The Frenkel line and isotope effect

It has been recently proposed that supercritical fluids experience a crossover between two different regimes of the particle dynamics. The crossover takes place at the Frenkel line which can be defined from vanishing of oscillations of the velocity autocorrelation functions (vacfs). Here, we present a novel detailed analysis of vacfs of binary isotopic mixtures of Lennard-Jones and Soft Spheres particles and consider temperature dependence of vacfs of individual components as well as mixtures. We show that partial vacfs can be very different: for the mass ratio of about 2, the temperature at which the oscillatory behavior of vacf vanishes differs eightfold. Notably, oscillatory behavior of vacfs of the whole mixture vanishes at about the same temperature as in the one-component fluid. We conclude that the criterion to locate the Frenkel line on the basis of vacfs is general and applies to mixtures, reflecting more fundamental changes of system dynamics such as the disappearance of transverse mode from the excitation spectrum.