2016/01/31 by R. Milkus, A. Zaccone
Physics and Astronomy · #cond-mat.dis-nn #cond-mat.soft #cond-mat.stat-mech
paper · pdf · doi:10.1103/physrevb.93.094204
published as Phys. Rev. B 93, 094204 (2016) · 10 pages, 7 figures
arxiv created 2016/03/21 · arxiv updated 2016/03/22
It is well known that amorphous solids display a phonon spectrum where the Debye ∼ ω2 law at low frequency melds into an anomalous excess-mode peak (the boson peak) before entering a quasi-localized regime at higher frequencies dominated by scattering. The microscopic origin of the boson peak has remained elusive despite various attempts to put it in a clear connection with structural disorder at the atomic/molecular level. Using numerical calculations on model systems, we show that the microscopic origin of the boson peak is directly controlled by the local breaking of center-inversion symmetry. In particular, we find that both the boson peak and the nonaffine softening of the material display a strong positive correlation with a new order parameter describing the local inversion symmetry of the lattice. The standard bond-orientational order parameter, instead, is shown to be a poor correlator and cannot explain the boson peak in randomly-cut crystals with perfect bond-orientational order. Our results bring a unifying understanding of the boson peak anomaly for model glasses and defective crystals in terms of a universal local symmetry-breaking principle of the lattice.