Spectrum Of Collective Excitations Research Articles

Electromagnetically-induced-transparency–like phenomenon with two atomic ensembles in a cavity

We study the spectra of collective low excitations of two atomic ensembles which couple with a single-mode cavity field. When the left ensemble is driven with an external optical field, its corresponding response spectrum to the incident optical light shows an electromagnetically-induced-transparency--like (EIT-like) phenomenon when the layers are arranged in the sequence of node antinode but not in the sequence of antinode node. In the case of antinode-antinode sequence, the response spectrum shows an EIT-like phenomenon with two transparent windows. We also investigate the fluctuation spectra of the atomic collective excitation modes and show similar EIT-like phenomena.

Magnetoplasmons in Rotating Dusty Plasmas

A rotating dusty plasma apparatus was constructed to provide the possibility of experimental emulation of extremely high magnetic fields by means of the Coriolis force, observable in a corotating measurement frame. We present collective excitation spectra for different rotation rates with a magnetic induction equivalent of up to 3200 T. We identify the onset of magnetoplasmon-equivalent mode dispersion in the rotating macroscopic two-dimensional single-layer dusty plasma. The experimental results are supported by molecular dynamics simulations of 2D magnetized Yukawa systems.

Coherence of two-dimensional electron-hole systems: Spontaneous breaking of continuous symmetries: A review

The spontaneous breaking of the continuous symmetries of a two-dimensional electron-hole system in a strong magnetic field perpendicular to the plane leads to the formation of new ground states and determines the energy spectrum of collective elementary excitations that appear above these new ground states. In this review, the main attention is paid to the electron-hole system formed from coplanar magnetoexcitons under conditions of Bose-Einstein condensation in the ground state with the wave vector k = 0 taking into account the influence of excited Landau levels, when exciton-type elementary excitations coexist with plasmon-type oscillations. At the same time, the properties of a two-component system consisting of a two-dimensional electron gas and a two-dimensional hole gas spatially separated in a double quantum well under conditions of the fractional quantum Hall effect are of great interest, because these properties can affect the quantum states of magnetic excitons that are formed when the distance between the layers tends to zero. Bilayer electron systems are also considered under conditions of the fractional quantum Hall effect with the one-half filling factor for each layer and the total filling factor equal to unity for both layers. The coherence between the electron states in the two layers is equivalent to the formation of excitons in a macroscopic coherent state. This makes it possible to compare the energy spectrum of collective elementary excitations of Bose-Einstein condensed excitons under conditions of the quantum Hall effect and coplanar magnetoexcitons. The breaking of the global gauge symmetry or of the continuous rotational symmetry leads to the formation of a gapless spectrum of the Nambu-Goldstone type, whereas the breaking of the local gauge symmetry is accompanied by the appearance of a gap in the energy spectrum (Higgs phenomenon). These phenomena are equivalent to the formation of massless and massive particles in the relativistic physics. The application of the Nielsen-Chadha theorem, which determines the number of Nambu-Goldstone modes as a function of the number of broken symmetry operators, is demonstrated using the example of Bose-Einstein condensation of spinor atomic gases in an optical trap. This example is presented for a better understanding of the results obtained in the case of a Bose-Einstein condensation of coplanar magnetoexcitons. The Higgs phenomenon leads to the formation of composite particles under conditions of the fractional quantum Hall effect. Their description is given in terms of the Ginzburg-Landau theory. The possibilities for the appearance of spontaneous coherence in a system of indirect excitons in structures with a double quantum well are analyzed. The experimental attempts to create these conditions, the main results obtained in this field, and the accumulated knowledge are reviewed. The basic properties of the energy spectrum of magnetoexciton polaritons in a microcavity are formulated. A hypothesis is put forward about the possibility of forming two-dimensional magnetoexcitons and two-dimensional magnetoexciton polaritons of high density with attached point quantum vortices, i.e., about the possibility of forming new composite particles.

Spinovi khvyli u masyvakh mahnitnykh nanotochok, pov"yazanykh mahnitodypol'noyu vzayemodiyeyu

A general theory of collective spin-wave excitations in finite and infinite periodic arrays of magnetic nanodots with magnetodipolar coupling has been developed. Non-uniform profiles of static and dynamic magnetizations in a dot are taken into account. The theory allows the spectra of collective excitations, their damping rates, excitation efficiencies by an external microwave field, and so on to be calculated and the stability of a stationary magnetic array configuration to be analyzed. An efficient technique has been proposed to calculate the spin-wave spectra in periodic arrays using the method of projection onto the eigenmodes of a solitary nanodot. The results obtained are compared with experimental data.

Quantum-dot supercrystals for future nanophotonics

The study of supercrystals made of periodically arranged semiconductor quantum dots is essential for the advancement of emerging nanophotonics technologies. By combining the strong spatial confinement of elementary excitations inside quantum dots and exceptional design flexibility, quantum-dot supercrystals provide broad opportunities for engineering desired optical responses and developing superior light manipulation techniques on the nanoscale. Here we suggest tailoring the energy spectrum and wave functions of the supercrystals' collective excitations through the variation of different structural and material parameters. In particular, by calculating the excitonic spectra of quantum dots assembled in two-dimensional Bravais lattices we demonstrate a wide variety of spectrum transformation scenarios upon alterations in the quantum dot arrangement. This feature offers unprecedented control over the supercrystal's electromagnetic properties and enables the development of new nanophotonics materials and devices.

Quantum Hall ferromagnetic phases in the Landau levelN=0of a graphene bilayer

In a Bernal-stacked graphene bilayer, an electronic state in Landau level $N=0$ is described by its guiding-center index $X$ (in the Landau gauge) and by its valley, spin, and orbital indices $\ensuremath{\xi}=\ifmmode\pm\else\textpm\fi{}K,\phantom{\rule{0.28em}{0ex}}\ensuremath{\sigma}=\ifmmode\pm\else\textpm\fi{}1,$ and $n=0,1.$ When Coulomb interaction is taken into account, the chiral two-dimensional electron gas (C2DEG) in this system can support a variety of quantum Hall ferromagnetic ground states where the spins and/or valley pseudospins and/or orbital pseudospins collectively align in space. In this work, we give a comprehensive account of the phase diagram of the C2DEG at integer filling factors $\ensuremath{\nu}\ensuremath{\in}[\ensuremath{-}3,3]$ in Landau level $N=0$ when an electrical potential difference ${\ensuremath{\Delta}}_{B}$ between the two layers is varied. We consider states with or without layer, spin, or orbital coherence. For each phase, we discuss the behavior of the transport gap as a function of ${\ensuremath{\Delta}}_{B},$ the spectrum of collective excitations, and the optical absorption due to orbital pseudospin-wave modes. We also study the effect of an external in-plane electric field on a coherent state that has both valley and spin coherence and show that it is possible, in such a state, to control the spin polarization by varying the strength of the external in-plane electric field.

Classical behavior of strongly correlated electron systems of solids near a quantum critical point

Abstract The spectra of collective excitations of electron systems situated near a Fermi-liquid quantum critical point are analyzed. Phonon-like soft and weakly damped branches of the transverse zero-sound mode are found that have extremely low effective Debye temperatures. Their contributions to the collision integral are shown to drive electron transport in the vicinity of the critical point toward a classical regime.

Self-consistent approach for Bose-condensed atoms in optical lattices

Bose atoms in optical lattices are considered at low temperatures and weak interactions, when Bose-Einstein condensate is formed. A self-consistent approach, based on the use of a representative statistical ensemble, is employed, ensuring a gapless spectrum of collective excitations and the validity of conservation laws. In order to show that the approach is applicable to both weak and tight binding, the problem is treated in the Bloch as well as in the Wannier representations. Both these ways result in similar expressions that are compared for the self-consistent Hartree-Fock-Bogolubov approximation. A convenient general formula for the superfluid fraction of atoms in an optical lattice is derived.

Ab initio study of collective excitations in a disparate mass molten salt

Ab initio molecular dynamics simulations and the approach of generalized collective modes are applied for calculations of spectra of longitudinal and transverse collective excitations in molten LiBr. Dispersion and damping of low- and high-frequency branches of collective excitations as well as wave-number dependent relaxing modes were calculated. The main mode contributions to partial, total, and concentration dynamic structure factors were estimated in a wide region of wave numbers. A role of polarization effects is discussed from comparison of mode contributions to concentration dynamic structure factors calculated for molten LiBr from ab initio and classical rigid ion simulations.

Collective dynamics in binary liquids: a molecular dynamics study of the composition dependence of the spectra of collective excitations

The spectra of longitudinal and transverse collective excitations in liquid binary metallic NacK1−c alloys are studied for pure components and four different concentrations. A theoretical generalized collective modes approach is used to analyze the concentration dependence of the dispersion of acoustic and optic branches in a wide region of wavenumbers. The dispersion of longitudinal collective excitations in binary alloys is estimated from the eight-variable thermo-viscoelastic dynamic model with full account of thermal fluctuations. It is found that the longitudinal and transverse branches show different dependences on concentration in the short-wavelength region. The issue of ‘positive dispersion’ of acoustic excitations in liquid binary alloys on the boundary of the hydrodynamic regime is discussed. It is shown that the coupling between longitudinal acoustic and optic modes is responsible for an increase of the ‘positive dispersion’ close to equimolar composition.

Collective excitations and stability of the excitonic phase in the extended Falicov-Kimball model

We consider the excitonic insulator state (often associated with electronic ferroelectricity), which arises on the phase diagram of an extended spinless Falicov-Kimball model (FKM) at half-filling. Within the Hartree--Fock approach, we calculate the spectrum of low-energy collective excitations in this state up to second order in the narrow-band hopping and/or hybridization. This allows to probe the mean-field stability of the excitonic insulator. The latter is found to be unstable when the case of the pure FKM (no hybridization with a fully localized band) is approached. The instability is due to the presence of another, lower-lying ground state and not to the degeneracy of the excitonic phase in the pure FKM. The excitonic phase, however, may be stabilized further away from the pure FKM limit. In this case, the low-energy excitation spectrum contains important information about the properties of the excitonic condensate (including the strongly suppressed critical temperature).

Interaction-enhanced magnetically ordered insulating state at the edge of a two-dimensional topological insulator

We develop a theory of the correlated magnetically ordered insulating state at the edge of a two-dimensional topological insulator. We demonstrate that the gapped spin-polarized state, induced by the application of the magnetic field $B$, is naturally facilitated by electron interactions, which drive the critical easy-plane ferromagnetic correlations in the helical liquid. As the key manifestation, the gap $\De$ in the spectrum of collective excitations, which carry both spin and charge, is enhanced and exhibits a scaling dependence $\De \propto B^{1/(2-K)}$, controlled by the Luttinger liquid parameter $K$. This scaling dependence could be probed through the activation behavior $G \sim (e^2/h) \exp(- \De/T)$ of the longitudinal conductance of a Hall-bar device at lower temperatures, providing a straightforward way to extract the parameter $K$ experimentally. Our findings thus suggest that the signatures of the interaction-driven quantum criticality of the helical liquid could be revealed already in a standard Hall-bar measurement.

Collective excitations in molten iron above the melting point: A generalized collective-mode analysis of simulations with embedded-atom potentials

It is shown, that the embedded-atom potential nicely describing structural properties of high pressure Fe can be successfully used for description of collective dynamics of liquid iron. A combination of molecular dynamics simulations and a fit-free analysis based on the approach of generalized collective modes (GCM) is used for calculations of spectra of collective excitations and relaxing modes at 1843 K. The obtained spectrum of acoustic excitations in the long-wavelength region perfectly agrees with the experimental speed of sound and reproduces the dispersion estimated from inelastic X-ray scattering (IXS) experiments. Heat fluctuations in liquid Fe were studied and resulted in calculated ratio of specific heats I³-1.40 being in agreement with the IXS-experiment estimate. We report analysis of the wave-number dependence of relaxation processes and their contributions to dynamic structure factors. This permits estimation of most important relaxation processes contributing to the shape of dynamic structure factors of liquid Fe in different regions of wave numbers.

Role of band structure and local-field effects in the low-energy collective electronic excitation spectra of 2H-NbSe2

We present a study of the electron dynamics in the layered compound 2$H$-NbSe${}_{2}$. First-principles calculations are used to obtain the band structure employed in the evaluation of the loss function with inclusion of local-field (LF) effects. Two different symmetry directions [(100) and (010)] were explored in the hexagonal basal plane. In both cases, a low-energy charge-carrier plasmon (CCP) at $\ensuremath{\sim}$1 eV presenting a negative dispersion over a wide momentum transfer range is found, in agreement with recent experimental results [Wezel et al., Phys. Rev. Lett. 107, 176404 (2011)]. On the contrary, in the (001) perpendicular direction, the CCP has negative dispersion at small momenta only, presenting strong positive dispersion at larger momenta. Our calculations reveal that this behavior can be explained without invoking many-body effects, as long as band structure effects are properly included in the evaluation of the excitation spectra. In addition to this CCP mode, we find another one with an arclike oscillating dispersion along the perpendicular direction, as well as the appearance of a CCP replica at high momenta due to LF effects.

Statistical description of hydrodynamic processes in ionic melts while taking into account polarization effects

A statistical description of hydrodynamic processes for molten salts is proposed taking into account polarization effects caused by the deformation of external ionic shells. This description is conducted by means of the Zubarev nonequilibrium statistical operator method, appropriate for researches of both strong and weak nonequilibrium processes. The nonequilibrium statistical operator and the generalized hydrodynamic equations that take into account the polarization processes are acquired for the ion-polarization model of ionic molten salts when the nonequilibrium averaged values of densities of ions number, their momentum, dipole momentum, and total energy are chosen for the reduced description parameters. A spectrum of collective excitations is investigated within the viscoelastic approximation for the ion-polarization model of ionic melts.

Drag in a resonantly driven polariton fluid

We study the linear response of a coherently driven polariton fluid in the pump-only configuration scattering against a point-like defect and evaluate analytically the drag force exerted by the fluid on the defect. When the system is excited near the bottom of the lower polariton dispersion, the sign of the interaction-renormalised pump detuning classifies the collective excitation spectra into three different categories (Ciuti and Carusotto 2005 Phys. Status Solidi b 242 2224): linear for zero, diffusive-like for positive and gapped for negative detuning. We show that both cases of zero and positive detuning share a qualitatively similar crossover of the drag force from the subsonic to the supersonic regime as a function of the fluid velocity, with a critical velocity given by the speed of sound found for the linear regime. In contrast, for gapped spectra, we find that the critical velocity exceeds the speed of sound. In all cases, the residual drag force in the subcritical regime depends on the polariton lifetime only. Also, well below the critical velocity, the drag force varies linearly with the polariton lifetime, in agreement with previous work (Cancellieri et al 2010 Phys. Rev. B 82 224512), where the drag was determined numerically for a finite-size defect.

The Roton Minimum: Is it a General Feature of Strongly Correlated Liquids?

AbstractThe roton minimum is a deep minimum in the collective excitation spectrum of the liquid, forming around fairly high k ‐values. We have discovered, through MD simulations, that this appears to be a general feature of strongly coupled liquids and is ubiquitous in 2D and 3D Yukawa liquids. We suggest that the physical origin of the roton minimum has to be sought in the quasi‐localization of particles in a strongly correlated liquid and in the ensuing formation of local microcrystals whose averaged frequency dispersion would show roton minimum‐like feature (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the problem of a consistent description of kinetic and hydrodynamic processes in dense gases and liquids: Collective excitations spectrum

Based on the generalized non-Markovian equations obtained earlier for a nonequilibrium one-particle distribution function and potential part of the averaged enthalpy density [Markiv B.B., Omelyan I.P, Tokarchuk M.V., Condens. Matter Phys., 2010, 13, 23005] a spectrum of collective excitations is investigated, where the potential of interaction between particles is presented as a sum of the potential of hard spheres and a certain long-range potential.

Effect of time-odd mean fields on inertial parameters of the quadrupole collective Hamiltonian

Most current implementations of the quadrupole collective Hamiltonian model do not include the contributions of time-odd mean fields to the moments of inertia and mass parameters (Thouless-Valatin dynamical rearrangement contributions). A hybrid model is introduced that allows a quantitative estimate of these contributions to the inertial functions of a five-dimensional collective Hamiltonian based on microscopic energy density functionals. Fully self-consistent constrained relativistic Hartree-Bogoliubov calculations of triaxial energy surfaces in the $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ plane are used to determine the parameters of an equivalent pairing-plus-quadrupole ($P+Q$) Hamiltonian. This Hamiltonian is employed in constrained Hartree-Fock-Bogoliubov (CHFB) plus local quasiparticle random-phase approximation (LQRPA) calculation of the deformation-dependent corrections to the Inglis-Belyaev moments of inertia and cranking mass parameters. This hybrid model is used to evaluate the influence of time-odd mean fields on vibrational and rotational collective masses and investigate their effect on the low-energy collective excitation spectra and transition rates of $\ensuremath{\gamma}$-soft Xe and Ba nuclei.

Correlation effect on the roton minimum of the rotating dipolar Fermi gas

The collective-excitation spectrum from a filled Landau level in a rotating dipolar Fermi gas is investigated through the equation of motion method. The predicted excitation spectrum exhibits a roton structure which is affected by the strength of dipolar interactions and trapping potential. The correlation effect on the roton minimum is observed by varying the dipolar interaction strength. An increase of the dipolar interaction strength leads to a deeper roton minimum. The roton minimum can even become zero, where the system exhibits instability, as the dipolar strength approaches a critical value. As the dynamical stability of the system is studied through the roton structure, we find that this zero-value roton minimum is a precursor of the occurrence of a bubble crystal.