Atomic-scale origin of dynamic viscoelastic response and creep in disordered solids
  Viscoelasticity has been described since the time of Maxwell as an
interpolation of purely viscous and purely elastic response, but its
microscopic atomic-level mechanism in solids has remained elusive. We studied
three model disordered solids: a random lattice, the bond-depleted fcc lattice,
and the fcc lattice with vacancies. Within the harmonic approximation for
central-force lattices, we applied sum-rules for viscoelastic response derived
on the basis of non-affine atomic motions. The latter motions are a direct
result of local structural disorder, and in particular, of the lack of
inversion-symmetry in disordered lattices. By defining a suitable quantitative
and general atomic-level measure of nonaffinity and inversion-symmetry, we show
that the viscoelastic responses of all three systems collapse onto a master
curve upon normalizing by the overall strength of inversion-symmetry breaking
in each system. Close to the isostatic point for central-force lattices,
power-law creep $G(t)\sim t^{-1/2}$ emerges as a consequence of the interplay
between soft vibrational modes and non-affine dynamics, and various analytical
scalings, supported by numerical calculations, are predicted by the theory.
