FisMat2017 - Submission - View

Abstract's title: Exponential mode analysis of time autocorrelation functions: a new route to fluid dynamics
Submitting author: Ubaldo Bafile
Affiliation: Istituto dei Sistemi Complessi-CNR
Affiliation Address: ISC-CNR v. Madonna del Piano 10, I-50019 Sesto Fiorentino (FI)
Country: Italy
Oral presentation/Poster (Author's request): Oral presentation
Other authors and affiliations: Stefano Bellissima (ISC-CNR e Dipartimento di Fisica e Astronomia, Universita' di Firenze, Sesto Fiorentino), Eleonora Guarini (Dipartimento di Fisica e Astronomia, Universita' di Firenze, Sesto Fiorentino), Martin Neumann (Fakultaet fuer Physik der Universitaet Wien, Vienna, Austria), Fabrizio Barocchi (Dipartimento di Fisica e Astronomia, Universita' di Firenze, Sesto Fiorentino)

We have applied the recently demonstrated exponential mode expansion method to the velocity autocorrelation function (VAF), a key quantity in the atomic-scale dynamics of fluids that has provided the first paradigmatic example of a long-time tail phenomenon. The new approach shows that there is much more to the VAF than simply the evidence of this long-time dynamics and allows for a full account and understanding of the basic dynamical processes encompassed by the VAF. By consistently exploiting the interpretation of its frequency spectrum as a global density of states in the fluid, we assign specific and unambiguous physical meanings to groups of modes related to the longitudinal and transverse collective dynamics, respectively. The high-frequency oscillating component of the VAF is then clearly related to acoustic waves. As for the transverse modes, the multi-exponential expansion reveals a transition marking the onset of propagating excitations when the density is increased beyond a threshold value, a result in agreement with the recent literature debating the issue of dynamical crossover boundaries such as the Frenkel line. This will also help obtain a still missing full account of transverse dynamics, in both its nonpropagating and propagating aspects which are linked through dynamical transitions depending on both the thermodynamic states and the excitation wavevectors.