Density of phonon states in nanostructured copper

Neutron spectroscopy analysis has been used to study the vibrational states in samples of coarse-grained and nanostructured copper. It has been revealed that the phonon spectrum for the nanostructured copper is characterized by a decrease in the local densities of states corresponding to the longitudinal ($\sim$28 meV) and transverse ($\sim$19 meV) vibrational modes. The total contribution to the molar heat capacity of the oscillators having the dispersion laws of coarse-grained copper decreases by about 18%. The experimentally observed increase in the energy (heat capacity) of the crystal in the nanostructured state is attributed to the spectrum transformation in the form of the transfer of the energy from the high-frequency phonons to the long-wavelength region, where the new acoustic branches of oscillations along the grain boundaries (the “second” ion sound effect) are developed. This means that the features of the scattering spectra of the slow neutrons in the samples are determined by the presence of two types of the scatterers.