Soft Transverse Phonons in an Amorphous Metal
In the search for high-transition-temperature superconductors it has been recognized that low atomic vibrational frequencies can lead to an enhancement of T c .1 Experiments have shown that thin-film superconductors prepared in a micro-crystalline or amorphous state, generally by vacuum deposition at cryogenic temperatures, can have a higher T c than the corresponding crystalline films.2 It has been proposed that the changes in T c in these films arise primarily from the lowering of vibrational frequencies upon disordering.3, 4 There are, however, a number of distinct mechanisms for lowering these frequencies, so that in any given film one or more mechanisms may be operative. For example, in both microcrystalline or amorphous films low vibrational frequencies may arise from localized modes at surfaces or internal structural discontinuities (grain boundaries or voids) or from propagating surface modes. Another source of low frequencies is the high defect concentrations of evaporated films, which tend to lower bulk phonon frequencies via a change in average mass density.
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