Dielectric matrix and plasmon dispersion in strongly coupled electronic bilayer liquids

We develop a dielectric matrix and analyze plasmon dispersion in strongly coupled charged-particle bilayers in the quantum domain. The formulation is based on the classical quasi-localized charge approximation (QLCA) and extends the QLCA formalism into the quantum domain. Its development, which parallels that of 2D companion paper [Phys. Rev. E 70, 026406 (2004)] by three of the authors, generalizes the single-layer scalar formalism therein to a bilayer matrix formalism. Using pair correlation function data generated from diffusion Monte Carlo simulations, we calculate the dispersion of the in-phase and out-of-phase plasmon modes over a wide range of in-layer coupling values and layer spacings. The out-of-phase spectrum exhibits an exchange-correlation induced long-wavelength energy gap in contrast to earlier predictions of acoustic dispersion softened by exchange-correlations. The energy gap is similar to what has been previously predicted for classical charged-particle bilayers and subsequently confirmed by recent molecular dynamics computer simulations.Comment: 53 pages including 15 Figures with their captions. Submitted to Physical Review

Factorization of 3-point static structure functions in 3D Yukawa liquids

In many-body systems the convolution approximation states that the 3-point static structure function, $S^{(3)}(\textbf{k}_{1},\textbf{k}_{2})$, can approximately be "factorized" in terms of the 2-point counterpart, $S^{(2)}(\textbf{k}_{1})$. We investigate the validity of this approximation in 3-dimensional strongly-coupled Yukawa liquids: the factorization is tested for specific arrangements of the wave vectors $\textbf{k}_{1}$ and $\textbf{k}_{2}$, with molecular dynamics simulations. With the increase of the coupling parameter we find a breakdown of factorization, of which a notable example is the appearance of negative values of $S^{(3)}(\textbf{k}_{1},\textbf{k}_{2})$, whereas the approximate factorized form is restricted to positive values. These negative values -- based on the quadratic Fluctuation-Dissipation Theorem -- imply that the quadratic part of the density response of the system changes sign with wave number. Our simulations that incorporate an external potential energy perturbation clearly confirm this behavior

Collective Modes in Two Dimensional Binary Yukawa Systems

We analyze via theoretical approaches and molecular dynamics simulations the collective mode structure of strongly coupled two-dimensional binary Yukawa systems, for selected density, mass and charge ratios, both in the liquid and crystalline solid phases. Theoretically, the liquid phase is described through the Quasi-Localized Charge Approximation (QLCA) approach, while in the crystalline phase we study the centered honeycomb and the staggered rectangular crystal structures through the standard harmonic phonon approximation. We identify "longitudinal" and "transverse" acoustic and optic modes and find that the longitudinal acoustic mode evolves from its weakly coupled counterpart in a discontinuous non-perturbative fashion. The low frequency acoustic excitations are governed by the oscillation frequency of the average atom, while the high frequency optic excitation frequencies are related to the Einstein frequencies of the systems.Comment: 35 pages, 17 figures, submitted to Phys. Rev.

Is the Vlasov equation valid for Yukawa plasmas?

We analyze the Vlasov dispersion relation for Yukawa plasmas in three dimensions for the purpose of identifying coupling parameter domains where the Vlasov approach is justified and the existence of a well-developed RPA type collective excitation is allowed. We establish a rigorous lower bound for the coupling parameter, below which there can be no real solution to the Vlasov dispersion relation. In the coupling domain, where weakly damped solutions do exist, we have focused on the long-wavelength acoustic regime where we establish more restrictive estimates for the lower bound of the coupling parameter. We also derive a general formula for the corresponding acoustic phase velocity, valid over a wide range of coupling parameter/screening parameter ratios above the lower bound. We conclude that the Vlasov approach is tenable only above a critical coupling value. Comparison with Molecular Dynamics simulation results further highlights the limitations of the Vlasov approximation for weakly coupled Yukawa plasmas

Linear and quadratic static response functions and structure functions in Yukawa liquids

We compute linear and quadratic static density response functions of three-dimensional Yukawa liquids by applying an external perturbation potential in molecular dynamics simulations. The response functions are also obtained from the equilibrium fluctuations (static structure factors) in the system via the fluctuation-dissipation theorems. The good agreement of the quadratic response functions, obtained in the two different ways, confirms the quadratic fluctuation-dissipation theorem. We also find that the three-point structure function may be factorizable into two-point structure functions, leading to a cluster representation of the equilibrium triplet correlation function

Sound speed in Yukawa one-component plasmas across coupling regimes

A many-body system of charged particles interacting via a pairwise Yukawa potential, the so-called Yukawa one-component plasma (YOCP), is a good approximation for a variety of physical systems. Such systems are completely characterized by two parameters: the screening parameter, κ, and the nominal coupling strength, . It is well known that the collective spectrum of the YOCP is governed by a longitudinal acoustic mode, both in the weakly and strongly coupled regimes. In the long-wavelength limit, the linear term in the dispersion (i.e., ω = sk) defines the sound speed s. We study the evolution of this latter quantity from the weak- through the strong-coupling regimes by analyzing the dynamic structure function S(k, ω) in the low-frequency domain. Depending on the values of and κ and w = s/vth (i.e., the ratio between the phase velocity of the wave and the thermal speed of the particles), we identify five domains in the (κ, ) parameter space in which the physical behavior of the YOCP exhibits different features. The competing physical processes are the collective Coulomb- like versus binary-collision-dominated behavior and the individual particle motion versus quasilocalization. Our principal tool of investigation is molecular dynamics (MD) computer simulation from which we obtain S(k, ω). Recent improvements in the simulation technique have allowed us to obtain a large body of high-quality data in the range = {0.1−10 000} and κ = {0.5−5}. The theoretical results based on various models are compared in order to see which one provides the most cogent physical description and the best agreement with MD data in the different domains

The Collider Phenomenology of Technihadrons in the Technicolor Straw Man Model

We discuss the phenomenology of the lightest SU(3)_C singlet and non-singlet technihadrons in the Straw Man Model of low-scale technicolor (TCSM). The technihadrons are assumed to be those arising in topcolor--assisted technicolor models in which topcolor is broken by technifermion condensates. We improve upon the description of the color--singlet sector presented in our earlier paper introducing the TCSM (hep-ph/9903369). These improvements are most important for subprocess energies well below the masses of the technirho and techniomega, and, therefore, apply especially to e+e- colliders such as LEP and a low--energy linear collider. In the color--octet sector, we consider mixing of the gluon, the coloron V_8 from topcolor breaking, and four isosinglet color--octet technirho mesons. We assume, as expected in walking technicolor, that these technirhos decay into qbar-q, gg, and g-technipion final states, but not into technipion pairs. All the TCSM production and decay processes discussed here are included in the event generator Pythia. We present several simulations appropriate for the Tevatron Collider, and suggest benchmark model lines for further experimental investigation.Comment: 42 pages, 7 figure