Exciton pairing and superconductivity in layered compounds

A new type of electronic phase transition in low-dimensional (layered) crystals with alternating layers of conduction electrons and holes is predicted; it is related to pairing by spatially separated carriers. We show the existence of “longitudinal superfluidity,” diamagnetic response to a longitudinal magnetic field, and “transverse superfluidity,” the analogs of de and ac quantum interference in a transverse field. These effects occur when both correlation (Coulomb) and tunneling (Josephson) interactions of adjacent layer electrons are included, while tunneling transitions (T12) under the condition T12≪ Δ (Δ is the energy gap) lead only to generation of critical field values E⊥, H‖ . At these values “destabilization” starts of the phase of the order parameter and of related quantum coherence effects (“superfluidity”).

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