Singleparticle excitation-spectra in the Hubbard model on a kagom'e lattice

The effects of frustration in spin systems have been studied for many years. Recently, the transport properties of the layered cobalt oxide NaxCoO2 and of related oxides have generated new interest in the frustrated systems. In the cobalt oxides, the Co ions form a triangular lattice. The hopping matrix element of electrons in the cobalt 3d orbitals is not isotropic, and we have shown [PRL91, 257003] that the triangular CoO2 lattice consists of four coupled kagom'e sublattices. For this reason, here, we examine the single-particle excitation spectrum of the Hubbard model on the kagom'e lattice, and study the motion of a carrier in this frustrated system. We use the quantum Monte Carlo and the exact-diagonalization methods. The dispersion relation of the tight-binding model on the kagom'e lattice has a flat dispersion at the top or the bottom of the energy band depending on the sign of hopping-matrix element t. This causes a two-fold degeneracy at the gamma point where th...