title: Collective excitations in supercritical fluids: analytical and molecular dynamics study of "positive" and "negative" dispersion.
abstract: The approach of generalized collective modes is applied to the study of dispersion curves of collective excitations along isothermal lines of supercritical pure Lennard-Jones fluid. An effect of structural relaxation and other nonhydrodynamic relaxation processes on the dispersion law is discussed. A simple analytical expression for the dispersion law in the long-wavelength region of acoustic excitations is obtained within a three-variable viscoelastic model of generalized hydrodynamics. It is shown that the deviation from the linear dependence in the long-wavelength region can be either "positive" or "negative" depending on the ratio between the high-frequency (elastic) and isothermal speed of sound. An effect of thermal fluctuations on positive and negative dispersion is estimated from the analytical solution of a five-variable thermoviscoelastic model that generalizes the results of the viscoelastic treatment. Numerical results are reported for a Lennard-Jones supercritical fluid along two isothermal lines T(*)=1.71,4.78 with different densities and discussed along the theoretical expressions derived.
authors.affiliation: Institute for Condensed Matter Physics, National Academy of Sciences of Ukraine, UA-79011 Lviv, Ukraine. bryk@icmp.lviv.ua
authors.forename: Taras
authors.initials: T
authors.lastname: Bryk
authors.forename: Ihor
authors.initials: I
authors.lastname: Mryglod
authors.forename: Tullio
authors.initials: T
authors.lastname: Scopigno
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Ruocco
authors.forename: Federico
authors.initials: F
authors.lastname: Gorelli
authors.forename: Mario
authors.initials: M
authors.lastname: Santoro