Ferromagnetism in the asymmetric Hubbard model

The density matrix renormalization group and quantum Monte Carlo calculations are used to study ferromagnetism in the one- and two-dimensional asymmetric Hubbard model. The model is examined for a wide range of electron concentrations n, Coulomb interactions U and down-spin electron hopping integrals t↓ changing from t↓ = 0 (the case of the Falicov-Kimball model) to t↓ = 1 (the case of the conventional Hubbard model). The critical value of the down-spin electron hopping integral t↓c below which the ferromagnetic state becomes stable is calculated numerically and the ground-state phase diagram of the model (in the t↓-U plane) is presented for physically the most interesting cases (n = 1 / 4,1 / 2 and 3/4). It is shown that at fixed U the ferromagnetic state is stabilized with increasing concentration of holes (1 − n) in the system, and at fixed n the ferromagnetic state is generally stabilized with increasing U.