Single-particle spectral weight of a ferromagnetic polymer chain: Cluster perturbation theory

By the cluster perturbation theory (CPT), the single-particle spectra and the density of state for an infinite Hubbard bipartite lattice are studied. At half-filling, the spectral functions exhibit three up-spin and three down-spin bands, and the ferrimagnetic ground state is obtained. The detail of the spectra in the CPT is quite different from that in the Hatree-Fock approximation (HFA). The spectra are broadened by the on-site interaction U and thus the band gap in the HFA disappears in the CPT for small U although the gap still exists for large U. The spectral weights with different spins are quite different. Besides the quasiparticle spectral weight, there exists an incoherent part in the spectra. The effect of hole doping is to remove spectral weights from two sides to the inside of the gap. With increasing doping, the polarization of the spectra decreases. With increasing U, the polarization of the spectra in the gap is reduced. Due to topological structure of the system, the distributions of the spectral weights created by removing and adding an electron on the different sites of a unit cell are different.