Observation of fast sound in two-dimensional dusty plasma liquids

Equilibrium molecular dynamics simulations are performed to study two-dimensional (2D) dusty plasma liquids. Based on the stochastic thermal motion of simulated particles, the longitudinal and transverse phonon spectra are calculated, and used to determine the corresponding dispersion relations. From there, the longitudinal and transverse sound speeds of 2D dusty plasma liquids are obtained. It is discovered that, for wavenumbers beyond the hydrodynamic regime, the longitudinal sound speed of a 2D dusty plasma liquid exceeds its adiabatic value, i.e., the so-called fast sound. This phenomenon appears at roughly the same length scale of the cutoff wavenumber for transverse waves, confirming its relation to the emergent solidity of liquids in the nonhydrodynamic regime. Using the thermodynamic and transport coefficients extracted from the previous studies, and relying on the Frenkel theory, the ratio of the longitudinal to the adiabatic sound speeds is derived analytically, providing the optimal conditions for fast sound, which are in quantitative agreement with the current simulation results.

Automated summary

Equilibrium molecular dynamics simulations were used to investigate 2D dusty plasma liquids. The phonon spectra and dispersion relations of the longitudinal and transverse waves were calculated based on the stochastic thermal motion of the simulated particles. It was found that the longitudinal sound speed of the 2D dusty plasma liquid exceeds its adiabatic value for wavenumbers beyond the hydrodynamic regime, indicating the presence of fast sound. The analytical derivation of the ratio of the longitudinal to the adiabatic sound speeds, using previous studies and the Frenkel theory, agreed quantitatively with the simulation results.