Coherent dipolar correlations in the ground-state of Kagome frustrated antiferromagnets

Abstract

We propose a new model for the nature of the low temperature phase of a geometrically frustrated antiferromgnet (AFM) with a Kagome lattice, SrCr$_{8-x}$Ga$_{4+x}$O$_{19}$. We propose that the long-range dipolar interaction between the magnetic Cr$^{3+}$ ions introduces correlations in their dynamics. The dipolar ground-state has the spins performing correlated zero-point oscillations in a coherent state with a well defined global phase and a complex order-parameter (i.e. Off-Diagonal Long Range Order). We calculate the magnon excitations of such a dipolar array and we find good agreement with the spin-wave velocities infered from measurements of the specific-heat. Various experimental properties of these materials are naturally explained by such a model.