A strong diffusive ion mode in dense ionized matter predicted by Langevin dynamics

Abstract

The state and evolution of planets, brown dwarfs and neutron star crusts is determined by the properties of dense and compressed matter. Due to the inherent difficulties in modelling strongly coupled plasmas, however, current predictions of transport coefficients differ by orders of magnitude. Collective modes are a prominent feature, whose spectra may serve as an important tool to validate theoretical predictions for dense matter. With recent advances in free electron laser technology, X-rays with small enough bandwidth have become available, allowing the investigation of the low-frequency ion modes in dense matter. Here, we present numerical predictions for these ion modes and demonstrate significant changes to their strength and dispersion if dissipative processes are included by Langevin dynamics. Notably, a strong diffusive mode around zero frequency arises, which is not present, or much weaker, in standard simulations. Our results have profound consequences in the interpretation of transport coefficients in dense plasmas.

Ab initiosimulations of the dynamic ion structure factor of warm dense lithium

“…This feature has been observed before [42]. In future work the DSF can be used to determine more material properties from the hydrodynamic model [19,43,44] or an extended hydrodynamic model [45] and in this sense check a possible influences of the used thermostat [46], which has to be further benchmarked; i.e. against available measurements of dense liquids [20][21][22][23][24][25].…”

Ion modes in dense ionized plasmas through nonadiabatic molecular dynamics

“…One approach that is able to fully capture the strongly coupled behavior of the ions has been molecular dynamics (MD), i.e., a fully atomistic simulation of the ion dynamics that results from numerical integration of the equations of motion [5]. As MD simulations explicitly calculate the ion trajectories they are able to provide transport properties, such as viscosity and thermal diffusivity [6], acoustic properties, such as the sound speed [7,8], and thermodynamic variables, including the equation of state [5,7]. The accuracy and predictive power of such simulations is encompassed within the calculation of the interatomic potential, with increased accuracy coming at the expense of computational cost.…”

“…The BO approximation thus neglects important details of the interaction and, of particular relevance to high energy density matter, the dynamics of electron-ion collisions are ignored [17]. Although justified for many equilibrium properties, like the equation of state [8,18], the adiabatic treatment prohibits direct energy transfer between electrons and ions and is therefore problematic for the modeling of transport properties and nonequilibrium matter [17,19,20].…”

“…Recent work has investigated the applicability of a Langevin thermostat to capture the effect of electron-ion interactions on the ion dynamics [8,18,21]. In this phenomenological approach, an additional stochastic Gaussian force is added to the equations of motion with a single collision frequency, or friction parameter, that must be determined a priori [20].…”

“…A number of classical (Rayleigh scattering [22]) and quantum (Born approximation [23]) models exist, but their applicability in the WDM state is unknown. The inclusion of these collisions has been shown to have a profound effect on the ion dynamics [8], including a strongly decreased adiabatic sound speed and increased diffusive mode around ω = 0.…”