VeranstaltungenDetails

Vortragende Person/Vortragende Personen:
Prof. Dr. Walter Schirmacher

Due to the absence of long-range symmetries the vibrational excitationsof glasses differ very much from those of crystals (phonons). Only at lowfrequencies phonon-like waves exist, because their wavelength is so largethat they are not sensitive to the structural disorder. At high frequencies (in the THz range) and/or in small computer-generated samples the vibrational excitations differ very much from phonons. They resemble more theeigenstates of random matrices.
Investigating the continuum limit of the Hessian of a glass, using a novelcoarse-graining procedure, we identify two different types of non-phononicvibrational excitations. A first type (Type-I.) arises from spatially fluctuating elastic constants and are responsible for vibrational anomalies, whichare observed in glasses in the THz range (“boson peak”) [1]. A secondtype (Type-II.) becomes distinct in very small samples, in which the low-frequency waves are suppressed. These Type-II excitations are caused bylocal stresses and exhibit non-irrotationaldisplacement patterns (vortices).We demonstrate that the frequency scaling of the Type-II excitations arenon-universal and depend on the details of the underlying interaction potentials. Recent small-sample findings of non-phononicspectra, whichscale with the fourth power of the frequency are shown to be an artifactof the smooth potential cutoff used in the simulations.

[1] W. Schirmacher and G. Ruocco, Heterogeneous Elasticity: The tale ofthe boson peak, arxiv:2009.05970.