Kenneth I. Golden

Quasilocalized charge approximation in strongly coupled plasma physics

The quasilocalized charge approximation (QLCA) was proposed in 1990 [G. Kalman and K. I. Golden, Phys. Rev. A 41, 5516 (1990)] as a formalism for the analysis of the dielectric response tensor and collective mode dispersion in strongly coupled Coulomb liquids. The approach is based on a microscopic model in which the charges are quasilocalized on a short-time scale in local potential fluctuations. The authors review the application of the QLC approach to a variety of systems which can exhibit strongly coupled plasma behavior: (i) the one-component plasma (OCP) model in three dimensions (e.g., laser-cooled trapped ions) and (ii) in two dimensions (e.g., classical 2D electron liquid trapped above the free surface of liquid helium), (iii) binary ionic mixture in a neutralizing uniform background (e.g., carbon–oxygen white dwarf interiors), (iv) charged particle bilayers (e.g., semiconductor electronic bilayers), and (v) charged particles in polarizable background (e.g., laboratory dusty plasmas).

Dynamics of two-dimensional dipole systems

Using a combined analytical/molecular dynamics approach, we study the current fluctuation spectra and longitudinal and transverse collective mode dispersions of the classical two-dimensional (point) dipole system (2DDS) characterized by the ϕ{D}(r)=μ{2}/r{3} repulsive interaction potential; μ is the electric dipole strength. The interest in the 2DDS is twofold. First, the quasi-long-range 1/r{3} interaction makes the system a unique classical many-body system, with a remarkable collective mode behavior. Second, the system may be a good model for a closely spaced semiconductor electron-hole bilayer, a system that is in the forefront of current experimental interest. The longitudinal collective excitations, which are of primary interest for the liquid phase, are acoustic at long wavelengths. At higher wave numbers and for sufficiently high coupling strength, we observe the formation of a deep minimum in the dispersion curve preceded by a sharp maximum; this is identical to what has been observed in the dispersion of the zero-temperature bosonic dipole system, which in turn emulates so-called roton-maxon excitation spectrum of the superfluid 4He . The analysis we present gives an insight into the emergence of this apparently universal structure, governed by strong correlations. We study both the liquid and the crystalline solid state. We also observe the excitation of combination frequencies, resembling the roton-roton, roton-maxon, etc. structures in 4He .

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.

COLLECTIVE MODES IN STRONGLY COUPLED ELECTRONIC BILAYER LIQUIDS

We present the first reliable calculation of the collective mode structure of a strongly coupled electronic bilayer. The calculation is based on a classical model through the 3rd frequency-moment-sum rule preserving quasi-localized-charge approximation, using the recently calculated hypernetted-chain pair correlation functions. The out-of-phase spectrum shows an energy gap at k=0 and the absence of a previously conjectured dynamical instability. {copyright} {ital 1999} {ital The American Physical Society}

Correlational origin of the roton minimum

We present compelling evidence supporting the conjecture that the origin of the roton in Bose-condensed systems arises from strong correlations between the constituent particles. By studying the two-dimensional bosonic dipole systems paradigm, we find that classical molecular-dynamics (MD) simulations provide a faithful representation of the dispersion relation for a low-temperature quantum system. The MD simulations allow one to examine the effect of coupling strength on the formation of the roton minimum and to demonstrate that it is always generated at a sufficiently high enough coupling. Moreover, the classical images of the roton-roton, roton-maxon, etc. states also appear in the MD simulation spectra as a consequence of the strong coupling.

The Quasilocalized Charge Approximation

The quasilocalized charge approximation (QLCA) has been used for some time as a formalism for the calculation of the dielectric response and for determining the collective mode dispersion in strongly coupled Coulomb and Yukawa liquids. The approach is based on a microscopic model in which the charges are quasilocalized on a short-time scale in local potential fluctuations. We review the conceptual basis and theoretical structure of the QLC approach and together with recent results from molecular dynamics simulations that corroborate and quantify the theoretical concepts. We also summarize the major applications of the QLCA to various physical systems, combined with the corresponding results of the molecular dynamics simulations and point out the general agreement and instances of disagreement between the two.

Second plasmon and collective modes in binary Coulomb systems

In a system consisting of two different charged species we identify the excitation of a second, low-frequency plasmon. At strong coupling the doublet of high-frequency (first) and low-frequency (second) plasmons replaces the single-plasmon excitation that prevails at weak coupling. We observe the formation of the second plasmon from the acoustic Goldstone-type mode associated with a short-range interaction as the range is extended to infinity.

Strong coupling effects in binary Yukawa systems.

We analyze the acoustic collective excitations in two- and three-dimensional binary Yukawa systems, consisting of two components with different masses. A theoretical analysis reveals a profound difference between the weakly and strongly correlated limits: at weak coupling the two components interact via the mean field only and the oscillation frequency is governed by the light component. In the strongly correlated limit the mode frequency is governed by the combined mass, where the heavy component dominates. Computer simulations in the full coupling range extend and confirm the theoretical results.

Collective Excitations in Electron-Hole Bilayers

We report a combined analytic and molecular dynamics analysis of the collective mode spectrum of a bipolar (electron-hole) bilayer in the strong coupling classical limit. A robust, isotropic energy gap is identified in the out-of-phase spectra, generated by the combined effect of correlations and of the excitation of the bound dipoles. In the in-phase spectra we identify longitudinal and transverse acoustic modes wholly maintained by correlations. Strong nonlinear generation of higher harmonics of the fundamental dipole oscillation frequency and the transfer of harmonics between different modes is observed.

Simulations of strongly coupled charged particle systems: static and dynamical properties of classical bilayers

This paper reviews our recent molecular dynamics simulation studies of the static and dynamical behaviour of classical bilayers in their liquid phase. The pair correlation functions obtained in the static calculations make it possible to trace the structural changes of the system as well as to calculate the energy and static structure functions of the bilayer. The dynamical calculations show the existence of two (in-phase and out-of-phase) longitudinal and two (in-phase and out-of-phase) transverse collective modes. We present the full dispersion relations for these modes at different layer separations. At low layer separations the out-of-phase modes are found to possess a finite frequency at wave numbers k → 0, confirming the existence of the long-wavelength energy gap in the bilayer system predicted by the quasi-localized charge approximation. It is only at higher layer separations that the dominant portion of the longitudinal out-of-phase mode is well approximated by the acoustic behaviour, resulting from the random phase approximation theory.