EXCITONIC PHASE-TRANSITIONS IN ELECTRONIC SYSTEMS

Although predicted theoretically, excitonic phase transitions have nev er actually been observed. Recently it has been claimed that they can occur in some strongly correlated materials. We study the possibility of an excitonic transition in a two-band model and show that a true ph ase transition never occurs in the presence of hybridization. We sugge st an alternative interpretation for these experiments based on the op ening or closing of a hybridization gap at a critical pressure.