A theory of the phase separation (PS) is proposed for a system with fluctuational short-range order resulting from the proximity of the system to interband excitonic (charge-transfer) instability. A functional integral approach for a description of local dynanfical correlations and the formation of short-range order regions is developed.
The PS, which has been directly observed by neutron diIfmction in La2CuO4+ 8 [1], manifests itself in different HTSC. The band approach to a theory of the PS was recently proposed by the present authors [2,3]. This approach allows systematic interpretation in a framework of a PS theory of a number of experiments on different HTSC, which seem *o be quite separate from each others [3]. In the band picture the possibility of the PS is connected with the existence of dielectric order which is suppressed with increasing concentration of excess particle n. It results in rapid reduction of electronic band edge with doping and a possible decrease of the chemical potential rt with increasing n. In the latter case the condition of absolute instability of spatially homogeneous state (~/z/~,~n < 0) is satisfied. This physical idea was realized in [2] in the meanfield approximation. However in HTSC long-range dielectric order is suppressed in superconducting regions of the phase diagram and dielectric correlations manifest itself in the form of dinamical fluctuatio,ls forming the short-range order regions. In the present paper we show that a scenario of tt.e PS based on the suppression of dielectric correlations can take place even in the systems with shortrange order in the absence of true long-range order. We consider the systems with interband excitonic instability (charge-transfer instability for example). A possibility of such instability in CuO 2 plane of HTSC has been