The influence of nonlocal interactions on valence transitions and formation of excitonic bound states in the generalized Falicov-Kimball model

We use the density-matrix-renormalization-group (DMRG) method to study the combined effects of nonlocal interactions on valence transitions and the formation of excitonic bound states in the generalized Falicov-Kimball model. In particular, we consider the nearest-neighbour Coulomb interaction U-nn between two d, two f, d and f electrons as well as the so-called correlated hopping term U-ch and examine their effects on the density of conduction n(d) (valence n(f)) electrons and the excitonic momentum distribution N(q). It is shown that U-nn and U-ch exhibit very strong and fully different effects on valence transitions and the formation (condensation) of excitonic bound states. While the nonlocal interaction U-nn suppresses the formation of zero momentum condensate (N(q = 0)) and stabilizes the intermediate valence phases with n(d) 0.5, n(f) 0.5, the correlated hopping term U-ch significantly enhances the number of excitons in the zero-momentum condensate and suppresses the stability region of intermediate valence phases. The physically most interesting results are observed if both U-nn and U-ch are nonzero, when the combined effects of U-nn and U-ch are able to generate discontinuous changes in n(f), N(q = 0) and some other ground-state quantities.