Charged domain walls in the two-dimensional Hubbard model

In this paper we numerically study the single-band Hubbard model on a two-dimensional square lattice using a mean-field iteration approach. We calculate the energies and configurations of finite-size systems for various doping densities up to 20%. The results show that there exists an attractive interaction among the holes, which forces the holes to form domain walls. The most stable configuration of domain-wall structure is determined, and its stability against transverse spin fluctuations is studied. When the nearest-neighbor repulsion V is considered, the system becomes richer in phases. The binding energy is reduced and vanishes completely at ${\mathit{V}}_{\mathit{c}}$=0.17t (for U/t=5, with t and U being the hopping integral and the on-site Hubbard repulsion, respectively). For small value (but larger than ${\mathit{V}}_{\mathit{c}}$) of V, we find that the holes form well-defined spin bags, which are separated from each other for a very small doping density. For a doping density larger than a critical value, they form domain walls. The physical implications of these results and their relation to experiment are discussed.