DIMENSIONAL CROSSOVER IN SPIN-DIFFUSION - A MANIFESTATION OF THE QUANTUM ZENO EFFECT

The quantum Zeno effect (QZE) implies that a too frequent (omega(phi)- ->infinity) observation of a quantum system would trap it in its initi al state, even though it would be able to evolve to some other state i f not observed. in our scheme, interacting spins in a three-dimensiona l cubic lattice, ''observe'' each other with a frequency omega(phi)pro portional to root J(x)(2)+J(y)(2)+J(z)(2)/(h) over bar, where the J's are the coupling constants. This leads to a ''diffusive'' spread of a local excitation characterized by the constants D(mu)proportional to J (mu)(2)/omega(phi). Thus, a strongly asymmetric interaction (e.g., J(y )/J(x(z)) much greater than 1), would hinder diffusion in the perpendi cular directions (D-x(z)-->0) manifesting the QZE. We show that this e ffect is present in numerical solutions of simple two-dimensional syst ems. This reduction in the diffusion kinetics was experimentally obser ved in paramagnetic compounds where the asymmetry of the interaction n etwork manifests through an exchange narrowed linewidth. Experimental designs are proposed.