-2 irreducible representation. In addition, the usual S = 1/2 Dirac cones emerge at the P point. By focusing the hoppings between the nearest-neighbor sites, we show an analytic form of the tight-binding Hamiltonian with a 4 x 4 matrix, and we give an explicit derivation of the S = 1 and S = 1/2 Dirac-cone dispersions. We also analyze the effect of the spin-orbit coupling to examine how the degeneracies at Dirac points are lifted. At the S = 1 Dirac points, the spin-orbit coupling lifts the energy level with sixfold degeneracy into two energy levels with two-dimensional (E) over bar (2) and four-dimensional (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., AMER PHYSICAL SOC, Nov. 2016, PHYSICAL REVIEW B, 94 (19), 195426, doi;web_of_scienceScientific journal

Phase Competition, Solitons, and Domain Walls in Neutral-Ionic Transition Systems

Phase competition and excitations in the one-dimensional neutral-ionic transition systems are theoretically studied comprehensively. From the semiclassical treatment of the bosonized Hamiltonian, we examine the competition among the neutral (N), ferroelectric-ionic (I-ferro), and paraelectric-ionic (I-para) states. The phase transitions between them can become first-order when the fluctuation-induced higher-order commensurability potential is considered. In particular, the description of the first-order phase boundary between N and Iferro enables us to analyze N-I-ferro 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 domain-wall excitations are classified in terms of the topological charge and spin. The relevance to the experimental observations in the molecular neutral-ionic transition systems is discussed. We ascribe the pressure-induced crossover in tetrathiafulvalene-p-chloranil (TTF-CA) at a high-temperature region to that from the N to the I-para state, and discuss its consequence., PHYSICAL SOC JAPAN, Oct. 2016, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 85 (10), 104705, doi;web_of_scienceScientific journal

Functional renormalization group study of orbital fluctuation mediated superconductivity: Impact of the electron-boson 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 two-orbital Hubbard model. The vertex corrections for the electron-boson coupling (U-VC), which are dropped in the Migdal-Eliashberg gap equation, are obtained by solving the RG equation. We reveal that the dressed electron-boson 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 U-VC 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 spin-channel U-VC, 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 U-VC, such as in Fe-based superconductors., AMER PHYSICAL SOC, Sep. 2016, PHYSICAL REVIEW B, 94 (11), 115155, doi;web_of_scienceScientific journal

Discovery of the K-4 Structure Formed by a Triangular pi Radical Anion

The K-4 structure was theoretically predicted for trivalent chemical species, such as sp(2) carbon. However, since attempts to synthesize the K-4 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. X-ray crystal analysis of the obtained crystals revealed the K-4 structure, which was formed by the unique intermolecular pi overlap directed toward three directions from the triangular-shape NDI-Delta radical anions: Electron paramagnetic resonance and static magnetic measurements confirmed the radical anion state of NDI-Delta 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., AMER CHEMICAL SOC, Jun. 2015, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 137 (24), 7612 - 7615, doi;web_of_scienceScientific journal

Spin-triplet superconductivity in Sr2RuO4 due to orbital and spin fluctuations: Analyses by two-dimensional renormalization group theory and self-consistent vertex-correction 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 random-phase approximation, called the RG + cRPA method. Thanks to the vertex correction (VC) for the susceptibility, which is dropped in the mean-field-level approximations, strong orbital and spin fluctuations at Q approximate to (2 pi/3,2 pi/3) emerge in the quasi-one-dimensional Fermi surfaces (FSs) composed of d(xz) + d(yz) orbitals. Due to the cooperation of both fluctuations, we obtain the triplet superconductivity in the E-u representation, in which the superconducting gap is given by the linear combination of (Delta(x)(k), Delta y(k)) similar to (sin 3k(x), sin 3k(y)). Very similar results are obtained by applying the diagrammatic calculation called the self-consistent VC method. Thus, the idea of "orbital + spin fluctuation mediated TSC" is confirmed by both the RG + cRPA method and the self-consistent VC method. We also revealed that the large superconducting gap on the d(xy)-orbital Fermi surface is induced from gaps on the quasi-one-dimensional FSs, in consequence of the large orbital mixture due to the 4d spin-orbit interaction., AMER PHYSICAL SOC, Apr. 2015, PHYSICAL REVIEW B, 91 (15), 155103, doi;web_of_scienceScientific journal

Composite pairing and superfluidity in a one-dimensional resonant Bose-Fermi mixture

We study the ground-state properties of one-dimensional 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 low-energy excitations, and the remaining system can be described by a two-component Tomonaga-Luttinger liquid. Using a bosonization scheme, we derive the effective low-energy Hamiltonian for the system, which has a similar form as that of the two-chain problem of coupled Tomonaga-Luttinger liquids. With the help of improved perturbative renormalization group analysis of the latter problem, we determine the ground-state 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., 16 Oct. 2013, Physical Review A - Atomic, Molecular, and Optical Physics, 88 (4), 043620, doiScientific journal

Orbital Nematic Instability in the Two-Orbital Hubbard Model: Renormalization-Group plus Constrained RPA Analysis

Motivated by the nematic electronic fluid phase in Sr3Ru2O7, we develop a combined scheme of the renormalization-group method and the random-phase-approximation-type 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 ferro-orbital instability near the magnetic or superconducting quantum criticality, due to the Aslamazov-Larkin-type 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 Fe-based superconductors., AMER PHYSICAL SOC, Jul. 2013, PHYSICAL REVIEW LETTERS, 111 (5), 057003, doi;web_of_scienceScientific journal

Cation dependence of the electronic states in molecular triangular lattice system β'-X[Pd(dmit)

The electronic structure of an isostructural series of molecular conductors, β'-X[Pd(dmit)2]2, is systematically studied by a first-principles method based on the density-functional theory. The calculated band structures are fitted to the tight-binding 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 face-to-face 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., Mar. 2013, Journal of the Physical Society of Japan, 82 (3), 033709, doi;scopus;scopus_citedby2 ]2 : A first-principles studyScientific journal

Spin-fluctuation-driven orbital nematic order in Ru-oxides: Self-consistent vertex correction analysis for two-orbital model

To reveal the origin of the "nematic electronic fluid phase" in Sr3Ru2O7, we apply the self-consistent vertex correction analysis to the (dxz, dyz)-orbital Hubbard model. It is found that the Aslamazov-Larkin type vertex correction causes the strong coupling between spin and orbital fluctuations, which corresponds to the Kugel-Khomskii spin-orbital 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 random-phaseapproximation (RPA). This study provides a natural explanation for the close relationship between the magnetic quantum criticality and nematic state in Sr3Ru2O7 and Fe-based superconductors. © 2013 The Physical Society of Japan., Jan. 2013, Journal of the Physical Society of Japan, 82 (1), 013707, doiScientific journal

Ab initio derivation of multi-orbital extended Hubbard model for molecular crystals

From configuration interaction (CI) ab initio calculations, we derive an effective two-orbital extended Hubbard model based on the gerade (g) and ungerade (u) molecular orbitals (MOs) of the charge-transfer molecular conductor (TTM-TTP)I-3 and the single-component 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 tight-binding parameters, we analyze the band structure to confirm that two bands overlap and mix in together, supporting the multi-band feature. Furthermore, using a fragment decomposition, we derive the effective model based on the fragment MOs and show that the staking TTM-TTP molecules can be described by the zig-zag two-leg ladder with the inter-molecular transfer integral being larger than the intra-fragment transfer integral within the molecule. The inter-site interactions between the fragments follow a Coulomb law, supporting the fragment decomposition strategy. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3678307], AMER INST PHYSICS, Jan. 2012, JOURNAL OF CHEMICAL PHYSICS, 136 (4), 044519, doi;web_of_scienceScientific journal

Spectral functions in a two-velocities Tomonaga-Luttinger Liquid

We obtain the spectral functions of a two-velocities Tomonaga-Luttinger liquid at T = 0K in terms of the Appell hypergeometric functions F-1 and F-2. 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., IOP PUBLISHING LTD, 2012, INTERNATIONAL CONFERENCE ON STRONGLY CORRELATED ELECTRON SYSTEMS (SCES 2011), 391, 012163, doi;web_of_scienceInternational conference proceedings

Spectral functions of two-band spinless fermion and single-band spin-1/2 fermion models

We examine zero-temperature one-particle spectral functions for the one-dimensional two-band spinless fermions with different velocities and general forward-scattering interactions. By using the bosonization technique and diagonalizing the model to two Tomonaga-Luttinger 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 two-band fermions, corresponding to the SU(2) symmetric spin-1/2 fermions with repulsive interactions, the spectral functions can be expressed in terms of the Gauss hypergeometric function and are shown to recover the double-peak structure suggesting the well-known "spin-charge" separation. By tuning the difference in velocities for the two-band fermions, we clarify the crossover in spectral functions from the spin-charge separation to the decoupled fermions. We discuss the relevance of our results to the spin-1/2 Hubbard model under a magnetic field, which can be mapped onto two-band spinless fermions., AMER PHYSICAL SOC, Oct. 2011, PHYSICAL REVIEW B, 84 (16), 165128, doi;web_of_scienceScientific journal

Mean-Field Theory of Intra-Molecular Charge Ordering in (TTM-TTP)I-3

We examine an intra-molecular charge-ordered (ICO) state in the multi-orbital molecular compound (TTM-TTP)I-3 on the basis of an effective two-orbital model derived from ab initio calculations. Representing the model in terms of the fragment molecular-orbital (MO) picture, the ICO state is described as the charge disproportionation on the left and right fragment MOs. By applying the mean-field theory, the phase diagram of the ground state is obtained as a function of the inter-molecular Coulomb repulsion and the intra-molecular transfer integral. The ICO state is stabilized by large inter-fragment Coulomb interactions, and the small intra-molecular transfer energy between two fragment MOs. Furthermore, we examine the finite-temperature phase diagram. The relevance to the experimental observations in the molecular compound of (TTM-TTP)I-3 is also discussed., PHYSICAL SOC JAPAN, Feb. 2011, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 80 (2), 024707, doi;web_of_scienceScientific journal

Multi-orbital molecular compound (TTM-TTP)I3: Effective model and fragment decomposition

The electronic structure of the molecular compound (TTM-TTP)I3, which exhibits a peculiar intra-molecular charge ordering, has been studied using multi-configuration ab initio calculations. First we derive an effective Hubbard-type model based on the molecular orbitals (MOs) of TTM-TTP we set up a two-orbital 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 TTM-TTP molecules can be described by a two-leg ladder model, while the inter-fragment 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 low-energy properties of this compound. © 2011 The Physical Society of Japan., Jan. 2011, Journal of the Physical Society of Japan, 80 (1), 013703, doiScientific journal

Finite-Temperature Properties across the Charge Ordering Transition-Combined Bosonization, Renormalization Group, and Numerical Methods-

We theoretically describe the charge ordering (CO) metal-insulator transition based on a quasi-one-dimensional extended Hubbard model, and investigate the finite temperature (T) properties across the transition temperature, T-CO. In order to calculate T dependence of physical quantities such as the spin susceptibility and the electrical resistivity, both above and below T-CO, 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 finite-T is determined by quantum Monte Carlo simulations. The results show that the spin susceptibility does not show a steep singularity at T-CO, 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 T-CO, 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., PHYSICAL SOC JAPAN, Sep. 2010, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 79 (9), 094714, doi;web_of_scienceScientific journal

Intramolecular charge ordering in the multi molecular orbital system (TTM-TTP)I-3

Starting from the structure of the (TTM-TTP)I-3 molecular-based 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 second-highest-occupied molecular orbitals are close to each other, i.e., this compound should be regarded as a two-orbital system. By dividing virtually the [TTM-TTP] molecule into three fragments, an effective model is constructed to rationalize the origin of this picture. In order to investigate the low-temperature, symmetry breaking experimentally observed in the crystal, the electronic distribution in a pair of [TTM-TTP] molecules is analyzed from CASPT2 calculations. Our inspection supports and explains the speculated intramolecular charge ordering which is likely to give rise to low-energy magnetic properties. (C) 2010 American Institute of Physics. [doi:10.1063/1.3432764], AMER INST PHYSICS, Jun. 2010, JOURNAL OF CHEMICAL PHYSICS, 132 (21), 214705, doi;web_of_scienceScientific journal

Competition of superfluidity and density waves in one-dimensional Bose-Fermi mixtures

We study a mixture of one-dimensional 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 current-current interactions. We obtain the exact exponents for the various response functions, and show that superfluid fluctuations are enhanced by the effective boson-fermion density-density interaction and suppressed by the effective boson-fermion current-current interaction. In the case of a bosonized model with purely density-density interaction, when the effective boson-fermion density-density 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 density-wave exponent and the exponent for fermionic superfluidity are monotonic functions of the velocity ratio. © 2010 The American Physical Society., 24 May 2010, Physical Review A - Atomic, Molecular, and Optical Physics, 81 (5), doiScientific journal

Competition of superfluidity and density waves in one-dimensional Bose-Fermi mixtures

We study a mixture of one-dimensional 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 current-current interactions. We obtain the exact exponents for the various response functions, and show that superfluid fluctuations are enhanced by the effective boson-fermion density-density interaction and suppressed by the effective boson-fermion current-current interaction. In the case of a bosonized model with purely density-density interaction, when the effective boson-fermion density-density 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 density-wave exponent and the exponent for fermionic superfluidity are monotonic functions of the velocity ratio., AMER PHYSICAL SOC, May 2010, PHYSICAL REVIEW A, 81 (5), 053626, doi;web_of_scienceScientific journal

Correlation function for the one-dimensional extended Hubbard model at quarter filling

We examine the density-density correlation function in the Tomonaga-Luttinger liquid state for the one-dimensional extended Hubbard model with the on-site 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 renormalization-group methods, we obtain the generalized analytical form of the correlation function. We show that, in the proximity to the gapped charge-ordered phase, the correlation function exhibits anomalous crossover between the pure power-law behavior and the power-law behavior with logarithmic corrections, depending on the length scale. Such a crossover is also confirmed by the highly accurate numerical density-matrix renormalization-group method., AMER PHYSICAL SOC, Feb. 2010, PHYSICAL REVIEW B, 81 (8), 085116, doi;web_of_scienceScientific journal

Mechanism for the Singlet to Triplet Superconductivity Crossover in Quasi-One-Dimensional Organic Conductors

Superconductivity of quasi-one-dimensional organic conductors with a quarter-filled band is investigated using the two-loop renormalization group approach to the extended Hubbard model for which both the single electron hopping t(perpendicular to) and the repulsive interaction V-perpendicular to perpendicular to the chains are included. For a four-patches Fermi surface with deviations to perfect nesting, we calculate the response functions for the dominant fluctuations and possible superconducting states. By increasing V-perpendicular to, it is shown that a d-wave (singlet) to f-wave (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., PHYSICAL SOC JAPAN, Oct. 2009, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 78 (10), 104702, doi;web_of_scienceScientific journal

Peierls ground state and excitations in the electron-lattice correlated system (EDO-TTF)(2)X

We investigate the exotic Peierls state in the one-dimensional organic compound (EDO-TTF)(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 one-dimensional chain. Such a Peierls state, wherein the interplay between the electron correlation and the electron-phonon interaction takes an important role, is examined based on an extended Peierls-Hoistein-Hubbard 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 (EDO-TTF)(2)PF(6)., AMER PHYSICAL SOC, May 2008, PHYSICAL REVIEW B, 77 (19), 195128, doi;web_of_scienceScientific journal

Charge ordering and sliding-induced instability in thiazyl-radical organic crystal of (NT)3X

We examine exotic properties of the charge-ordered (CO) state which has been found even at room temperature in the 3:1 organic compound (NT) 3GaCl4 (NT=naphtho[2,1-d:6,5-d']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 two-chain extended Hubbard model. Within the mean-field 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 density-wave 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 non-linear conductivity found in the compound is also discussed. © 2008 IOP Publishing Ltd., Institute of Physics Publishing, 2008, Journal of Physics: Conference Series, 132, 012009, doiScientific journal

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 one-dimensional quarter-filled Peierls-Hubbard model with the nearest-neighbor repulsive interaction V and the electron-phonon interaction (alpha 1/K with K being the elastic constant). From the mean-field approach, we obtain the phase diagram for the ground state on the plane of parameters V and K. The coexistent state of the spin-density wave and the charge ordering is realized for large V and K. With decreasing K, it exhibits a first-order phase transition to the unconventional Peierls state which is described by the bond-centered charge-density-wave 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 (EDO-TTF)(2)PF6 compound., PHYSICAL SOC JAPAN, Nov. 2007, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 76 (11), 114709, doi;web_of_scienceScientific journal

Charge-ordered state versus Dimer-Mott insulator at finite temperatures

We theoretically investigate the competition between charge-ordered state and Mott insulating state at finite temperatures in quarter-filled quasi-one-dimensional 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 mean-field approximation. The results show that lattice dimerization, especially in the critical region, and frustration in the interchain Coulomb interactions reduce the charge-ordering phase transition temperature and enlarge the dimer-Mott insulating phase. We also derive a general formula of the Knight shift in the charge-ordered phase and its implication to experiments is discussed., PHYSICAL SOC JAPAN, Oct. 2007, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 76 (10), 103701, doi;web_of_scienceScientific journal

Interchain-frustration-induced metallic state in quasi-one-dimensional Mott insulators

The mechanism that drives a metal-insulator transition in an undoped quasi-one-dimensional 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 nearest-neighbor chains. By applying an N-perpendicular to-chain renormalization group method at the two-loop 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 half-filled molecular conductors., AMER PHYSICAL SOC, Sep. 2007, PHYSICAL REVIEW LETTERS, 99 (12), 126404, doi;web_of_scienceScientific journal

Effect of interchain frustration in quasi-one-dimensional conductors at half-filling

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 N-perpendicular 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., IOP PUBLISHING LTD, Apr. 2007, JOURNAL OF PHYSICS-CONDENSED MATTER, 19 (14), 145228, doi;web_of_scienceScientific journal

Role of fluctuations in density wave and superconductivity of quasi-one-dimensional conductors

The response functions, chi, for the spin density wave (SDW) and the superconductivity in the quasi-one-dimensional electron systems at quarter-filling are calculated by the newly developed N-chain renormalization group method. It is shown that the interchain hopping, t(perpendicular to), enhances chi of SDW having the dependence on temperature with the power-law even below the crossover temperature T-x similar to t(perpendicular to). Further, chi of the d-wave singlet superconductivity (dSS) is enhanced rapidly below T-x by t(perpendicular to), while there is no dominant behavior for dSS in the one-dimensional 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., ELSEVIER SCIENCE BV, Mar. 2007, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 310 (2), 1128 - 1129, doi;web_of_scienceScientific journal

Charge-ordered insulating state in beta-Na0.33V2O5

We discuss a possible scenario for the charge-ordered insulating state observed in the quasi-one-dimensional vanadium bronze beta-Na0.33V2O5 at ambient pressure, based on the picture of weakly-coupled two-leg ladders, proposed by Doublet and Lepetit [Phys. Rev. B 71 ( 2005) 075119]. The resultant charge-ordering pattern can have a 3-fold and/or 6-fold periodicity along the one-dimensional axis which is consistent with the X-ray 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 charge-ordering pattern reproduces well the results of the recent detailed NMR measurements. (c) 2006 Elsevier B.V. All rights reserved., ELSEVIER SCIENCE BV, Mar. 2007, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 310 (2), E200 - E202, doi;web_of_scienceScientific journal

Role of interchain hopping in the magnetic susceptibility of quasi-one-dimensional electron systems

A role of interchain hopping in quasi-one-dimensional (Q-1D) electron systems is investigated by extending the Kadanoff-Wilson renormalization group of one-dimensional (1D) systems to Q-1D 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 non-logarithmic 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 random-phase-approximation approach, which predicts an enhancement of chi(T) by t(perpendicular to) over the whole temperature range. The influence of both the long-range 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 Q-1D organic conductors, and propose a theoretical prediction for the effect of pressure on magnetic susceptibility., PHYSICAL SOC JAPAN, Jan. 2007, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 76 (1), 014709, doi;web_of_scienceScientific journal

Two-loop renormalization group theory for the quasi-one-dimensional Hubbard model at half filling

We derive two-loop renormalization group equations for the half filled one-dimensional Hubbard chains coupled by the interchain hopping. Our renormalization group scheme for the quasi-one-dimensional electron system is a natural extension of that for the purely one-dimensional systems in the sense that transverse-momentum dependences are introduced in the g-ological coupling constants and we regard the transverse momentum as a patch index. We develop symmetry arguments for the particle-hole symmetric half filled Hubbard model and obtain constraints on the g-ological 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 two-leg ladder system. By utilizing the field-theoretical Bosonization and Fermionization method, we derive low-energy effective theory and analyze the magnitude of all the excitation gaps in detail. It is shown that the low-energy excitations in the two-leg Hubbard ladder have SO(3)xSO(3)xU(1) symmetry when the interchain hopping exceeds the magnitude of the charge gap., AMER PHYSICAL SOC, Oct. 2006, PHYSICAL REVIEW B, 74 (15), 155109, doi;web_of_scienceScientific journal

Finite-temperature charge-ordering transition and fluctuation effects in quasi one-dimensional electron systems at quarter filling

Finite-temperature charge-ordering phase transition in quasi one-dimensional (ID) molecular conductors is investigated theoretically, based on a quasi I D extended Hubbard model at quarter filling with interchain Coulomb repulsion V-perpendicular to. The interchain term is treated within a mean-field 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 charge-ordered state at a finite temperature when V-perpendicular to is introduced: (i) a weak-coupling region where the system transforms from a metal to a charge-ordered insulator with a finite transition temperature at a finite critical value of V-perpendicular to, (ii) an intermediate region where this transition occurs by infinitesimal V-perpendicular to due to the stability of the inherent ID fluctuation, and (iii) a strong-coupling region where the charge ordered state is already realized in a purely 1D case, of which the transition temperature becomes finite with infinitesimal V-perpendicular to. An analytical formula for the V-perpendicular to dependence of the transition temperature is derived for each region., PHYSICAL SOC JAPAN, Jun. 2006, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 75 (6), 063706, doi;web_of_scienceScientific journal

Dimensionality effect in quasi-one-dimensional mott insulators

We examine the effect of interchain hopping on the quasi-one-dimensional Hubbard model at half filling. Treating interactions within the weak-coupling perturbation theory, we derive two-loop renormalization-group 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., SPRINGER/PLENUM PUBLISHERS, Feb. 2006, JOURNAL OF LOW TEMPERATURE PHYSICS, 142 (3-4), 651 - 654, doi;web_of_scienceScientific journal

Magnetic susceptibility in normal states of quasi-one-dimensional superconductors

The magnetic susceptibility chi(T) in normal states of the quasi-one-dimensional (Q1D) superconductor is calculated by using the N-chain renormalization group technique developed newly. For temperatures above T-cross being of the order of the interchain hopping t(perpendicular to), chi(T) of Q1D system agrees, with that qf one-dimensional systems. For temperatures below T-cross 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 "pseudo-gap like" behavior is discussed in terms of antiferromagnetic spin fluctuations., SPRINGER/PLENUM PUBLISHERS, Feb. 2006, JOURNAL OF LOW TEMPERATURE PHYSICS, 142 (3-4), 507 - 510, doi;web_of_scienceScientific journal

Effect of intersite repulsions on magnetic susceptibility of one-dimensional electron systems at quarter filling

The temperature dependence of magnetic susceptibility, chi(T), is investigated for one-dimensional interacting electron systems at quarter filling within the Kadanoff-Wilson renorrualization-group 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 nearest-neighbor interaction, an enhancement which surpasses that of the next-nearest-neighbor interaction. A connection between our predictions for chi(T) and experimental results for chi(T) in quasi-one-dimensional organic conductors is presented., PHYSICAL SOC JAPAN, Dec. 2005, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 74 (12), 3159 - 3162, doi;web_of_scienceScientific journal

Magnetic response and quantum critical behavior in the doped two-leg extended Hubbard ladder

We have investigated quantum critical behavior in the doped two-leg extended Hubbard ladder, by using a weak-coupling bosonization method. In the ground state, the dominant fluctuation changes from the conventional d-wave-like superconducting (SCd) state into density-wave states, with increasing nearest-neighbor repulsions and/or decreasing doping rate. The competition between the SCd state and the charge-density-wave state coexisting with the p-density-wave state becomes noticeable on the critical point, at which the gap for magnetic excitations vanishes. Based on the Majorana-fermion 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 power-law behavior. We discuss the commensurability effect due to the umklapp scattering and relevance to the two-leg ladder compounds Sr14-xCaxCu24O41., AMERICAN PHYSICAL SOC, Aug. 2005, PHYSICAL REVIEW B, 72 (7), 075121, doi;web_of_scienceScientific journal

Quantum phase transition between the spin-Peierls state and the antiferromagnetic state in the TMTTF organic compounds

A competition between the spin-Peierls (SP) state and the anti ferromagnetic (AF) state has been examined for the half-filled quasi -one-dimensional electron system coupled with dynamical phonon. By considering the anti-adiabatic limit of phonon, an effective one-dimensional Hamiltonian is derived which includes fluctuations of both the SP state and the AF state. We have shown, by using the renormalization-group 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 quasi-one-dimensional conductors, TMTTF compounds. (c) 2005 Elsevier B.V. All rights reserved., ELSEVIER SCIENCE BV, Apr. 2005, PHYSICA B-CONDENSED MATTER, 358 (1-4), 42 - 49, doi;web_of_scienceScientific journal

Spin-Peierls transition temperature in quarter-filled organic conductors

We examine the transition temperature of spin-Peierls state, T-SP, in the one-dimensional quarter-filled Peierls-Hubbard model with lattice dimerization and intersite repulsive interaction, V, by applying the bosonization and the renormalization-group method. It is shown that for large V the transition temperature T-SP is suppressed due to the appearance of the charge ordering. In the spin-Peierls state, the location of the spin-singlet state is shifted continuously from the bond of the lattice to the on-site 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 T-SP, which has been observed in the NMR measurement., PHYSICAL SOC JAPAN, Mar. 2005, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 74 (3), 983 - 987, doi;web_of_scienceScientific journal

Interplay of lattice dimerization and charge ordering in one-dimensional quarter-filled electron systems

The effect of charge ordering on lattice dimerization has been investigated in the one-dimensional extended Peierls-Hubbard model at quarter filling. We estimate the strength of the lattice dimerization by applying the bosonization method and the perturbative renormalization-group method. In the ground state, it is shown that a first-order phase transition takes place between the dimerized insulator and the charge-ordered insulator. The lattice dimerization disappears in the charge-ordered 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., PHYSICAL SOC JAPAN, Sep. 2004, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 73 (9), 2487 - 2493, doi;web_of_scienceScientific journal

Charge-density-wave formation in the doped two-leg extended hubbard ladder

We investigate the electronic properties of a doped two-leg Hubbard ladder with both the onsite and nearest-neighbor Coulomb repulsions by using the weak-coupling renormalization-group method. It is shown that, for strong nearest-neighbor repulsions, the charge-density-wave state coexisting with the p-density-wave state becomes a dominant fluctuation where spins form intrachain singlets. By increasing doping rate, we have also shown that the effects of the nearest-neighbor repulsions are reduced and the system exhibits a quantum phase transition into the d-wave-like (or rung-singlet) 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 two-leg ladder compound, Sr14-xCaxCu24O41, is discussed., PHYSICAL SOC JAPAN, Apr. 2004, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 73 (4), 804 - 807, doi;web_of_scienceScientific journal

Ground-state phase diagram of the one-dimensional half-filled extended Hubbard model

We revisit the ground-state phase diagram of the one-dimensional half-filled extended Hubbard model with on-site (U) and nearest-neighbor (V) repulsive interactions. In the first half of the paper, using the weak-coupling renormalization-group approach (g-ology) including second-order corrections to the coupling constants, we show that bond-charge-density-wave (BCDW) phase exists for Uapproximate to2V in between charge-density-wave (CDW) and spin-density-wave (SDW) phases. We find that the umklapp scattering of parallel-spin electrons disfavors the BCDW state and leads to a bicritical point where the CDW-BCDW and SDW-BCDW continuous-transition lines merge into the CDW-SDW first-order 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 CDW-BCDW transition from the Gaussian transition into the Ising transition., AMER PHYSICAL SOC, Jan. 2004, PHYSICAL REVIEW B, 69 (3), 035103, doi;web_of_scienceScientific journal

Staggered flux state in the two-leg Hubbard ladder at half filling

We investigate the staggered flux state in the half-filled extended Hubbard ladder using the strong-coupling perturbation theory. The staggered flux state has long-range order of currents on rungs flowing in the alternating direction. We derive a low-energy effective Hamiltonian and study an Ising-type quantum phase transition between the staggered flux state and the d-wave-pairing Mott insulating state. (C) 2003 Elsevier Science B.V. All rights reserved., ELSEVIER SCIENCE BV, May 2003, PHYSICA B-CONDENSED MATTER, 329, 981 - 982, doi;web_of_scienceScientific journal

Commensurate-incommensurate transition in a one-dimensional dimerized electron system close to quarter-filling

We investigate electronic states upon doping (i.e., by varying the chemical potential) for a quarter-filled one-dimensional electron system with a dimerization, on-site and nearest-neighbor repulsive interaction. By applying the renormalization group method to the bosonized Hamiltonian, the metal-insulator transition followed by commensurate-incommensurate 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., ELSEVIER SCIENCE SA, Mar. 2003, SYNTHETIC METALS, 133, 49 - 50, doi;web_of_scienceScientific journal

Generalized two-leg Hubbard ladder at half filling: Phase diagram and quantum criticalities

The ground-state phase diagram of the half-filled two-leg Hubbard ladder with intersite Coulomb repulsions and exchange coupling is studied by using the strong-coupling perturbation theory and the weak-coupling bosonization method. Considered here as possible ground states of the ladder model are four types of density-wave states with different angular momentum (s-density-wave state, p-density-wave state, d-density-wave state, and f-density-wave state) and four types of quantum disordered states, i.e., Mott insulating states (S-Mott, D-Mott, S'-Mott, and D'-Mott states, where S and D stand for s- and d-wave symmetry). The s-density-wave state, the d-density-wave state, and the D-Mott state are also known as the charge-density-wave state, the staggered-flux state, and the rung-singlet state, respectively. Strong-coupling approach naturally leads to the Ising model in a transverse field as an effective theory for the quantum phase transitions between the staggered-flux state and the D-Mott state and between the charge-density-wave state and the S-Mott state, where the Ising ordered states correspond to doubly degenerate ground states in the staggered-flux or the charge-density-wave state. From the weak-coupling bosonization approach it is shown that there are three cases in the quantum phase transitions between a density-wave state and a Mott state: the Ising (Z(2)) criticality, the SU(2)(2) criticality, and a first-order transition. The quantum phase transitions between Mott states and between density-wave states are found to be the U(1) Gaussian criticality. The ground-state phase diagram is determined by integrating perturbative renormalization-group equations. It is shown that the S-Mott state and the staggered-flux state exist in the region sandwiched by the charge-density-wave phase and the D-Mott phase. The p-density-wave state, the S'-Mott state, and the D'-Mott state also appear in the phase diagram when the next-nearest-neighbor repulsion is included. The correspondence between Mott states in extended Hubbard ladders and spin-liquid states in spin ladders is also discussed., AMER PHYSICAL SOC, Dec. 2002, PHYSICAL REVIEW B, 66 (24), 245106, doi;web_of_scienceScientific journal

Ising transition in a one-dimensional quarter-filled electron system with dimerization

We examine critical properties of the quarter-filled one-dimensional Hubbard model with dimerization and with the onsite and nearest-neighbor 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 charge-ordered insulating state. It is also shown that the dielectric permittivity exhibits a strong enhancement as decreasing temperature with power-law dependence at the Ising critical point. (C) 2002 Elsevier Science Ltd. All rights reserved., PERGAMON-ELSEVIER SCIENCE LTD, Jun. 2002, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 63 (6-8), 1459 - 1463, doi;web_of_scienceScientific journal

Phase diagram of the one-dimensional extended Hubbard model at half filling

We reexamine the ground-state phase diagram of the one-dimensional half-filled Hubbard model with on-site and nearest-neighbor repulsive interactions. We calculate second-order corrections to coupling constants in the weak-coupling renormalization-group approach (g-ology) to show that the bond-charge-density-wave (BCDW) phase exists for weak couplings in between the charge-density-wave (CDW) and spin-density-wave (SDW) phases. We find that the umklapp scattering of parallel-spin electrons destabilizes the BCDW state and gives rise to a bicritical point where the CDW-BCDW and SDW-BCDW continuous-transition lines merge into the CDW-SDW first-order transition line., AMERICAN PHYSICAL SOC, Feb. 2002, PHYSICAL REVIEW LETTERS, 88 (5), 056402, doi;web_of_scienceScientific journal

Crossover from quarter-filling to half-filling in a one-dimensional electron system with a dimerized and quarter-filled band

The interplay between quarter-filled and half-tilled umklapp scattering has been examined by applying the renormalization group method to a one-dimensional quarter-filled electron system with dimerization, on-site (U) and nearest-neighbor (V) repulsive interactions. The phase diagram on the U-V 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 quarter-filling to that of half-filling with decreasing temperature., PHYSICAL SOC JAPAN, Jun. 2001, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 70 (6), 1460 - 1463, doi;web_of_scienceScientific journal

Effects of next-nearest-neighbor repulsion on one-dimensional quarter-filled electron systems

We examine effects of the next-nearest-neighbor repulsion on electronic states of a one-dimensional interacting electron system which consists of quarter-filled band and interactions of on-site and nearest-neighbor repulsion. We derive the effective Hamiltonian for the electrons around wave number +/-k(F) (K-F: Fermi wave number) and apply the renormalization group method to the bosonized Hamiltonian. It is shown that the next-nearest-neighbor repulsion makes 4k(F)-charge ordering unstable and suppresses the spin fluctuation. Further the excitation gaps and spin susceptibility are also evaluated., PHYSICAL SOC JAPAN, Mar. 2001, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 70 (3), 762 - 773, doi;web_of_scienceScientific journal

Spinless fermion ladders at half filling

We study a half-filled 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 confinement-deconfinement 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)., AMER PHYSICAL SOC, Jan. 2001, PHYSICAL REVIEW B, 63 (4), 045121, doi;web_of_scienceScientific journal

Commensurate-incommensurate transition in two-coupled chains of nearly half-filled electrons

We investigate the physical properties of two coupled chains of electrons, with a nearly half-filled band, as a function of the interchain hopping t(perpendicular to) and the doping. Mie show that upon doping, the system undergoes a metal-insulator transition well described by a commensurate-incommensurate 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 confinement-deconfinement 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 metal-insulator transition the Luttinger parameter takes the universal value K-rho* = 1, in agreement with previous results on special limits of this model., SPRINGER-VERLAG, Jan. 2001, EUROPEAN PHYSICAL JOURNAL B, 19 (2), 185 - 193, doi;web_of_scienceScientific journal

Electronic states in correlated three-coupled chains

Three-coupled chains of a quarter-filled 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 two-coupled chains. (C) 2000 Elsevier Science Ltd. All rights reserved., PERGAMON-ELSEVIER SCIENCE LTD, Jan. 2001, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 62 (1-2), 427 - 430, doi;web_of_scienceScientific journal

Commensurability in one-dimensional electron systems at quarter-filling

We examine the role of commensurability in electronic states and excitations for a one-dimensional interacting electron system at quarter-filling with on-site (U), nearest-neighbor (V-1) and next-nearest-neighbor (V-2) repulsive interactions, By applying the renormalization group method to the bosonized model, it is shown that the next-nearest-neighbor repulsion stabilizes 2k(F) density waves and induces the excitation gap in spin fluctuation for V-2 > U/2, (C) 2000 Elsevier Science Ltd. All rights reserved., PERGAMON-ELSEVIER SCIENCE LTD, Jan. 2001, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 62 (1-2), 419 - 422, doi;web_of_scienceScientific journal

Mechanism of confinement in low-dimensional organic conductors

Confinement-deconfinement transition in quarter-filled two-coupled 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 half-filling. 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., PERGAMON-ELSEVIER SCIENCE LTD, Jan. 2001, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 62 (1-2), 93 - 97, doi;web_of_scienceScientific journal

Spinless fermion ladders at half filling

We study a half-filled 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 confinement-deconfinement 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, Physical Review B - Condensed Matter and Materials Physics, 63 (4), 451211 - 451218, doiScientific journal

Correlation gap in one-dimensional quarter-filled electron system

Umklapp scattering in an one-dimensional quarter-filled electron system with both on-site U(> 0) and nearest-neighbor 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 two-fold periodicity. (C) 2000 Elsevier Science B.V. All rights reserved., ELSEVIER SCIENCE BV, Jul. 2000, PHYSICA B, 284, 1547 - 1548, doi;web_of_scienceScientific journal

Confinement by charge gap in organic conductor Bechgaard salts

In Bechgaard salts, which are effectively half-filling 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 two-coupled chains of quarter-filled 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., ELSEVIER SCIENCE BV, Jun. 2000, PHYSICA B, 281, 684 - 685, doi;web_of_scienceScientific journal

Correlation effects in a one-dimensional quarter-filled electron system with repulsive interactions

A one-dimensional electron system at quarter-filling has been examined by applying the renormalization group method to a bosonized model with on-site (U) and nearest-neighbor (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) mol-es 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 next-nearest-neighbor repulsion is also discussed., PHYSICAL SOC JAPAN, Mar. 2000, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 69 (3), 651 - 654, doi;web_of_scienceElectronic states in half-filled correlated system with alternating potential

The effect of an alternating potential on a one-dimensional half-tilled 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., PHYSICAL SOCIETY JAPAN, Dec. 1999, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 68 (12), 3966 - 3974, doi;web_of_scienceScientific journal

Theoretical study on the charge gap of organic conductor - Bechgaard salts

The charge gap in a quarter-filled organic conductor, which is induced hy dimerization and/or alternating potential is studied by applying the renormalization group method to the bosonized one-dimensional Kamiltonian. The interplay of dimerization and alternating potential gives rise to the unconventional enhancement of the charge gap in addition to the on-site repulsive interaction. The experimental results of resistivity and optical conductivity in Bechgaard salts are examined based on the present calculation., PHYSICAL SOC JAPAN, Jun. 1999, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 68 (6), 1809 - 1812, doi;web_of_scienceCompetition between SDW and SC states in two-Hubbard 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., ELSEVIER SCIENCE SA, Jun. 1999, SYNTHETIC METALS, 103 (1-3), 2189 - 2190, doi;web_of_scienceScientific journal

Confinement by umklapp scattering in two coupled chains

The effect of umklapp scattering on two coupled chains with both half-filled 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., ELSEVIER SCIENCE SA, Jun. 1999, SYNTHETIC METALS, 103 (1-3), 2191 - 2192, doi;web_of_scienceScientific journal

Confinement-deconfinement transition in two coupled chains with umklapp scattering

A role of umklapp scattering has been examined for two-coupled 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 confinement-deconfinement transition is calculated as the function of interchain hopping and intrachain interactions. A crossover from one-dimensional regime into that of coupled chains is also shown with decreasing temperature. [S0163-1829(99)11019-1]., AMER PHYSICAL SOC, May 1999, PHYSICAL REVIEW B, 59 (19), 12326 - 12337, doi;web_of_scienceScientific journal

Renormalized inter-chain hopping vs charge gap in two coupled chains

The role of intra-chain interactions, which renormalize inter-chain hopping, has been examined for two coupled chains with umklapp scattering by use of the renormalization group method. It is clarified that the confinement-deconfinement transition occurs when the charge gap becomes larger than the renormalized inter-chain hopping., PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE, Mar. 1999, PROGRESS OF THEORETICAL PHYSICS, 101 (3), 763 - 768, doi;web_of_scienceScientific journal

Two-coupled chains with spin-anisotropic backward scattering

By applying renormalization group method to the bosonized Hamiltonian of two-coupled chains with repulsive intrachain interaction, we have examined a role of backward scattering with a spin-anisotropy 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 g-ology. (C) 1998 Elsevier Science B.V. All rights reserved., ELSEVIER SCIENCE BV, Jul. 1998, PHYSICA C, 303 (3-4), 246 - 256, doi;web_of_scienceScientific journal

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 half-filled band. By analyzing in terms of the renormalization-group 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 spin-density-wave state in organic conductors., AMER PHYSICAL SOC, Jun. 1998, PHYSICAL REVIEW B, 57 (24), 15040 - 15043, doi;web_of_scienceScientific journal

Crossover between high and low energy-states in two-coupled chains of Tomonaga model

By applying the renormalization group method to two-coupled 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 one-dimensional 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., KYOTO UNIV, Nov. 1997, PROGRESS OF THEORETICAL PHYSICS, 98 (5), 1045 - 1062, doi;web_of_scienceScientific journal

Electron nematic transitions in cuprate superconductors

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