Diamagnetic properties of the electron-exciton bound states in charge-transfer organic solids

Abstract

An effective Hamiltonian is established which describes the excitation spectrum of independent quasiparticles arising from the coherent electron-exciton pairing in molecular solids of the charge-transfer type. This Hamiltonian is then used to study the interaction of the quasiparticles with an external transverse electromagnetic field. An expression for the dielectric function is derived which describes the dielectric properties of the system. In the presence of electron-exciton pairing and in the long-wavelength limit, the diamagnetic current is finite and is of the London type, indicating the existence of the Meissner effect. The corrections to the London value for the diamagnetic current depend on the magnitudes of the binding energy of the exciton, the gap function, and the energy difference between the excited and ground states of the system. In the absence of electron-exciton pairing, the diamagnetic current vanishes and the dielectric function corresponds to that for the exciton band.