Monthly Archives: January 2015

Resolving structural transitions in spherical dust clusters. (arXiv:1410.2393v2 [physics.plasm-ph] UPDATED)

Finite systems in confining potentials are known to undergo structural transitions similar to phase transitions. However, these systems are inhomogeneous, and their "melting" point may depend on the position in the trap and vary with the particle number. Focusing on three-dimensional Coulomb systems in a harmonic trap a rich physics is revealed: in addition to radial melting we demonstrate the existence of intrashell disordering and inter-shell angular melting. Our analysis takes advantage of a novel melting criterion that is based on the spatial two and three-particle distribution functions and the associated reduced entropy which can be directly measured in complex plasma experiments.

A Review of Studies on Strongly-Coupled Coulomb Systems Since the Rise of DFT and SCCS-1977

The conferences on “Strongly Coupled Coulomb Systems” (SCCS) arose from the “Strongly Coupled Plas-mas” meetings, inaugurated in 1977. The progress in SCCS theory is reviewed in an ‘author-centered’ frame to limit its scope. Our efforts, i.e., with François Perrot, sought to apply density functional theory (DFT) to SCCS calculations. DFT was then poised to become the major computational scheme for condensed matter physics. The ion-sphere models of Salpeter and others evolved into useful average-atom models for finite-T Coulomb systems, as in Lieberman's Inferno code. We replaced these by correlation-sphere models that exploit the description of matter via density functionals linked to pair-distributions. These methods provided practical computational means for studying strongly interacting electron-ion Coulomb systems like warm-dense matter (WDM). The staples of SCCS are wide-ranged, viz., equation of state, plasma spectroscopy, opacity (absorption, emission), scattering, level shifts, transport properties, e.g., electrical and heat conductivity, laser- and shock-created plasmas, their energy relaxation and transient properties etc. These calculations need pseudopotentials and exchange-correlation functionals applicable to finite-T Coulomb systems that may be used in ab initio codes, molecular dynamics, etc. The search for simpler computational schemes has proceeded via proposals for orbital-free DFT, statistical potentials, classical maps of quantum systems using classical schemes like HNC to include strong coupling effects (CHNC). Laughlin's classical plasma map for the fractional quantum Hall effect (FQHE) is a seminal example where we report new results for graphene. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-Density Crystals in Charged Colloids: Comparison with Yukawa Theory

Charged colloids can behave as Yukawa systems, with similar phase behaviour. Using particle-resolved studies, we consider a system with an unusually long Debye screening length which forms crystals at low colloid volume fraction φ ≈ 0.01. We quantitatively compare this system with the Yukawa model and find that its freezing point is compatible with the theoretical prediction but that the crystal polymorph is not always that expected. In particular we find body-centred cubic crystals where face-centred cubic crystals are expected. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Waves in a Lennard-Jones Dusty Plasma Liquid

Recently it has been proposed to alter the interaction between dust grains in a plasma by applying ac electric fields to distort the Debye spheres surrounding the grains, which may lead to interaction potentials with a Lennard-Jones form under certain conditions (e.g., Kompaneets et al., Phys. Plasmas 16, 043705 (2009)). Motivated by this, we consider the dispersion relations of longitudinal and transverse waves in a dusty plasma liquid where the grains interact via a Lennard-Jones potential. We use the Quasi-Localized Charge approximation combined with an analytic expression for the pair distribution function for Lennard-Jones fluids (Matteoli and Mansoori, J. Chem. Phys. 103, 4672 (1995)). Possible dusty plasma experimental parameters are considered. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fano-like anti-resonances in strongly coupled binary Coulomb systems

Molecular-dynamics (MD) simulations of a strongly coupled binary ionic mixture have revealed the appearance of sharp minima in the species-resolved dynamical density fluctuation spectra. This phenomenon is reminiscent of the well-known Fano anti-resonance, occurring in various physical processes. We give a theoretical analysis using the quasi-localized charge approximation, and demonstrate that the observed phenomenon in the equilibrium spectrum is a novel manifestation of the Fano mechanism, that occurs at characteristic frequencies of the system different from the conventional classical Fano frequencies.

Equilibrium and Non-Equilibrium Melting of Two-Dimensional Plasma Crystals

Recent progress in experimental and theoretical studies of melting of two-dimensional (2D) plasma crystals is summarized. Several generic, equilibrium and non-equilibrium processes which can be observed and investigated in 2D complex plasmas are discussed, such as KTHNY, grain-boundary-induced, and shear-induced melting. Furthermore, the key features of the dominant plasma-specific mechanism of melting operating in 2D plasma crystals, the mode-coupling instability, are presented. The onset of the instability, which is characterized by threshold values of the experimental parameters, identifies the “dividing line” between the regimes when 2D complex plasmas can be employed to study generic strong-coupling phenomena, and the situations when the melting occurs due to specific processes peculiar to complex plasmas. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Versatile particle collection concept for correlation of particle growth and discharge parameters in dusty plasmas

The feasibility of collecting nanoparticles from a dusty plasma by means of the neutral drag force is investigated. The nanoparticles are formed in a capacitively coupled asymmetric discharge running in an Ar/C 2 H 2 —mixture at a frequency of 13.56 MHz and an RF-power of 9 W. By opening a gate valve between the plasma reactor and a vacuum chamber at a lower pressure at any desired point of the growth cycle of the dust particles a neutral gas flux is induced that drags the particles out of the plasma onto a substrate. By changing the parameters of the collection process, e.g. the substrate positioning or the substrate temperature, the efficiency of the collection process can be adjusted. Information about the particle size distributions is obtained by performing ex situ SEM measurements. As the collection process creates a time stamp in the in situ recorded control parameters, e.g. the self-bias voltage or the process gas pressure, a direct and pre...

Wave-particle dynamics of waveform and defect evolutions in undulated nonlinear self-excited dust acoustic waves

The wave-particle dynamics for the evolutions of defects and surrounding pitchfork type waveforms of a weakly disordered self-excited dust acoustic wave is experimentally investigated in an rf dusty plasma system. Particle trajectories are tracked and correlated with waveform evolution to construct an Eulerian-Lagrangian wave-particle dynamical picture. It is found that the local accumulation and depletion of particles in the wave crest and rear, respectively, determines the local crest speed, and the growth and decay of the local crest height, which in turn determine the waveform evolution. The local crest height and the focusing and defocusing of particle trajectories due to the transverse force fields from the tilted wave crest and the non-uniform crest height along the wave crest are the key factors to determine the above particle accumulation and depletion. It explains the observations such as the lower speed of smaller crests, the straightening of the leading front of the pitchfork waveform associated with the transverse motion of defect to the open side, and the vertical defect gliding in the wave frame through the detachment of the strongly kinked pitchfork branch followed by its reconnection with the trailing crest.