Nanostructures often appear to play by a different set of rules from those governing bulk materials. Mechanically, they tend to be stronger, more ductile, and more flexible than their bulk counterparts (see Physics Today, November 2013, page 14). Shrinking a material’s scale can also change its thermal properties. In a bulk semiconductor or insulator, for instance, those properties are primarily determined by phonon–phonon and phonon–defect interactions; in nanostructures, phonon–surface interactions dominate.

A precise description of phonon–surface interactions has proven difficult to pin down from first principles. It’s generally agreed that phonons with wavelengths much larger than a surface’s characteristic roughness length scale should reflect specularly, like light off a smooth mirror, and that those with wavelengths much shorter than the roughness length scale should scatter diffusively. Theories differ, however, as to what happens at intermediate wavelengths.

Richard Robinson and coworkers at Cornell University have introduced a tool that...

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