Stamatios Kyrkos

Collective Dynamics of Complex Plasma Bilayers

A classical dusty plasma experiment was performed using two different dust grain sizes to form a strongly coupled asymmetric bilayer (two closely spaced interacting monolayers) of two species of charged dust particles. The observation and analysis of the thermally excited particle oscillations revealed the collective mode structure and dispersion (wave propagation) in this system; in particular, the existence of the theoretically predicted k=0 energy (frequency) gap was verified. Equilibrium molecular-dynamics simulations were performed to emulate the experiment, assuming Yukawa-type interparticle interaction. The simulations and analytic calculations based both on lattice summation and on the quasilocalized charge approximation approach are in good agreement with the experimental findings and help in identifying and characterizing the observed phenomena.

Molecular Dynamics Studies of Solid–Liquid Phase Transition in 2-D Yukawa Systems

We present systematic studies aimed at investigating the precise details of solid-liquid phase transition in 2-D classical many-particle systems interacting with the Yukawa potential. This is done by introducing and analyzing a variety of indicators, such as the bond angular order parameter, the angular distribution of the Einstein oscillations, local angular correlations, global positional correlations, and the variation of internal energy in the vicinity of the melting temperature. Our results consequently show rapid changes around Gamma=415 for kappamacr=2 of the investigated quantities

Phonons in Yukawa lattices and liquids

The understanding of the theoretical structure of phonon dispersion in Yukawa lattices and the relationship between these perfect lattice phonons on the one hand, and the excitations in the disordered and liquid states on the other, is an important issue in analysing experimental and simulation results on plasma crystals. As the first step in this programme, we have numerically calculated the full phonon spectrum for 2D triangular Yukawa lattices, for a wide range of κ (screening parameter) values and along different propagation angles. Earlier calculations of the excitation spectra of the 2D and 3D Yukawa liquids were based on the quasilocalized charge approximation (QLCA), whose implicit premise is that the spectrum of an average distribution (governed by the isotropic liquid pair correlation function) is a good representation of the actual spectrum. To see the implications of this model more clearly, we compare the high r (near crystallization) QLCA phonon spectra with the angle-averaged phonon spectra of the lattice phonons.

Collective Modes in 2-D Yukawa Solids and Liquids

We report comparative studies on collective excitations in 2-D complex plasmas, in which particles interact through the Yukawa potential, encompassing both the solid and the strongly coupled liquid states. Dispersion and polarization of the collective modes in the solid state are calculated through the lattice summations, while in the liquid state, through molecular dynamics (MD) simulations in conjunction with the theoretical quasi-localized charge approximation analysis. The latter closely emulates the dispersion, resulting from an angular averaging in the lattice. In general, however, the lattice dispersion is substantially different from that of the liquid. The MD simulations show the dramatic transformation of the anisotropic phonon dispersion of the crystal lattice near the solid-liquid transition into the isotropic liquid dispersion

Beam–plasma interaction in strongly coupled plasmas

The well-known problem of beam–plasma instability acquires new aspects when one or both of the two components (the beam and the plasma) are strongly interacting. We have now theoretically considered the case when the plasma is in the solid phase and forms a lattice. In this situation, the inherent anisotropy of the lattice leads to a coupling between the longitudinal and transverse polarizations. One of the novel features of the beam–plasma instability in this scenario is the possible excitation of transverse modes, which should be an experimentally observable signature of the instability. We have initially concentrated on a 2D toy model with the beam lying in the lattice plane. At the same time, we have initiated a molecular dynamics simulation program for studying various aspects of the penetration of a beam into a plasma lattice. The beam parameters can be adjusted in order to see the effects of increasing coupling strength within the beam and to distinguish between collective phenomena and scattering on individual particles. When both components are strongly interacting, a number of remarkable phenomena—trapping of beam particles, creation of dislocations, local melting of the lattice—may be observed.

Beam–Plasma Interaction in Two-Dimensional Yukawa Lattices

The beam-plasma instability displays new properties when either the beam or the plasma, or both, are strongly interacting. We have considered theoretically the case when the plasma is in the crystalline phase and forms a lattice, and the beam is moving in the lattice plane. We consider a 2-D plasma crystal in which both the grains and the beam particles interact through a realistic Yukawa potential. The beam particles are assumed to be weakly coupled to each other and to the lattice. Using the full phonon spectrum for a 2-D hexagonal Yukawa lattice, we determine and compare the transverse and longitudinal growth rates. The relationship between the beam speed and the longitudinal and transverse sound speeds, and the direction of the beam with respect to the principle axes of the lattice determine the qualitative behavior of the growth rates. For beam speeds between the longitudinal and transverse sound speeds, the transverse instability could be more important, because it appears at lower wavenumbers

Compressibilities in bilayers of charged particles

The isothermal compressibility plays a central role in determining the characteristics of the static response in plasma systems. In a charged particle bilayer this role is assumed by L ij , the matrix of inverse compressibilities. For weak coupling, the inverse compressibilities of a bilayer of charged particles can be calculated analytically in the Debye limit from the equation of state through the chemical potential. There are two different charging procedures to obtain the latter. We present the results of a rather lengthy analytical calculation, exploring both approaches. The limits of the validity of the Debye description are discussed, and we compare the weak coupling results with L ij values inferred from S(k→0) through the compressibility sum rule, where the structure function S(k) is generated for strong coupling both through molecular dynamics simulations and by HNC calculations.

Impurity screening in strongly coupled plasma systems

We present an overview of the problem of screening of an impurity in a strongly coupled one-component plasma within the framework of the linear response (LR) theory. We consider 3D, 2D and quasi-2D layered systems. For a strongly coupled plasma the LR can be determined by way of the known S(k) structure functions. In general, an oscillating screening potential with local overscreening and antiscreening regions emerges. In the case of the bilayer, this phenomenon becomes global, as overscreening develops in the layer of the impurity and antiscreening in the adjacent layer. We comment on the limitations of the LR theory in the strong coupling situation.