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  • Letter

Gap opening mechanism for correlated Dirac electrons in organic compounds α−(BEDT−TTF)2I3 and α−(BEDT−TSeF)2I3

Daigo Ohki, Kazuyoshi Yoshimi, Akito Kobayashi, and Takahiro Misawa
Phys. Rev. B 107, L041108 – Published 25 January 2023
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    Abstract

    To determine how electron correlations open a gap in two-dimensional massless Dirac electrons in the organic compounds α−(BEDT−TTF)2I3 [α−(ET)2I3] and α−(BEDT-TSeF)2I3 [α−(BETS)2I3], we derive and analyze ab initio low-energy effective Hamiltonians for these two compounds. We find that the horizontal stripe charge ordering opens a gap in the massless Dirac electrons in α−(ET)2I3, while an insulating phase without explicit symmetry breaking appears in α−(BETS)2I3. We clarify that a combination of anisotropic transfer integrals and electron correlations induces a dimensional reduction in the spin correlations, i.e., one-dimensional spin correlations develop in α−(BETS)2I3. We show that one-dimensional spin correlations open a gap in the massless Dirac electrons. Our finding paves the way for opening gaps for massless Dirac electrons using strong electron correlations.

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    • Received 12 September 2022
    • Revised 9 January 2023
    • Accepted 10 January 2023
    • Corrected 6 February 2023

    DOI:https://doi.org/10.1103/PhysRevB.107.L041108

    ©2023 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Charge density wavesDirac fermionsMagnetic order
    1. Physical Systems
    Molecular solidsOrganic compoundsStrongly correlated systems
    Condensed Matter, Materials & Applied Physics

    Corrections

    6 February 2023

    Correction: Reference [50] contained minor typographical errors and have been fixed.

    Authors & Affiliations

    Daigo Ohki1, Kazuyoshi Yoshimi2, Akito Kobayashi1, and Takahiro Misawa3,*

    • 1Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
    • 2Institute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan
    • 3Beijing Academy of Quantum Information Sciences, Beijing 100193, China

    • *tmisawa@baqis.ac.cn

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    Vol. 107, Iss. 4 — 15 January 2023

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    Images

    • Figure 1
      Figure 1

      Crystal structures and real-space distribution of MLWFs for (a) α−(ET)2I3 and (b) α−(BETS)2I3 at 30 K drawn by vesta [52]. Four ET (BETS) molecules (labeled by A, A′, B, and C sites) exist in the unit cell indicated by the black lines. The A and A′ are crystallographically equivalent. Energy band structures for (c) α−(ET)2I3 and (d) α−(BETS)2I3 at 30 K. The solid lines are obtained by DFT calculations, while the squares are obtained from the MLFWs. Here, we define Γ≡(0,0,0), M′≡(−π,π,0), Y≡(0,π,0), X≡(π,0,0). The bandwidth for the four bands of α−(ET)2I3 is approximately 3/4 times smaller than that of α−(BETS)2I3.

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    • Figure 2
      Figure 2

      Schematic diagrams of α-type organic conductors for (a) α−(ET)2I3 and (b) α−(BETS)2I3 at 30 K. Transfer integrals and Coulomb interactions for the nearest-neighbor sites are also shown. The shaded pink parallelogram shows a unit cell. We also show a schematic picture of the ground state obtained by mVMC for (a) α−(ET)2I3: Horizontal stripe charge order (HCO) with spin dimer on strong transfer tb2, and (b) α−(BETS)2I3: One-dimensional antiferromagnetism (AFM) correlations develop in the A−A′ chain. Molecules surrounded by shaded purple and green rectangles indicate bonds with a strong spin singlet correlation, and molecules with a shaded yellow circle are electron-rich sites.

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    • Figure 3
      Figure 3

      Spin structure factors for (a) α−(ET)2I3 and (b) α−(BETS)2I3. We map the lattice structures into 2L×2L square lattices. The superposition of the (π,0) and (π,π) spin structures is consistent with the schematic images in Fig. 2. (c) System size dependence of peak values of spin structure factors. The dashed curves show the results of fitting using the function a(1/L)+b(1/L)2.

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    • Figure 4
      Figure 4

      Doping dependence of chemical potential for (a) α−(ET)2I3 and (b) α−(BETS)2I3, where μ0=[μ(N0+1)−μ(N0−1)]/2 and N0/Ns=1.5. For α−(ET)2I3 and α−(BETS)2I3, the estimated charge gap is Δc∼0.1eV and Δc∼0.07eV. For comparison, we plot the chemical potential for noninteracting systems for L=12 (brown crosses). (c) Size dependence of the spin gap. We fit the data for L≥8 using the linear function a+b(1/L) to reduce the finite-size effects.

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