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Fig. 1 The upper panel
displays DOS obtained in
approximation I and the
lower one—obtained in the
present approximation for
Hubbard model (correlated
hopping is absent).
Semi-elliptic bare DOS is
used. Black (the most inner)
line with thin quasiparticle
peak is for U/w = 1.6; blue
curve is for U/w = 1.8 and
red (the most outer) curve for
U/w = 2.1
In a system with electron–hole asymmetry (Fig. 2), the proposed approximation
gives lower critical value for metal-to-insulator transition, wider upper subband. By
comparing plots for near-critical values of Coulomb correlations with correlated
hopping taken into account, we note that the improved approximation estimates
the effect of the correlated hopping more conservatively. Subband widths decrease
by far slower and, correspondingly, maximum DOS values are lower than in the
approximation I. Whatever approximation is used, the correlated hopping of electrons
enhances localization, which is apparent from the spectacular reduction of the upper
bandwidth.