Macroscopic quantum dynamics of toroidal moment in Ising-type rare-earth clusters

We study the quantum dynamics of polygonal rare-earth molecular clusters with Ising-type ion magnetization. It is shown that the ground state of such systems is a non-magnetic quasi-doublet of states with oppositely twisted ion spins. The states differ in sign of toroidal moment, which is a natural physical quantity to characterize the spin chirality of the clusters. The possibility of macroscopic quantum tunneling of toroidal moment between the states is predicted. The effects of an external current is considered, both in equilibrium and in the frames of the Landau-Zener-St\"uckelberg tunneling model. The special treatment is given for the most important case of triangular rare-earth clusters.