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Strong Coupling Nature of the Excitonic Insulator State in Ta2NiSe5

Koudai Sugimoto, Satoshi Nishimoto, Tatsuya Kaneko, and Yukinori Ohta
Phys. Rev. Lett. 120, 247602 – Published 14 June 2018
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    Abstract

    We analyze the measured optical conductivity spectra using the density-functional-theory-based electronic structure calculation and density-matrix renormalization group calculation of an effective model. We show that, in contrast to a conventional description, the Bose-Einstein condensation of preformed excitons occurs in Ta2NiSe5, despite the fact that a noninteracting band structure is a band-overlap semimetal rather than a small band-gap semiconductor. The system above the transition temperature is therefore not a semimetal but rather a state of preformed excitons with a finite band gap. A novel insulator state caused by the strong electron-hole attraction is thus established in a real material.

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    • Received 30 August 2017
    • Revised 29 April 2018

    DOI:https://doi.org/10.1103/PhysRevLett.120.247602

    © 2018 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    ExcitonsPhase transitionsSemiconductors
    1. Physical Systems
    Strongly correlated systems
    1. Techniques
    Density functional calculationsDensity matrix renormalization groupHubbard modelMean field theory
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    Koudai Sugimoto1, Satoshi Nishimoto2,3, Tatsuya Kaneko4, and Yukinori Ohta5

    • 1Center for Frontier Science, Chiba University, Chiba 263-8522, Japan
    • 2Department of Physics, Technical University Dresden, 01069 Dresden, Germany
    • 3Institute for Theoretical Solid State Physics, IFW Dresden, 01171 Dresden, Germany
    • 4Computational Condensed Matter Physics Laboratory, RIKEN, Wako, Saitama 351-0198, Japan
    • 5Department of Physics, Chiba University, Chiba 263-8522, Japan

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    Vol. 120, Iss. 24 — 15 June 2018

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    Images

    • Figure 1
      Figure 1

      Schematic representation of the conventional phase diagram of an excitonic insulator [8]. The band gap Eg in the noninteracting band structure is either positive (semiconducting) or negative (semimetallic). The crossover between the semimetallic and semiconducting states at finite temperatures (T) is indicated by the dashed line.

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    • Figure 2
      Figure 2

      Calculated optical conductivity spectra of Ta2NiSe5 (purple line) and Ta2NiS5 (green line) for the electric field E parallel to (a) the crystallographic a axis and (b) the c axis. Vertical lines indicate the positions of the major peaks for Ta2NiSe5, which are labeled α, β, γ, and δ following Ref. [15] in (a). The same labels are put in (b) but have no relation to the labels in (a).

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    • Figure 3
      Figure 3

      Calculated optical conductivity spectra σ(ω) of the extended Falicov-Kimball model using an L×2=100×2 cluster. We assume tc=1 and tf=−0.5, and D is chosen to keep ⟨ci†ci⟩=0.1 for each V. The inset shows the calculated single-particle spectrum A(k,ω) of the same model with L×2=60×2 and its enlargement near the Fermi level, where the broadening is η=0.05.

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    • Figure 4
      Figure 4

      Ground-state phase diagrams of (a) the two-chain Hubbard model with tc=t, tf=−0.5t, and U=2V and (b) the three-chain Hubbard model with tc=0.8  eV, tf=−0.4  eV, and U=4V calculated by the mean-field approximation, neglecting the Hartree shift [32]. Corresponding quasiparticle band dispersions are also illustrated schematically.

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