Modified Debye spectra in disordered solids

Low-energy modes, as manifested by an excess phonon heat capacity, are ubiquitous in glasses and is often thought to be a signature of a glassy or disordered state. However, this feature is also observed in negative thermal expansion crystals, thermoelectrics, and shape-memory alloys. We compare the peaks in metallic glasses and shape-memory alloys as a function of disorder and crystallinity.