Structural relaxation in metallic glasses: reversible and irreversible changes in a two-level systems model

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Abstract
The small reversible property changes seen during annealing of metallic glasses happen more quickly than the irreversible changes simultaneously observed. By modelling isothermal property changes as the sum of contributions from many two-level systems in thermal contact, the authors attribute this experimental result to a correlation, in metallic glasses, between the barrier heights and the energy splittings of the two-level systems. A previously published (activation energy spectrum) treatment is the starting point in the authors' derivation; they remove an important limitation and show that (i) the previously published treatment is restricted to the analysis of irreversible changes; and (ii) glasses which exhibit relatively large reversible structural changes must contain relatively many two-level systems, with small energy splittings.