Complex singularities of the fluid velocity autocorrelation function
Abstract
There are intensive debates regarding the nature of supercritical fluids: if their evolution from liquidlike to gaslike behavior is a continuous multistage process or there is a sharp welldefined crossover. Velocity autocorrelation function Z is the established detector of evolution of fluid particles dynamics. Usually, complex singularities of correlation functions give more information. For this reason, we investigate Z in complex plane of frequencies using numerical analytic continuation. We have found that naive picture with few isolated poles fails describing Z(ω) of onecomponent LennardJones (LJ) fluid. Instead, we see the singularity manifold forming branch cuts extending approximately parallel to the real frequency axis. That suggests LJ velocity autocorrelation function is a multivalued function of complex frequency. The branch cuts are separated from the real axis by the welldefined "gap" whose width corresponds to an important time scale of a fluid characterizing crossover of system dynamics from kinetic to hydrodynamic regime. Our working hypothesis is that the branch cut origin is related to competition between oneparticle dynamics and hydrodynamics. The observed analytic structure of Z is very stable under changes in the temperature; it survives at temperatures two orders of magnitude higher than the critical one.
 Publication:

Soviet Journal of Experimental and Theoretical Physics Letters
 Pub Date:
 November 2015
 DOI:
 10.1134/S0021364015220038
 Bibcode:
 2015JETPL.102..643C