Direct observation of the discrete character of intrinsic localized modes in an antiferromagnet

In a strongly nonlinear discrete system, the spatial size of an excitation can become comparable to, and influenced by, the lattice spacing. Such intrinsic localized modes (ILMs)-also called 'discrete breathers' or 'lattice solitons'-are responsible for energy localization in the dynamics of discrete nonlinear lattices1-5. Their energy profiles resemble those of localized modes of defects in a harmonic lattice but, like solitons, they can move ( although, unlike solitons, some energy is exchanged during collisions between them). The manipulation of these localized energy 'hotspots' has been achieved in systems as diverse as annular arrays of coupled Josephson junctions(6,7), optical waveguide arrays(8), two-dimensional nonlinear photonic crystals(9) and micromechanical cantilever arrays(10). There is also some evidence for the existence of localized excitations in atomic lattices(11-15), although individual ILMs have yet to be identified. Here we report the observation of countable localized excitations in an antiferromagnetic spin lattice by means of a nonlinear spectroscopic technique. This detection capability permits the properties of individual ILMs to be probed; the disappearance of each ILM registers as a step in the time-dependent signal, with the surprising result that the energy staircase of ILM excitations is uniquely defined.