In order to understand the low-energy vibrational excitations common to amorphous solids, we have studied their evolution in ion-implanted crystalline silicon by measuring internal friction and heat conduction. The spectral density of these low-energy excitations evolves with increasing dose exactly towards that observed in the amorphous phase. More importantly, this evolution is unrelated to that of the amorphicity. We conclude that the defects in the crystal should be used to model the excitations in the amorphous silicon, rather than the amorphous structure itself. [S0031-9007(98)07337-2]
A two-dimensional diffraction grating for phonons is described. It scatters thermal phonons strongly when their dominant wavelength is equal to the grating's lattice constant. Through measurements of the Kapitza resistance between the silicon crystal carrying the grating and liquid helium, it is shown that the scattering by the grating is predominantly coherent, as expected for phonon diffraction.
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