Simple Model for Some Dense Magnetic Exciton Systems

Abstract

The (triplet-state) TCNQ ion radical salts and also Würster's blue perchlorate exhibit deviations from the normal behavior of typical molecular solids. These systems are believed to contain triplet excitons which are restricted to essentially a unidirectional propagation. A simple phenomenological model is presented which attempts to describe the gross aspects of the observed anomalous behavior in terms of the relatively high exciton concentrations indicated. The model derives its interesting features from a quadratic dependence of the free energy on the excitation density (exciton concentration); the presence of such a term is made plausible by considerations of exciton—exciton and exciton—lattice interactions. The model accounts for the occurrence of phase transitions similar to those observed in (φ3PCH3)+(TCNQ)2— and Würster's blue perchlorate and indicates that under appropriate conditions one may achieve unusual magnetic phenomena in molecular solids.