Excitonic insulator state in the two-orbital Hubbard model: Variational cluster approach

Kaneko, Tatsuya; Seki, Koh‐ichi; Ohta, Y.

doi:10.1103/physrevb.85.165135

Cited by 70 publications

(92 citation statements)

References 38 publications

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“…Evidently, excitonic CDW and SDW states are degenerate at J = J = 0 and λ = 0 42,43 . Any finite J and/or λ lifts this degeneracy.…”

Section: Excitonic Sdw Statementioning

confidence: 99%

“…For simplicity, we assume U f = U c = U and employ U = 2U − J to suppress the Hartree shift 46 . In this choice, the EI state is stabilized between the band-insulator and Mott-insulator states 42,43 . Moreover, we consider a dispersionless Einstein phonon ω q = ω and a momentum-independent electron-phonon coupling constant g q = g. Since the strength of the electron-phonon coupling appears in the form λ = g 2 /ω in the mean-field approximation used below, we take λ as the electron-phonon coupling parameter in what follows.…”

Section: A Model Hamiltonianmentioning

confidence: 99%

“…The conduction-band bottom at k = (0, 0) gives rise to an electron pocket, while the valence-band top produces a hole pocket at k = (π, π), resulting in the modulation vector of the density wave Q = (π, π); see Ref. 42 for the band dispersion and Fermi surface in the Brillouin zone of the square lattice. For simplicity, we assume U f = U c = U and employ U = 2U − J to suppress the Hartree shift 46 .…”

Section: A Model Hamiltonianmentioning

confidence: 99%

“…Excitonic phases in strongly correlated spinful systems can be discussed in the framework of two-band Hubbard-type models. Including thereby the Hund's rule coupling is known to stabilize the spin-triplet excitonic phase in the otherwise degenerate spin-singlet and spintriplet excitonic phases [18][19][20]28,42,43 . On the other hand, we have shown in our previous work 43 that taking into account electronic interactions only, a spin-singlet excitonic phase cannot be stabilized, which may however be realized in 1T -TiSe 2 and Ta 2 NiSe 5 , where the importance of electron-phonon coupling was recently pointed out [10][11][12][13][14]23 .…”

Section: Introductionmentioning

confidence: 99%

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Competition between excitonic charge and spin density waves: Influence of electron-phonon and Hund's rule couplings

et al. 2015

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We analyze the stability of excitonic ground states in the two-band Hubbard model with additional electron-phonon and Hund's rule couplings using a combination of mean-field and variational cluster approaches. We show that both the interband Coulomb interaction and the electron-phonon interaction will cooperatively stabilize a charge density wave (CDW) state which typifies an "excitonic" CDW if predominantly triggered by the effective interorbital electron-hole attraction or a "phononic" CDW if mostly caused by the coupling to the lattice degrees of freedom. By contrast, the Hund's rule coupling promotes an excitonic spin density wave. We determine the transition between excitonic charge and spin density waves and comment on a fixation of the phase of the excitonic order parameter that would prevent the formation of a superfluid condensate of excitons. The implications for exciton condensation in several material classes with strongly correlated electrons are discussed.

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“…Evidently, excitonic CDW and SDW states are degenerate at J = J = 0 and λ = 0 42,43 . Any finite J and/or λ lifts this degeneracy.…”

Section: Excitonic Sdw Statementioning

confidence: 99%

Section: A Model Hamiltonianmentioning

confidence: 99%

Section: A Model Hamiltonianmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Competition between excitonic charge and spin density waves: Influence of electron-phonon and Hund's rule couplings

et al. 2015

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“…[33][34][35][36][37][38][39][40][41] The multiband Hubbard model, taking into account the spin de- * ohta@faculty.chiba-u.jp grees of freedom, has also been used to discuss, for example, the relative stability of the spin-singlet and spintriplet excitonic phases. [42][43][44][45][46][47][48][49] We have so far studied the excitonic phases of the EFKM and multiband Hubbard models using the variational cluster approximation (VCA) based on the exact diagonalization of small clusters. 50,51) However, some difficulty arises in such a small-cluster approach, particularly when we calculate physical quantities as a function of the model parameters.…”

Section: Introductionmentioning

confidence: 99%

Excitonic Insulator State of the Extended Falicov–Kimball Model in the Cluster Dynamical Impurity Approximation

et al. 2017

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We comparatively study the excitonic insulator state in the extended Falicov-Kimball model (EFKM, a spinless two-band model) on the two-dimensional square lattice using the variational cluster approximation (VCA) and the cluster dynamical impurity approximation (CDIA). In the latter, the particle-bath sites are included in the reference cluster to take into account the particle-number fluctuations in the correlation sites. We thus calculate the particle-number distribution, order parameter, ground-state phase diagram, anomalous Green's function, and pair coherence length, thereby demonstrating the usefulness of the CDIA in the discussion of the excitonic condensation in the EFKM. IntroductionThe excitonic phases, often referred to as excitonic insulators (EIs), are the states where the valence and conduction bands are hybridized spontaneously by the interband Coulomb interaction, and have been predicted to occur near the semimetal-semiconductor phase boundary as the quantum condensation of electron-hole pairs (excitons). [1][2][3][4][5][6] In the semimetallic region, where the Coulomb interaction is largely screened by free carriers, the excitonic phase is described in analogy to the BCS theory of superconductivity, whereas in the semiconducting region, it is described as the Bose-Einstein condensation (BEC) of preformed excitons (or strongly bound electron-hole pairs). Thus, the BCS-BEC crossover is expected to occur by controlling the band gap from a negative value to a positive one. [7][8][9] In recent years, the possible realization of spinsinglet excitonic condensation has been suggested for transition-metal chalcogenides such as 1T -TiSe 2 and Ta 2 NiSe 5 . 10-23) The spin-triplet excitonic condensation has also been suggested to occur in the high-spin/lowspin crossover region of some cobalt oxide materials with the cubic perovskite structure. 24-29) Because these materials are among transition-metal chalcogenides and oxides, where the effects of electron correlations are strong, one must reconsider the excitonic phases from the standpoint of strongly correlated electron systems. 30-32) Thus, the lattice models such as Hubbard models, rather than the gas models, are appropriate for use. The spinless extended Falicov-Kimball model (EFKM) is the simplest lattice model for describing the excitonic phases, and has been used to discuss, for example, the BCS-BEC crossover of the excitonic condensation. 33-41) The multiband Hubbard model, taking into account the spin de-

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Combined effects of local and nonlocal hybridization on formation and condensation of excitons in the extended Falicov-Kimball model

Farkašovský

2017

Solid State Communications

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We study the combined effects of local and nonlocal hybridization on the formation and condensation of the excitonic bound states in the extended FalicovKimball model by the density-matrix-renormalization-group (DMRG) method. Analysing the resultant behaviours of the excitonic momentum distribution N (q) we found, that unlike the local hybridization V , which supports the formation of the q = 0 momentum condensate, the nonlocal hybridization V n supports the formation of the q = π momentum condensate. The combined effect of local and nonlocal hybridization further enhances the excitonic correlations in q = 0 as well as q = π state, especially for V and V n values from the charge-density-wave (CDW) region. Strong effects of local and nonlocal hybridization are observed also for other ground-state quantities of the model such as the f -electron density, or the density of unbound d-electrons, which are generally enhanced with increasing V and V n . The same calculations performed for nonzero values of f -level energy E f revealed that this model can yield a reasonable explanation for the pressure-induced resistivity anomaly observed experimentally in T mSe 0.45 T e 0.55 compound.

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Excitonic insulator state in the two-orbital Hubbard model: Variational cluster approach

Cited by 70 publications

References 38 publications

Competition between excitonic charge and spin density waves: Influence of electron-phonon and Hund's rule couplings

Competition between excitonic charge and spin density waves: Influence of electron-phonon and Hund's rule couplings

Excitonic Insulator State of the Extended Falicov–Kimball Model in the Cluster Dynamical Impurity Approximation

Combined effects of local and nonlocal hybridization on formation and condensation of excitons in the extended Falicov-Kimball model

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