Two-dimensional Plasma Crystals Research Articles

Nonlinear longitudinal waves in a two-dimensional screened Coulomb crystal.

Nonlinear interactions of longitudinal waves were observed in a two-dimensional plasma crystal, i.e., a lattice composed of highly charged microspheres immersed in a plasma. The waves were launched by radiation pressure of a laser, and wave spectra in omega-k space were analyzed at various amplitudes of waves. At a sufficiently large amplitude of wave, the second and third wave harmonics satisfying a dispersion relation were observed. As the second harmonic propagates from the excitation region, it was amplified for a small distance, and then damped. The experimental results were compared to a nonlinear wave theory and to a molecular dynamic simulation.

Ground State Configurations of Two-dimensional Plasma Crystals under Long-range Attractive Particle Interaction Force

The configurational properties of two-dimensional clusters of charged dust particles in dusty plasma interacting with each other via a model potential with a short-range repulsion and a long-range attraction are investigated through the Monte Carlo simulation technique. The particles are confined by a harmonic potential. For the small number of particles, a hexagonal structure of the ground state changes into a shell configuration with increasing value of the attraction parameter α. For larger clusters, the increased attraction changes the defect locations and more particles energetically suit in the center of the cluster.

Defects related self-diffusion in a two-dimensional dusty plasma crystal

Single-particle motion and self-diffusion in a two-dimensional dusty plasma are investigated by molecular dynamic simulation. The velocity autocorrelation function indicates that the single-particle motion is strongly coupled to the collective motion in the dusty plasma when the coupling parameter is high enough. The point defects play an important role in the particle self-diffusion process. A simple point defect motion model is developed to explain the mechanism of self-diffusion and estimate the self-diffusion coefficient in the dusty plasma. The value of the self-diffusion coefficient calculated by using the model is in agreement with the result obtained from mean square displacement for a two-dimensional dusty plasma crystal.

Experiments and molecular-dynamics simulation of elastic waves in a plasma crystal radiated from a small dipole source.

The radiation of elastic waves from a localized source is observed experimentally in a two-dimensional plasma crystal. An initial shear stress applied by a laser forms a small dipole source. The emerging complex wave pattern is shown to consist of outgoing compressional and shear wave pulses. Subsequent structures are identified as inward-going waves due to the finite size of the source region, which reappear on the opposite side. The compressional wave forms a trailing wave train due to strong dispersion, while the nondispersive shear wave evolves into a vortex-antivortex pair on either side. The experiments are compared with a molecular-dynamics simulation.

Phonon spectrum in a plasma crystal.

The Fourier spectra of longitudinal and transverse waves corresponding to random particle motion were measured in a two-dimensional plasma crystal. The crystal was composed of negatively charged microspheres immersed in a plasma at a low gas pressure. The phonons were found to obey a dispersion relation that assumes a Yukawa interparticle potential. The crystal was in a nonthermal equilibrium, nevertheless phonon energies were almost equally distributed with respect to wave number over the entire first Brillouin zone.

2-D plasma dust crystal compression in a parabolic well

A spherically curved (locally parabolic) electrode with a conformal plasma sheath can be used to directly test two-dimensional (2-D) plasma crystal particle-particle interaction models and to derive the screening length and the charge on the particles. By using such a well, the lateral interaction potential was isolated and accessible. Good agreement was obtained between measured and calculated single particle trajectories using measured gas densities and temperatures to determine the neutral gas drag. Plasma crystal compression in the parabolic potential was demonstrated to be in good agreement with that predicted by an equation of state for the 2-D dust layer incorporating Debye shielding of the charges.

Low-frequency modes in two-dimensional Debye-Yukawa plasma crystals

In a Debye-Yukawa system, charged micro-particles interact with each other through a screened Coulomb potential φ=( Q/ r)exp{− r/λ D} where Q is the charge and λ D is the screening length. In a strongly coupled Debye-Yukawa system (where Q is much larger than the magnitude of an electron charge), a plasma crystal may be formed. One important feature of such a strongly coupled system is the existence of transverse waves (or shear waves) in low frequency modes. In this paper we present an analytic treatment of longitudinal (or compressional) and transverse low frequency modes in two-dimensional plasma crystals. Dispersion relations of the modes are obtained for two-dimensional triangular lattices. It is also found that the modes (the compressional and the transverse) are strongly damped near the ‘negative dispersion’ regime. Theoretical predictions agree quantitatively with recent experiments, and the effects of neutral gas damping are also discussed.

The phonon wake behind a charge moving relative to a two-dimensional plasma crystal

In a recent experiment a wake was created in a two-dimensional lattice of charged dust grains by a charge moving parallel to the lattice plane. Multiple “Mach cones” were observed in the wake. This paper describes a linear theory of the phonon wake caused by a charge moving relative to a crystalline lattice. The theory predicts multiple structures in the wake that are qualitatively similar to those observed in the experiments. These structures are caused by constructive interference of compressional phonons excited by the moving charge, combined with the strongly dispersive nature of these phonons.

Laser-excited dust lattice waves in plasma crystals

Abstract Laser-excited waves in a two-dimensional dust plasma crystal have been observed experimentally in a parallel plate rf discharge. The measured dispersion relation is compared with theoretical models. Agreement is found with dust lattice waves, whereas deviations from dust acoustic waves exist. From the dispersion relation of a dust lattice wave the screening of the particles in the rf sheath is determined.