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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 lattices1,2,3,4,5. 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 junctions6,7, optical waveguide arrays8, two-dimensional nonlinear photonic crystals9 and micromechanical cantilever arrays10. There is also some evidence for the existence of localized excitations in atomic lattices11,12,13,14,15, 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.

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Figure 1: Schematic diagrams of the experimental procedure and the nonlinear process.
Figure 2: Snapshot of the mixing spectrum versus the probe oscillator frequency.
Figure 3: Experimental demonstration of ILM energy quantization.


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We acknowledge conversations with P. Brouwer, L. English, B. Hubbard, D. Mills, J. Page, J. Parpia, J. Sethna and R. Silsbee. This work was supported by NSF-DMR and by the Cornell Center for Materials Research.

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Sato, M., Sievers, A. Direct observation of the discrete character of intrinsic localized modes in an antiferromagnet. Nature 432, 486–488 (2004).

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