Title: Extended Falicov-Kimball model at weak onsite and intersite Coulomb interactions
Author(s):

	K.J. Kapcia (Institute of Nuclear Physics, Polish Academy of Sciences, ulica W. E. Radzikowskiego 152, PL-31342 Kraków, Poland),
	J. Krawczyk (Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ulica Okólna 2, PL-50422 Wrocław, Poland),
	R. Lemański (Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ulica Okólna 2, PL-50422 Wrocław, Poland)

We analyze in detail a behavior of the order parameter in the half-filled extended Falicov-Kimball model for small Coulomb interactions (both onsite U and intersite V). The parameter is defined as the difference of localized electron concentrations in both sublattices of the Bethe lattice (in the limit of large coordination number). Using two methods, namely, the dynamic mean field theory and the Hartree-Fock approximation, we found the ranges of U and V for which the anomalous temperature dependence of the order parameter, characterized by the sharp reduction near T ≈ T_C /2, occurs (T_C is the temperature of the continuous order-disorder transition). In order to quantitatively describe this anomaly, we defined a function that measures the departure of the order parameter dependence from the standard mean-field S=1/2 Ising-like curve. We determined the U-dependent critical value VT_C of V above which the anomaly disappears. Indicators of the anomalous behavior of the parameter dependence can be also observed in the temperature dependence of the specific heat.

Key words: extended Falicov-Kimball model, correlated electron systems, lattice models in condensed matter, mean-field theories, intersite interactions, Bethe lattice

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