Regular Article

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

Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 043 53 Košice, Slovakia

^a e-mail: farky@saske.sk

Received: 30 January 2019
Received in final form: 21 March 2019
Published online: 1 July 2019

Abstract

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.