Coherent behaviour without magnetic order of the triangular lattice antiferromagnet NiGa₂S₄

Abstract

We present experimental evidence for the unconventional coherent state in two dimensions, found in the geometrically frustrated antiferromagnet NiGa2S4 with a triangular Bravais lattice. Despite strong antiferromagnetic coupling, S = 1 spins only show nanoscale order at low temperatures. Thermodynamic measurements, however, indicate a gapless linearly dispersive mode and suggest coherence beyond the two-spin correlation length.