Phase diagram of the asymmetric Hubbard model

Farkašovský, Pavol

doi:10.1103/physrevb.77.085110

Cited by 48 publications

(98 citation statements)

References 15 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…From the theoretical side, the extended FalicovKimball model was considered as a paradigmatic model to describe the EI formation and the closely related phenomenon of electronic ferroelectricity [30][31][32][33][34][35][36][37][38][39][40][41] . Here the spin degrees of freedom were not taken into account, however.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2015

View full text Add to dashboard Cite

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.

show abstract

Section: Introductionmentioning

confidence: 99%

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

et al. 2015

View full text Add to dashboard Cite

show abstract

“…While the CDW ground state is stable for all ratios t f /t c at δ = 0, it becomes rapidly suppressed for δ > 0, especially if the c-and f-bandwidths are comparable. 17 Since we are interested in the (homogeneous) condensed excitonic phase only, we adjust the parameters δ, t f , and U accordingly. To make contact with previous Hartree-Fock approaches 17,18 , we use the equation of motion method for the anticommutator Green functions 20 a kσ ; a † kσ ω and a kσ ; a † k,−σ ω , and perform a decoupling that allows for the description of the FE EI phase by the order parameter…”

mentioning

confidence: 99%

Bound state formation and the nature of the excitonic insulator phase in the extended Falicov-Kimball model

et al. 2008

View full text Add to dashboard Cite

Motivated by the possibility of pressure-induced exciton condensation in intermediate-valence Tm[Se,Te] compounds we study the Falicov-Kimball model extended by a finite f-hole valence bandwidth. Calculating the Frenkel-type exciton propagator we obtain excitonic bound states above a characteristic value of the local interband Coulomb attraction. Depending on the system parameters coherence between c-and f-states may be established at low temperatures, leading to an excitonic insulator phase. We find strong evidence that the excitonic insulator typifies either a BCS condensate of electron-hole pairs (weak-coupling regime) or a Bose-Einstein condensate (BEC) of preformed excitons (strong-coupling regime), which points towards a BCS-BEC transition scenario as Coulomb correlations increase.PACS numbers: 71.28.+d, 71.35.Lk, 71.30.+h, 71.28.+d. 71.27.+a That excitons in solids might condense into a macroscopic phase-coherent quantum state-the excitonic insulator-was theoretically proposed about more than four decades ago 1 , for a recent review see Ref.2. The experimental confirmation has proved challenging, because excitonic quasiparticles are not the ground state but bound electron-hole excitations that tend to decay on a very short timescale. Thus a large number of excitons has to be created, e.g. by optical pumping, with sufficiently long lifetimes as a steady-state precondition for the Bose-Einstein condensate (BEC) realizing process.The obstacles to produce a BEC out of the faroff-equilibrium situation caused by optical excitation might be circumvented by pressure-induced generation of excitons. Hints that pressure-sensitive, narrow-gap semiconducting materials, such as intermediate-valent TmSe 0.45 Te 0.55 , might host an excitonic BEC in solids came from a series of electric and thermal transport measurements. 3 Fine-tuning the excitonic level, by applying pressure, to the level of electrons in the narrow 4f-valence band, excitons can form near the semiconductor semimetal transition in thermodynamical equilibrium and might give rise to collective excitonic phases. A phase diagram has been deduced out of the resistivity, thermal diffusity and heat conductivity data, which contains, below 20 K and in the pressure range between 5 and 11 kbar, a superfluid Bose condensed state. 4 The experimental claims for excitonic condensation in TmSe 0.45 Te 0.55 have been analysed from a theoretical point of view. 5,6 Adapting the standard effective-mass, (statically) screened Coulomb interaction model to the Tm[Se,Te] electron-hole system, the valence-band-hole conduction-band-electron mass asymmetry was found to suppress the excitonic insulator (EI) phase on the semimetallic side, as observed experimentally. But also on the semiconducting side, the EI instability might be prevented-within this model-by either electron-hole liquid phases 6,7 or, at very large electron-hole mass ratios ( 100), by Coulomb crystallization. 8 The effective-mass Mott-Wannier-type exciton model neglects, however, important band structure effec...

show abstract

“…[33][34][35]41) At D = 0, where n f = n c = 0.5, the SOO phase appears, in which an electron occupies the c and f orbitals alternately. Comparison with the XXZ model obtained in the strong correlation limit of the EFKM indicates that the SOO phase becomes more stable when |t c |/|t f | ≪ 1.…”

Section: Extended Falicov-kimball Modelmentioning

confidence: 99%

“…[33][34][35][36][37][38][39][40][41] Assuming a two-dimensional square lattice, we define the Hamiltonian of the EFKM as…”

Section: Extended Falicov-kimball Modelmentioning

confidence: 99%

“…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][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.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

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

et al. 2017

View full text Add to dashboard Cite

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-

show abstract

Phase diagram of the asymmetric Hubbard model

Cited by 48 publications

References 15 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

Bound state formation and the nature of the excitonic insulator phase in the extended Falicov-Kimball model

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

Contact Info

Product

Resources

About