Is diffusion anomalous in two-dimensional Yukawa liquids?

There have recently been many predictions of "superdiffusion" in two-dimensional strongly coupled Yukawa systems, both by computer simulations and in dusty plasma experiments, with substantially varying diffusion exponents. Here we show that the results crucially depend on the strength of dissipation and the time instant of the measurement. For sufficiently large friction even subdiffusion is possible. However, there are strong indications that, in the long-time limit, anomalous diffusion vanishes for dissipative as well as for frictionless systems

Effective coupling parameter for 2D Yukawa liquids and non-invasive measurement of plasma parameters

We derive an effective coupling parameter for two-dimensional Yukawa systems based on the height of the first maximum of the pair distribution function. Two variants -- one valid in the high-coupling range, the other for arbitrary couplings of the liquid -- are derived. Comparison to previous approaches to Yukawa coupling parameters shows that the present concept is more general and more accurate. Using, in addition, dynamical information contained in the velocity autocorrelation function, we outline a reference data method that can be employed as a non-invasive measurement scheme of the plasma parameters -- the coupling strength and the screening length. This approach requires only input from a time-series of configuration snapshots and particle velocities with no recourse to additional information about the system. Our results should be directly applicable as a simple, yet reliable diagnostic method for a variety of experiments, including dusty plasmas, colloidal suspensions and ions in traps, and can be employed to facilitate comparisons between experiments, theory and simulations

Comment on: "Self-Diffusion in 2D Dusty-Plasma Liquids: Numerical-Simulation Results" [arXiv:0812.0338]

An analysis of superdiffusion in two-dimensional strongly correlated Yukawa liquids is presented. In particular, we report a regular, montonic behavior of the diffusion exponent on the screening parameter

Reply to comment of Shukla et al. on 'On "Novel attractive forces" between ions in quantum plasmas - failure of linearized quantum hydrodynamics'

In an earlier paper we demonstrated that the "novel attractive force" between protons in dense hydrogen reported by Shukla and Eliasson based on linearized quantum hydrodynamics (LQHD) is wrong. As benchmark results we used state of the art density functional theory (DFT) simulations. In their response to that paper Shukla et al. (arXiv:1112.5556) claimed that the disagreement is due to a failure of DFT. Here we show - by comparing the properties and limitations of DFT and QHD - that their statements have no scientific basis and that DFT is a suitable benchmark for more approximate approaches such as QHD and LQHD.Comment: third of three papers, to appear in Phys. Rev.

Towards ab initio thermodynamics of the electron gas at strong degeneracy

Recently a number of theoretical studies of the uniform electron gas (UEG) at finite temperature have appeared that are of relevance for dense plasmas, warm dense matter and laser excited solids and thermodynamic density functional theory simulations. In particular, restricted path integral Monte Carlo (RPIMC) results became available which, however, due to the Fermion sign problem, are confined to moderate quantum degeneracy, i.e. low to moderate densities. We have recently developed an alternative approach---configuration PIMC [T. Schoof {\em et al.}, Contrib. Plasma Phys. {\bf 51}, 687 (2011)] that allows one to study the so far not accessible high degeneracy regime. Here we present the first step towards UEG simulations using CPIMC by studying implementation and performance of the method for the model case of $N=4$ particles. We also provide benchmark data for the total energy

Comment on "Discussion on `Novel attractive force between ions in quantum plasmas -- failure of simulations based on a density functional approach"

In a recent article [P.K. Shukla, B. Eliasson and M. Akbari-Moghanjoughi, Physica Scripta {\bf 87}, 018202 (2013)] the authors criticized our analysis of the screened proton potential in dense hydrogen that was based on {\em ab initio} density functional theory (DFT) simulations [M. Bonitz, E. Pehlke, and T. Schoof, Phys. Rev. E {\bf 87}, 037102 (2013)]. In particular, they attributed the absence of the Shukla-Eliasson attractive force between protons in the DFT simulations to a failure of DFT. Here we discuss in detail their arguments and show that their conclusions are incorrect

Dynamics of two-dimensional complex plasmas in a magnetic field

We consider a two-dimensional complex plasma layer containing charged dust particles in a perpendicular magnetic field. Computer simulations of both one-component and binary systems are used to explore the equilibrium particle dynamics in the fluid state. The mobility is found to scale with the inverse of the magnetic field strength (Bohm diffusion) for strong fields. For bidisperse mixtures, the magnetic field dependence of the long-time mobility depends on the particle species providing an external control of their mobility ratio. For large magnetic fields, even a two-dimensional model porous matrix can be realized composed by the almost immobilized high-charge particles which act as obstacles for the mobile low-charge particles

Ab Initio Quantum Monte Carlo Simulations of the Uniform Electron Gas without Fixed Nodes

The uniform electron gas (UEG) at finite temperature is of key relevance for many applications in the warm dense matter regime, e.g. dense plasmas and laser excited solids. Also, the quality of density functional theory calculations crucially relies on the availability of accurate data for the exchange-correlation energy. Recently, new benchmark results for the N = 33 spin-polarized electrons at high density, r_s = r/a_B <= 4 and low temperature, have been obtained with the configuration path integral Monte Carlo (CPIMC) method [T. Schoof et al., Phys. Rev. Lett. 115, 130402 (2015)]. To achieve these results, the original CPIMC algorithm [T. Schoof et al., Contrib. Plasma Phys. 51, 687 (2011)] had to be further optimized to cope with the fermion sign problem (FSP). It is the purpose of this paper to give detailed information on the manifestation of the FSP in CPIMC simulations of the UEG and to demonstrate how it can be turned into a controllable convergence problem. In addition, we present new thermodynamic results for higher temperatures. Finally, to overcome the limitations of CPIMC towards strong coupling, we invoke an independent method|the recently developed permutation blocking path integral Monte Carlo approach [T. Dornheim et al., accepted for publication in J. Chem Phys., arXiv:1508.03221]. The combination of both approaches is able to yield ab initio data for the UEG over the entire density range, above a temperature of about one half of the Fermi temperature. Comparison with restricted path integral Monte Carlo data [E. W. Brown et al., Phys. Rev. Lett. 110, 146405 (2013)] allows us to quantify the systematic error arising from the free particle nodes

Ab initio thermodynamic results for the degenerate electron gas at finite temperature

The uniform electron gas (UEG) at finite temperature is of key relevance for many applications in dense plasmas, warm dense matter, laser excited solids and much more. Accurate thermodynamic data for the UEG are an essential ingredient for many-body theories, in particular, density functional theory. Recently, first-principle restricted path integral Monte Carlo results became available which, however, due to the fermion sign problem, had to be restricted to moderate degeneracy, i.e. low to moderate densities with $r_s={\bar r}/a_B \gtrsim 1$. Here we present novel first-principle configuration PIMC results for electrons for $r_s \leq 1$. We also present quantum statistical data within the $e^4$-approximation that are in good agreement with the simulations at small to moderate $r_s$.Comment: Revision in response to referee comments. New and more accurate data including extrapolation to macroscopic limi

\textit{Ab Initio} Path Integral Monte Carlo Results for the Dynamic Structure Factor of Correlated Electrons: From the Electron Liquid to Warm Dense Matter

The accurate description of electrons at extreme density and temperature is of paramount importance for, e.g., the understanding of astrophysical objects and inertial confinement fusion. In this context, the dynamic structure factor $S(\mathbf{q},\omega)$ constitutes a key quantity as it is directly measured in X-ray Thomson (XRTS) scattering experiments and governs transport properties like the dynamic conductivity. In this work, we present the first \textit{ab initio} results for $S(\mathbf{q},\omega)$ by carrying out extensive path integral Monte Carlo simulations and developing a new method for the required analytic continuation, which is based on the stochastic sampling of the dynamic local field correction $G(\mathbf{q},\omega)$. In addition, we find that the so-called static approximation constitutes a promising opportunity to obtain high-quality data for $S(\mathbf{q},\omega)$ over substantial parts of the warm dense matter regime