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Fermi Surface Deformation near Charge-Ordering Transition

Kato Group

The shape of a Fermi surface (FS) is an important factor in determining the electronic properties of a metal. For many organic conductors, band structures are obtained by the extended Hückel method, and give an important starting point to understand material properties. The shape of the FS is probed with high sensitivity in transport measurements such as angle-dependent magnetoresistance oscillations, the Shubnikov-de Haas effect and magnetoresistance.

Fig. 1. Spectral functions A(k, ω=0) for (a) V=1, (b) V=2, and V=2.48. Red circles indicate the Fermi surfaces. Charge-ordering transition takes place at V=2.52. (d) Scattering rates (the imaginary part of the self-energy) on the Fermi surface. Lifetimes of quasiparticles become short(long) in the hot(cold) region.

The shape of the FS may be modified by strong electronic exchange-correlation effects, which are neglected in band calculations. Deformation of the FS has been discussed on the basis of a single-band Hubbard model in the context of research of high-Tc superconductors. In several theoretical works, the FS was shown to be deformed so that its nesting condition improves at the wavenumber of the antiferromagnetic(AF) ordering. However, the degree of deformation obtained was small in these studies.

In this work, we study how the FS is deformed when the system approaches the charge-ordering(CO) transition, which is frequently observed in organic conductors. Starting with the extended Hubbard model on a square lattice and employing a fluctuation exchange (FLEX) approximation, we calculate one-particle Green’s function near the CO transition. The calculated spectral functions A(k, ω=0) are shown in Fig. 1 (a), (b), and (c). In the present calculation, electron hopping, on-site Coulomb interaction, and temperature are fixed as t=1, U=5, and kBT=0.1, respectively. We further change nearest-neighbor interaction V and second-nearest-neighbor interaction V’ along one of the two diagonal directions by assuming V=V’. As V(=V’) increases, charge fluctuations develope at the wave-number Q=(2π/3,2π,3), and finally become divergent at V=2.52, indicating phase transition into a charge-modulated phase with a wavenumber Q. As clearly seen in Fig. 1 (a)-(c), charge fluctuations develope near the CO transition change the FS shapes. Near the CO transition (see Fig. 1 (c)), the FS is deformed so that the wavenumber vector Q spans it well. We note that the FS is deformed in the Hartree-Fock(HF) approximation through hopping renormalization by off-diagonal Fock terms. However, the appearance of a flat part in the FS is not obtained in the HF approximation, since such a feature appears only when charge fluctuations are fed back into one-particle Green’s function as done in the FLEX approximation.

As a result of the FS deformation, the quasiparticle-scattering anisotropy strengthens because the anisotropic self-energy is enhanced near CO. In Fig. 1 (d), we show the quasiparticle scattering rate –ImΣR(k, ω=0) as a function of position on the FS. The inset of Fig. 1 (d) shows three FS points: the –kx direction (A), +ky direction (B) and +kx direction (C). The results show that the scattering rate is largely enhanced in the hot region BC (the flat part of the FS) as the system approaches the CO transition, whereas it remains small in the cold region AB. Formation of hot and cold regions reflects anisotropy in the exchange-correlation interaction potential, which becomes strong near the CO transition. This anisotropic quasi-particle scattering can be detected via the in-plane anisotropy of electronic, thermal conductivities, and magnetoresistances.

In summary, we have demonstrated that in contrast to AF spin fluctuations, the FS is largely modified near the CO transition. We emphasize that this remarkable change in FS originates from the large discrepancy between the CO wave vector and the nesting vector in a noninteracting system. As a result of FS deformation, quasiparticle properties become more anisotropic near the CO transition. We note that when the system is close to the CO transition, the FS is sensitive to changes in temperature and/or pressure because strength of charge fluctuations changes rapidly.


References
  • [1] H. Seo, J. Merino, H. Yoshioka, and M. Ogata, J. Phys. Soc. Jpn. 75, 051009 (2006).
  • [2] K. Yoshimi, T. Kato, and H. Maebashi, J. Phys. Soc. Jpn. 80, 123707 (2011).
Authors
  • K. Yoshimia,b, T. Kato, and H. Maebashi
  • aUniversity of Tokyo
  • bNanosystem Research Institute “RICS”, AIST