Abstract
It is well established that the low-temperature properties of amorphous solids are governed by structure-induced low-energy excitations (LEE). The LEE can be described by the two-level-system (TLS) tunneling model [1]. Cohen and Grest [2] correlate these tunneling states with the excess free volume vf of glasses frozen in during the quenching process: the TLS are assumed to arise from tunneling of single atoms or small groups of atoms into voids, i.e. local agglomerations of free volume. In this model, one expects the density of TLS to increase with increasing vf, or equivalently to decrease with increasing mass density ρ. Recently, we have demonstrated the similarity of the changes in vf and in the phonon scattering from LEE upon altering the amorphous structure by variation of the quenching rate [3], by structural relaxation [4.5] or by plastic deforming [3] amorphous Pd-Cu-Si. These findings give direct evidence for the validity of the free volume model to explain the presence of LEE in metallic glasses.
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