Origin of the boson peak in amorphous solids

It is widely known that the low-temperature physical properties, such as the heat capacity and thermal conductivity, of a disordered amorphous solid are markedly different from those of its ordered crystalline counterpart. However, the origin of this discrepancy is not known. One of the universal features of disordered solids is the excess vibrational density of states, known as the ‘boson peak’. Here we study the microscopic origin of the boson peak through numerical investigations of the dynamic structure factor of two-dimensional model glasses over a wide frequency–wavenumber range. We show that the boson peak originates from quasi-localized vibrations of string-like dynamical defects. Furthermore, we reveal that these dynamical defects provide a common structural origin for the three most fundamental dynamic modes of glassy systems: the boson peak, fast β relaxation and slow structural relaxation.