Synopsis: Solitons in an ionic crystal

Neutron scattering measurements indicate that intrinsic localized modes are found in thermal equilibrium in $\text{NaI}$, a simple ionic crystal.

Solitons are localized wave packets with finite energy that retain their shape over time. They are ubiquitous in driven nonlinear systems that are out of equilibrium, including optical fibers, magnets, micromechanical systems, and Josephson junctions. A long-standing question has been whether such modes exist in real solids at thermal equilibrium. The presence of these so-called “intrinsic localized modes” (ILMs) was first proposed in the 1980s, and many theoretical models have made similar predictions for solids that have a significant lattice anharmonicity, but experimental evidence has been lacking.

In an article appearing in Physical Review B, Michael Manley from Lawrence Livermore National Laboratory and collaborators from several other national laboratories and universities show that $\text{NaI}$, a simple three-dimensional ionic crystal, can support a single intrinsic localized mode in thermal equilibrium above 555 $\text{K}$. Inelastic neutron scattering measurements on both powders and single crystals show that the localized mode occurs at a single frequency of 299 $\text{meV}$, which lies near the center of a gap in the phonon spectrum. This mode’s energy does not depend on its wave vector, as expected of an ILM, and the mode gains energy with increasing temperature. These findings present the first observation of a three-dimensional intrinsic localized mode in a crystalline solid, and suggest an important role for such modes in the high-temperature physical properties of solids. – Sarma Kancharla

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