The effect of correlation on Peierls transition, which is accompanied by a dimerization, t d , of a bond alternation for transfer energy, has been examined for a half-filled one-dimensional electron system with on-site repulsive interaction (U ). By applying the renormalization group method to the interaction of the bosonized Hamiltonian, the dimerization has been calculated variationally and self-consistently with a fixed electron-phonon coupling constant (λ) and it is shown that t d takes a maximum as a function of U . The result is examined in terms of charge gap and spin gap and is compared with that of the numerical simulation by Hirsch [Phys. Rev. Lett 51 (1983) 296]. Relevance to the spin Peierls transition in organic conductors is discussed.
KEYWORDS: Peierls transition, Hubbard model, dimerization, renormalization, charge gap, spin gapPeierls transition for a one-dimensional half-filled electron system coupled with phonon has been studied extensively since the Su-Schrieffer-Heeger model was proposed for the quasi-one-dimensional conductor polyacetylene with a bond alternation.1) The role of correlation in such a model has been examined by introducing an onsite repulsive interaction, U . The calculation using the Hartree-Fock approximation leads to the Peierls state, which exists only for U smaller than a critical value of the order of the band width.2, 3) This study has been further developed by taking into account a one-dimensional quantum fluctuation. The numerical simulation exhibits an enhancement of dimerization in the presence of repulsive interaction. 4,5,6,7,8,9) A notable finding is that the dimerization takes a maximum at a value of U being nearly the band width 5,8,9) and that the charge gap becomes much larger than the dimerization gap.5) Regarding the case of weak coupling, the analytical method of a renormalization group (RG) has also exhibited the enhancement of t d as a function of U .10) The effect of finite phonon frequency has been explored using the RG method based on the bosonization.11, 12, 13) It has been shown that the half-filled case leads to a competition between the state with both spin and charge gaps and the state with only a charge gap on the plane of the phonon frequency and the electron-phonon coupling constant.13) In contrast, the case of strong coupling with large U has been examined in terms of the opposite approach, i.e., the expansion of 1/U ) and mapping the electron system into a spin 1/2 chain system, which leads to the spin Peierls transition. The successful treatment of the quantum fluctuation leads to the occurrence of the spin Peierls transition for an arbitrary magnitude of the electron-phonon coupling constant.