Ising spin chain models for the layered antiferroelectric squaric acid

Abstract

We start from a suitable vertex model for the molecular planes of squaric acid. Taking into account besides other terms the four spin interaction at a vertex and introducing the assumption that even above T_C only the ground states and the first excited states are thermally populated, it is shown that Ising spin chain models are suitable approximations for describing the in plane interactions. Dynamical processes are considered taking into account the in chain interactions exactly. The parameters of the model are related to experimental results known for this crystal and possible extensions of the model are discussed.