Damping Of Collective Excitations Research Articles

Collective excitations of a charged Fermi superfluid in the BCS-BEC crossover

We consider collective excitations in the superfluid state of Fermi condensed charged gases. The dispersion and damping of collective excitations at nonzero temperatures are examined, and the coexistence and interaction of different branches of collective excitations: plasma oscillations, pair-breaking Higgs modes, and Carlson–Goldman phonon-like excitations are taken into account. The path integral methods for superfluid Fermi gases and for Coulomb gas are combined into a unified formalism that extends the Gaussian fluctuation approximation to account for plasmonic modes. This approximation of Gaussian pair and density fluctuations is able to describe all branches of collective excitations existing in a charged superfluid. The spectra of collective excitations are determined in two ways: from the spectral functions and from the complex poles of the fluctuation propagator. A resonant avoided crossing of different modes is shown. It is accompanied by resonant enhancement of the response provided by the pair-breaking modes due to their interaction with plasma oscillations. This may facilitate the experimental observation of the pair-breaking modes.

Generalised hydrodynamic description of the time correlation functions of liquid metals: ab initio molecular dynamics study

A new methodology of calculation of dispersion and damping of collective excitations from ab initio molecular dynamics (AIMD) of liquid metals is proposed. It is suggested to use for the analysis of AIMD-derived time correlation functions a thermo-viscoelastic dynamic model within an approach of the generalised collective modes. The proposed scheme, in which the viscoelastic matrix elements of the generalised hydrodynamic matrix are directly calculated from AIMD and the matrix elements requiring knowledge of energy density fluctuations in the system are treated as fitting parameters, allows to recover AIMD time correlation functions in a wide range of wave numbers. The dispersion and damping of collective excitations are obtained as a complex-conjugated pair of the generalised hydrodynamic matrix. An issue of sum rules in the proposed scheme is discussed. The methodology is applied for calculations of dispersion and damping of generalised acoustic modes in molten lithium.

Hydrodynamic model for electron-hole plasma in graphene

We propose a hydrodynamic model describing steady-state and dynamic electron and hole transport properties of graphene structures which accounts for the features of the electron and hole spectra. It is intended for electron-hole plasma in graphene characterized by high rate of inter-carrier scattering compared to external scattering (on phonons and impurities), i.e., for intrinsic or optically pumped (bipolar plasma), and gated graphene (virtually monopolar plasma). We demonstrate that the effect of strong interaction of electrons and holes on their transport can be treated as a viscous friction between the electron and hole components. We apply the developed model for the calculations of the graphene dc conductivity; in particular, the effect of mutual drag of electrons and holes is described. The spectra and damping of collective excitations in graphene in the bipolar and monopolar limits are found. It is shown that at high gate voltages and, hence, at high electron and low hole densities (or vice-versa), the excitations are associated with the self-consistent electric field and the hydrodynamic pressure (plasma waves). In intrinsic and optically pumped graphene, the waves constitute quasineutral perturbations of the electron and hole densities (electron-hole sound waves) with the velocity being dependent only on the fundamental graphene constants.

Generalized collective excitations in supercritical argon

Dispersion and damping of collective excitations in supercritical argon along the isothermal line of 280 K are studied by a combination of molecular dynamics simulations and Generalized Collective Mode (GCM) analysis. ‘Positive dispersion’ of collective excitations and its dependence on density is discussed on the basis of GCM theory of ‘positive dispersion’.

Ratio of shear viscosity to entropy density for trapped fermions in the unitarity limit

We extract the shear viscosity to entropy density ratio $\ensuremath{\eta}/s$ of cold fermionic atoms in the unitarity limit from experimental data on the damping of collective excitations. We find that near the critical temperature $\ensuremath{\eta}/s$ is roughly equal to 1/2 in units of $\ensuremath{\hbar}/{k}_{B}$. With the possible exception of the quark gluon plasma, this value is closer to the conjectured lower bound $1/(4\ensuremath{\pi})$ than any other known liquid.

SOFT MODE AND SPIN-GLASS LIKE TRANSITION IN INSULATING GLASS

The spectrum and damping of collective excitations of a system of electric dipole centers with internal degrees of freedom in glass are calculated. It is shown that one of the collective modes becomes soft, signaling a spin-glass like phase transition in insulating glass. The contribution to the specific heat is determined. The possibility of phase transition from paraelectric to electric pseudospin glass phase in dielectric glass is considered.

Electronic Raman scattering in p-doped GaAs/Ga1-xAlxAs quantum-well structures: Scattering mechanisms and many-particle interactions.

By means of resonance Raman spectroscopy we have investigated intersubband transitions of quasi-two-dimensional (2D) hole gases in p-type modulation-doped GaAs/${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As quantum-well structures. The observed excitations have an essentially single-particle character due to Landau damping of collective excitations and due to single-particle scattering by energy-density fluctuations under conditions of extreme resonance. In samples with well widths of typically 100\char21{}200 \AA{} and 2D hole densities \ensuremath{\rho}\ensuremath{\sim}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, we observe a characteristic variation of intersubband-transition energies with laser frequency in depolarized and in polarized scattering configurations. This variation is caused by the nonparabolic subband dispersion of the 2D single-particle hole subbands. Experiments under variation of p, by illuminating the sample with photons that have energies above the band gap of the ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As barriers, allow an estimate of the relative strengths of direct and exchange Coulomb interactions. From these experiments a greater relative strength of the exchange interaction, in comparison to that found for 2D electron gases, can be deduced.

High-frequency damping of collective excitations in fermion systems. I. Plasmon damping and frequency-dependent local-field factor in a two-dimensional electron gas.

Starting from a general expression of Glick and Long [Phys. Rev. B 4, 3455 (1971)] for the imaginary part of the frequency- and wave-number-dependent dielectric function \ensuremath{\epsilon}(k,\ensuremath{\omega}) of an electron gas in a two-particle--hole pair excitation approximation, an asymptotic expression for ${\ensuremath{\epsilon}}_{2}$(k,\ensuremath{\omega}) is derived, which is valid for any wave number k provided \ensuremath{\omega} is much higher than a certain characteristic k-dependent frequency. The formula for ${\ensuremath{\epsilon}}_{2}$(k,\ensuremath{\omega}) is cast in a form suitable for application to any arbitrary potential v(k) in three-, two-, and one-dimensional space. In the case of D=3 and the Coulomb potential v(k) our result is the same as that of Glick and Long, although its region of validity is now larger. As a specific example, the damping of plasmons in a 2D electron gas has been calculated. Also, an interpolation formula for the complex local-field factor G(k,\ensuremath{\omega}) in a 2D electron gas has been derived. The latter immediately leads to an expression for the \ensuremath{\omega}-dependent exchange-correlation potential---a result analogous to the one derived by Gross and Kohn in the 3D case. The asymptotic expression for ${\ensuremath{\epsilon}}_{2}$(k,\ensuremath{\omega}) is used in paper II to calculate the damping of zero sound in normal liquid $^{3}\mathrm{He}$.

High-frequency damping of collective excitations in fermion systems. II. Damping of zero sound in normal liquid 3He.

High-frequency damping of collective excitations in fermion systems. II. Damping of zero sound in normal liquid 3He.

Phonon Scattering by Paraelectrical Centers in Crystals II. Lowest Order Approximation

AbstractOn the basis of formalism developed in a previous work, a theoretical study of the dynamic of paraelectric centers interacting both, with each other and with lattice vibrations is presented. Spectrum and damping of collective excitations are calculated. Frequency and center concentration dependences of phonon lifetime are analysed. An application of the results obtained from the analysis of thermal conductivity is discussed. A new thermal conductivity mechanism is proposed.

A Monte-Carlo calculation of the secondary electron emission of normal metals: II. Results for aluminium

A theoretical model of the secondary electron emission of pure normal metals is applied to aluminium. The calculated yields strongly depend on the plasmon damping mode assumed in the model. The separate contributions to the true secondary peak coming from the damping of collective excitations are evaluated. These contributions are important and one-pair processes must be invoked to explain the line shape of the experimental secondary peak of Al. The study of the depth dependence of the energy transfers in the solid gives an estimation of the primary beam range. The inner angular distribution of the true secondaries is nearly isotropic, leading to a cosine distribution in vacuum. However, the backscattered current shows a marked anisotropy which strongly depends on the primary energy.

Dynamical current correlations in simple liquids

Expressions for the longitudinal and transverse current correlation functions of simple liquids are obtained using Mori's formalism. These involve unknown damping functions. The use of damping functions for a free gas gives results which are in complete disagreement with experiments. The results obtained on the basis of a renormalized free particle memory function in which the collisional damping of collective excitations is incorporated show remarkable improvement over earlier results and are in reasonable agreement with the molecular dynamics data of Rahman. Therefore, the choice of the memory function is as important as the satisfaction of lower order moments.

Damping of collective excitations in nearly degenerate two-component plasmas

Damping of collective excitations in nearly degenerate two-component plasmas

Damping of collective excitations in nearly degenerate two-component plasmas

Damping of collective excitations in nearly degenerate two-component plasmas