Collective Mode Structure Research Articles

Collective excitations of a hot anisotropic QCD medium with the Bhatnagar-Gross-Krook collisional kernel within an effective description

Collective modes of an anisotropic hot QCD medium have been studied within the semi-classical transport theory employing Bhatnagar-Gross-Krook (BGK) collisional kernel. The modeling of the isotropic medium is primarily based on a recent quasi-particle description of hot QCD equation of state where the medium effects have been encoded in effective gluon and quark/anti-quark momentum distributions that posses non-trivial energy dispersions. The anisotropic distribution functions are obtained in a straightforward the way by stretching or squeezing the isotropic ones along one of the directions. The gluon self-energy is computed using these distribution functions in a linearized transport equation with Bhatnagar-Gross-Krook (BGK) collisional kernel. Further, the tensor decomposition of gluon self-energy leads to the structure functions which eventually controls the dispersion relations and the collective mode structure of the medium. It has been seen that both the medium effects and collisions induce appreciable modifications to the collective modes and plasma excitations in the hot QCD medium.

Tamm states at the interface between a conventional material and a one dimensional photonic crystal with metamaterials

Using the Green׳s function formalism, Tamm states of localized modes are investigated at the interface separating auniform conventional material and a one-dimensional semi-infinite photonic crystal consisting of a series of alternating conventional materials and metamaterials. We investigate how the presence of such metamaterials influences the band structure of collective modes that appear in the photonic crystal, with special attention to the power spectrum of collective excitations and the dispersion relations. It is shown that there is one localized backward TE mode with frequencies below the resonance frequency of the metamaterial magnetic permeability and above such frequency there are one forward TM, and two backward TM and TE localized modes.

Band structure of collective modes and effective properties of binary magnonic crystals

In this paper a theoretical study of the band structure of collective modes of binary ferromagnetic systems formed by a submicrometric periodic array of cylindrical cobalt nanodots partially or completely embedded into a permalloy ferromagnetic film is performed. The binary ferromagnetic systems studied are two-dimensional periodic, but they can be regarded as three-dimensional, since the magnetization is non uniform also along the z direction due to the contrast between the saturation magnetizations of the two ferromagnetic materials along the thickness. The dynamical matrix method, a finite-difference micromagnetic approach, formulated for studying the dynamics in one-component periodic ferromagnetic systems is generalized to ferromagnetic systems composed by F ferromagnetic materials. It is then applied to investigate the spin dynamics in four periodic binary ferromagnetic systems differing each other for the volume of cobalt dots and for the relative position of cobalt dots within the primitive cell. The dispersion curves of the most representative frequency modes are calculated for each system for an in-plane applied magnetic field perpendicular to the Bloch wave vector. The dependence of the dispersion curves on the cobalt quantity and position is discussed in terms of distribution of effective “surface magnetic charges” at the interface between the two ferromagnetic materials. The metamaterial properties in the propagative regime are also studied (1) by introducing an effective magnetization and effective “surface magnetic charges” (2) by describing the metamaterial wave dispersion of the most representative mode in each system within an effective medium approximation and in the dipole-exchange regime. It is also shown that the interchange between cobalt and permalloy does not necessarily lead to an interchange of the corresponding mode dispersion. Analogously to the case of electromagnetic waves in two-dimensional photonic crystals, the degree of localization of the localized collective modes is expressed in terms of an energy concentration factor.

Excitations and quasi-one-dimensionality in field-induced nematic and spin density wave states

We study the excitation spectrum and dynamical response functions for several quasi-one-dimensional spin systems in magnetic fields without dipolar spin order transverse to the field. This includes both nematic phases, which harbor "hidden" breaking of spin-rotation symmetry about the field and have been argued to occur in high fields in certain frustrated chain systems with competing ferromagnetic and antiferromagnetic interactions, and spin density wave states, in which spin-rotation symmetry is truly unbroken. Using bosonization, field theory, and exact results on the integrable sine-Gordon model, we establish the collective mode structure of these states, and show how they can be distinguished experimentally.

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 quasilocalized 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 nonperturbative 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.

Collective Modes in Classical Mass‐Asymmetric Bilayers

AbstractUsing the quasi‐localized charge approximation (QLCA), we analyze the effects of asymmetry on the long‐wavelength longitudinal collective mode dispersion in a variety of strongly correlated electronic bilayer liquids, most notably, the mass‐asymmetric electron‐hole bilayer in its Coulomb liquid and dipole liquid phases. We point out the marked differences between the strong coupling (QLCA) and weak coupling (random‐phase‐approximation) descriptions of the way the asymmetry affects the collective mode structure (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamical instability of a spin spiral in an interacting Fermi gas as a probe of the Stoner transition

We propose an experiment to probe ferromagnetic phenomena in an ultracold Fermi gas, while alleviating the sensitivity to three-body loss and competing many-body instabilities. The system is initialized in a small pitch spin spiral, which becomes unstable in the presence of repulsive interactions. To linear order the exponentially growing collective modes exhibit critical slowing down close to the Stoner transition point. Also, to this order, the dynamics are identical on the paramagnetic and ferromagnetic sides of the transition. However, we show that scattering off the exponentially growing modes qualitatively alters the collective mode structure. The critical slowing down is eliminated and in its place a new unstable branch develops at large wave vectors. Furthermore, long-wavelength instabilities are quenched on the paramagnetic side of the transition. We study the experimental observation of the instabilities, specifically addressing the trapping geometry and how phase-contrast imaging will reveal the emerging domain structure. These probes of the dynamical phenomena could allow experiments to detect the transition point and distinguish between the paramagnetic and ferromagnetic regimes.

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.

Coexistence of superfluid and Mott phases of lattice bosons

Recent experiments on strongly interacting bosons in optical lattices have revealed the coexistence of spatially separated Mott-insulating and number-fluctuating phases. We explore the condensate properties of the number fluctuating phase trapped between the Mott-insulating regions and derive the associated collective-mode structure. We discuss the crossover of the interlayer properties between two- and three-dimensional behavior as a function of the lattice parameters and estimate the critical temperatures for the transition from the normal to superfluid state.

Effects of nonlinear frequency pulling on the cavity phasing and the collective mode structure in phase-locked VCSEL arrays

Carrier and gain depletion due to cross-cavity interactions in actively coupled vertical-cavity surface-emitting laser arrays cause nonlinear-cavity frequency pulling. As a result the collective mode structure departs significantly from earlier studied linearly coupled eigenmodes. Theory and simulations find that array eigenmodes have uniform amplitude and density distribution over the interior array sites, accompanied with boundary-layer formation at the edges. Exact zero or pi phasing among adjacent sites is achieved, depending on the coupling-strength sign. In contrast, in linearly coupled array modes neglecting cross-cavity hole burning, the phase difference cannot be exactly zero or pi, yielding a sinusoidal amplitude distribution over the array sites. The exact zero phasing and the uniform intensity distribution in nonlinearly coupled arrays offer advantages for practical applications.

Effect of magnetic field on collective modes of two-dimensional electron liquids: A molecular dynamics study

We have produced molecular dynamics simulations of a classical two-dimensional electron liquid subject to a constant magnetic field perpendicular to its plane for various values of the coupling parameter and the magnetic field strength. The results were used to obtain the corresponding density and transverse current correlation functions and to study their collective mode structure, in particular, the dispersion curves of the longitudinal plasmon modes and the transverse shear modes. Our dispersion results have been compared with theoretical predictions of the quasilocalized charge approximation (QLCA) and its extensions, for zero and nonzero magnetic fields. We conclude that the QLCA theory explains quite well the plasmon dispersion for all values of the coupling parameter and magnetic field that we investigated; however, the theory is not as good in explaining the shear mode dispersion. We have also observed an additional frequency peak for a range of coupling parameters in both correlation functions, but only for nonzero magnetic fields.

Zero Sound in a Mixture of a Single-Component Fermion Gas and a Bose-Einstein Condensate

The resonant dynamics of mediated interactions supports zero sound in a cold atom degenerate mixture of a single-component fermion gas and a Bose-Einstein condensate. We characterize the onset of instability in the phase separation of an unstable mixture and we find a rich collective mode structure for stable mixtures with one undamped mode that exhibits an avoided crossing and a Landau-damped mode that terminates.

Fine structure of giant resonances revisited experimentally and theoretically

Several examples are presented how the fine structure of collective modes carries unique information on their basic physical properties and, for excitations above the particle thresholds, on the decay mechanisms. The nature of the electric pygmy dipole resonance observed near particle threshold in neutron-rich nuclei is clarified by a detailed study of208Pb Evidence for an orbital magnetic quadrupole twist mode is obtained from a comparison of high-resolution electron and proton scattering experiments on58Ni. An attempt to describe cross sections and spin observables of the 58Ni(p,p′) experiment by state-of-the-art calculations shows surprising deviations. Finally the fine structure of the isoscalar giant quadrupole and the Gamow-Teller resonance in medium-to-heavy nuclei has been studied and new methods to extract the properties of characteristic scales related to the decay properties by wavelet analysis techniques are presented.

Fine structure of giant resonances revisited experimentally and theoretically

Several examples are presented how the fine structure of collective modes carries unique information on their basic physical properties and, for excitations above the particle thresholds, on the decay mechanisms. The nature of the electric pygmy dipole resonance observed near particle threshold in neutron-rich nuclei is clarified by a detailed study of 208 Pb Evidence for an orbital magnetic quadrupole twist mode is obtained from a comparison of high-resolution electron and proton scattering experiments on 58 Ni. An attempt to describe cross sections and spin observables of the 58 Ni(p,p′) experiment by state-of-the-art calculations shows surprising deviations. Finally the fine structure of the isoscalar giant quadrupole and the Gamow-Teller resonance in medium-to-heavy nuclei has been studied and new methods to extract the properties of characteristic scales related to the decay properties by wavelet analysis techniques are presented.

Fine structure of giant resonances revisited experimentally and theoretically

Several examples are presented how the fine structure of collective modes carries unique information on their basic physical properties and, for excitations above the particle thresholds, on the decay mechanisms. The nature of the electric pygmy dipole resonance observed near particle threshold in neutron-rich nuclei is clarified by a detailed study of 208Pb Evidence for an orbital magnetic quadrupole twist mode is obtained from a comparison of high-resolution electron and proton scattering experiments on 58Ni. An attempt to describe cross sections and spin observables of the 58Ni(p,p′) experiment by state-of-the-art calculations shows surprising deviations. Finally the fine structure of the isoscalar giant quadrupole and the Gamow-Teller resonance in medium-to-heavy nuclei has been studied and new methods to extract the properties of characteristic scales related to the decay properties by wavelet analysis techniques are presented.

Identification of 3He-A by ultrasound experiments

On the basis of available experimental data Gould [Physica B 178 (1992) 266] has suggested that 3He-A, conventionally identified as the axial phase, may actually be an axi-planar phase. Some experiments have been done to clarify the situation [see, for example, T.R. Mullins et al., Phys. Rev. Lett. 72 (1994) 4177] but the problem is still open. While we take no position on the interpretation of the data, we note that the collective mode structure of the two phases differ and that appropriate measurements could resolve the issue. We investigate this problem within a simple, time-dependent Ginzburg–Landau (GL) model as well as by studying the second variation of the free energy functional. Both methods show that the spectrum in the axial phase is degenerate, while it is split in the axi-planar phase. This fact may serve as a sensitive test of the existence of the latter.

Collective modes in strongly correlated yukawa liquids: waves in dusty plasmas.

We determine the collective mode structure of a strongly correlated Yukawa fluid, with the purpose of analyzing wave propagation in a strongly coupled dusty plasma. We identify a longitudinal plasmon and a transverse shear mode. The dispersion is characterized by a low- k acoustic behavior, a frequency maximum well below the plasma frequency, and a high- k merging of the two modes around the Einstein frequency of localized oscillations. The damping effect of collisions between neutrals and dust grains is estimated.

Static and dynamic properties of Yukawa systems : Application to dusty plasmas

The pair correlation function g(r) for a strongly coupled Yukawa liquid has been generated by using an HNC code for a wide range of nominal coupling (Γ=βZ 2 e 2 /a) and range (κ =κa) values (a is the interparticle distance). We have analyzed the collective mode structure of the system via the Quasi Localized Charge Approximation (QLCA) which requires the input of the correlation function [2]. The availability of g(r) has now made it possible to extend the analysis to arbitrary wave lengths for high κ-values. Both mode frequencies approach the same asymptotic limit, the Einstein frequency of the localized mode.

Effect of correlations on the energy gap in electronic bilayers: Critical analysis of different approaches

This paper investigates the effect of correlations in electronic bilayers on the longitudinal collective mode structure. We employ the dielectric permeability constructed by means of the classical theory of moments. It is shown that the neglection of damping processes overestimates the role of correlations. We conclude that the correct account of damping processes leads to an absence of an energy gap.

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 $3^{rd}$ frequency-moment-sum-rule preserving Quasi Localized Charge Approximation, using the recently calculated Hypernetted Chain pair correlation functions. The spectrum shows an energy gap at $k=0$ and the absence of a previously conjectured dynamical instability.