The vibrational modes of matter
Instituto de Fisica Teorica
UAM-CISC, Madrid, Spain
Abstract: In the first part of the talk a theoretical description of the vibrational density of states (VDOS) of solids (both crystals and glasses) is presented starting from first principles. Without using any assumptions whatsoever about the existence and nature of 'disorder' in the material, it is shown analytically that the boson peak in the VDOS of both ordered crystals and glasses arises naturally from the competition between elastic mode propagation and viscous damping. The theory explains the recent experimental observations of boson peak in perfectly ordered crystals which cannot be explained based on previous theoretical frameworks. The theory also explains, for the first time, how the vibrational spectrum changes with the atomic density of the solid, and explains recent experimental observations of this effect.
In the second part, we will discuss the theoretical challenges introduced by the recent observation of low frequency solid-like elastic response in liquids. We will propose and motivate a possible resolution of the puzzle based on the so-called k-gap dispersion relation, providing a universal framework to understand the vibrational modes in both solids and liquids.
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