Electron-phonon interaction and coupled phonon-plasmon modes

The theory of Raman scattering by the coupled electron-phonon system in metals and heavily doped semiconductors is developed with Coulomb screening and the electron-phonon deformation interaction taken into account. The Boltzmann equation for carriers is applied. Phonon frequencies and optic coupling constants are renormalized due to interactions with carriers. The k-dependent semiclassical dielectric function is employed instead of the Lindhard-Mermin expression. The results of calculations are presented for various values of the carrier concentration and the electron-phonon coupling constant.     

Femtosecond formation of coupled phonon-plasmon modes in InP: Ultrabroadband THz experiment and quantum kinetic theory

The ultrafast transition of an optical phonon resonance to a coupled phonon-plasmon system is studied. After 10-fs photoexcitation of i-InP, the buildup of coherent beats of the emerging hybrid modes is directly monitored via ultrabroadband THz spectroscopy. The anticrossing is mapped out as a function of time and density. A quantum kinetic theory of microscopic carrier-carrier and carrier-LO-phonon interactions explains the delayed formation of the collective modes. The buildup time is quantitatively reproduced to scale with the oscillation cycle of the upper branch of the coupled resonance.     

Surface effects on optical phonons and on phonon-plasmon modes

The size and shape dependence of the properties of long wave optical phonons in polar crystals is discussed. The main effect occurring due to the presence of surfaces is the appearance of a surface optical phonon band in the frequency region between the frequencies ω_t and ω_l of the long wave transverse and longitudinal bulk modes. The surface modes give rise to strong absorption peaks in the infrared absorption spectra of small samples. For very small crystallites the surface mode absorption is dominant, and as the size of the specimens is increased, the ratio of surface to bulk mode absorption decreases. It is shown that the large spread in particle sizes usually encountered in experimental work and the increased damping of the phonon modes in small samples both tend to obscure the fine structure of the absorption spectrum. The surface phonon-plasmon modes in polar crystals containing free carriers are treated in an analogous manner.     

Anisotropy of phonon-plasmon modes in GaAs/AlAs(311) superlattices

Doped GaAs/AlAs superlattices grown on the (311)A and (311)B surfaces have been studied using Raman spectroscopy. Phonon and phonon-plasmon modes with different directions of the wave vectors in the superlattice plane (i.e., the modes propagating in different lateral directions) have been observed in back-scattering from the superlattice face with the use of a Raman scattering accessory. Lateral anisotropy of mixed phonon-plasmon modes associated with structural anisotropy of the superlattice grown on the faceted (311)A surface has been experimentally revealed for the first time.     

Delocalization of phonon-plasmon modes in GaAs/AlAs superlattices with tunnel-thin AlAs barriers

The technique of Raman spectroscopy has been used to investigate doped (n-type) and undoped GaAs/AlAs superlattices with AlAs barrier thicknesses from 17 to 1 monolayers. The peak corresponding to the scattering by a two-dimensional plasmon was found in the Raman spectrum of a doped superlattice with relatively thick barriers. The position of the experimental peak corresponded to the value calculated in the model of plasma oscillations in periodic planes of a two-dimensional electron gas. The electron tunneling effects played an increasingly prominent role as the AlAs barrier thickness decreased. The peaks corresponding to the scattering by coupled phonons with three-dimensional plasmons were found in the Raman spectra for a superlattice with an AlAs thickness of 2 monolayers; i.e., the delocalization of coupled modes was observed. In this case, the folding of acoustic phonons was observed in the superlattice under consideration, indicative of its good periodicity, while the localization of optical phonons in GaAs layers was observed in undoped superlattices with an AlAs thickness of 2 monolayers.     

Experimental discovery of the anisotropy of phonon-plasmon modes in GaAs/AlAs(100) superlattices

Doped (n-type) GaAs/AlAs superlattices with thicknesses of the GaAs and AlAs layers from 1.7 to 6.8 Å and 13.6 Å, respectively, have been studied by means of Raman spectroscopy. The use of a microattachment for Raman backscattering studies has allowed for the observation of the modes with the wave vector directed both across and along the superlattice layers (in the scattering from the side face of the superlattice). The theoretically predicted anisotropy of mixed phonon-plasmon modes caused by the anisotropy of the electron effective mass in the type II superlattices has been experimentally discovered.     

Damping of high-lying single-particle modes in heavy nuclei

The recent experimental and theoretical results on the damping of high-lying single-particle modes in heavy nuclei are reviewed. In one-nucleon transfer reactions these states manifest themselves as broad “resonance”-like structures superimposed on a large continuum. The advantages and the limitations of the transfer reaction approach will be presented using the results from neutron and proton pick-up and stripping reactions. The problem raised by the subtraction of the underlying background, the assumptions made to describe the reaction process and the method used to extract the strength distributions are presented. The existing empirical systematics is summarized for nuclei ranging from ⁹⁰Zr to ²⁰⁸Pb.The theoretical approaches used to explain the damping of the high-lying single-particle modes are based on the coupling between collective and single-particle degrees of freedom. In a first step the bare single-particle mode is spread over several doorway collective states due to the interaction with surface vibrations. In a second step the doorway states spread their strengths over many other degrees of freedom. These two steps of the damping mechanism are discussed in detail within the framework of the quasiparticle-phonon nuclear model. A large-scale comparison between the measured and calculated average energies, spreading widths and spectroscopic strengths of the high-lying single-particle (hole) states in heavy nuclei is presented. The systematic features of the damping (energy, angular momentum and isotopic dependence) are discussed. Recent advances of the experimental approaches, such as the γ-decay of the high-lying states or the use of heavy-ion transfer reactions at intermediate energies, are outlined.The detailed study of the damping mechanism of high-lying single-particle modes reveals new features and leads us to a new field in nuclear structure: “the spectroscopy of inner and outer subshells”.     

Phonon-plasmon coupled modes in hetero-superlattices

The spectrum of coupled phonon-plasmon modes is considered in a mesoscopic system of thin conducting planes separated by insulating layers. The reflectance of such a sample in the infrared region is calculated. Reflectance minima are determined by the longitudinal and transverse phonon frequencies in the insulating interlayers and by the van Hove singularities of the coupled modes. Measuring the differential Raman cross section allows the spectrum of these modes to be found directly.     

Entanglement preservation of two coupled modes

A scheme to control the evolution of any initial state in subspace {|1〉⊗|0〉,|0〉⊗|1〉} is presented. The physical system considered is the one of two coupled modes sharing one excitation. The scheme is based on unitary interactions with an auxiliary subsystem, and it can be used to preserve the initial entanglement of the system. A proposal for implementation of the scheme in the context of microwave cavity is presented.     

Damping of internal soliton modes in media with quadratic nonlinearity

Optical solitons in media with quadratic nonlinearity and frequency dispersion are theoretically analyzed. Internal soliton modes and the rate of their radiative damping as a function of detuning from the phase-matching condition and the nonlinear shift of phase velocity for different soliton dimensions (d=1, 2, 3) are found. The length of a nonlinear medium required to form a stationary soliton is determined.     

Spin-dependent coexistence of weakly coupled and strongly coupled modes in semiconductor microcavities

The dependence of the transition from the strong-coupling regime to the weak-coupling regime on the polariton spin orientation in a InGaAs semiconductor microcavity is experimental studied by means of time-resolved photoluminescence. Polaritons are created by non-resonant circularly-polarized optical excitation and the power intensity that breaks the strong coupling is found to be much lower for co-polarized polaritons than that for cross-polarized polaritons. Coulomb screening effects alone cannot explain the stronger loss of oscillator strength for majority excitons (co-polarized) and spin-dependent mechanisms are required to justify such behaviour.     

Superconducting cosmic strings with coupled zero modes

The physics of fermion zero modes on bosonic superconducting strings is investigated. The charged field at the core couples left movers to right movers, giving an effective theory of massive fermions on the string world sheet. Despite their mass, the fermions still contribute to the bosonic current through the two-dimensional anomaly. Even when the Higgs field is neutral and there is no anomaly, the string remains superconducting provided the sum of the charges of the zero modes do not vanish. The mechanism is closely analogous to a theory of superconductivity proposed by Fröhlich.     

Damping of collective modes in DIC: Quantal versus statistical fluctuations

A model is developed to describe the transformation of relative kinetic energy into intrinsic excitation energy in DIC. Energy dissipation is viewed as an indirect process, in which collective vibrational modes are first excited coherently and then damped due to the coupling to the remaining non-collective degrees of freedom. Both collective and intrinsic degrees of freedom are included explicitly, and the coupling between them is treated in a random-matrix model. Under certain assumptions it is shown that, in the weak-coupling limit, the collective probability distribution in phase space obeys a Fokker-Planek equation. This transport equation is used to derive equations of motion for the expectation values of some “macroscopic” quantities characterizing the process. Some numerical results are presented and a qualitative comparison with the Copenhagen model is attached.     

Magnetostatic modes in coupled antiferromagnet/ferromagnet bilayers

We study the influence of interface effects on the magnetostatic modes propagating in a coupled ferromagnetic/antiferromagnetic bilayer. We assume that the magnetic layers are thick enough to be described by the bulk parameters and they are coupled through the interaction between the magnetic moments located at the interface. We use a phenomenological approach taking into account the presence of different magnetic layers in the system to calculate the modified dynamical response of each material. We use the corrected magnetic permeability of the layers to obtain a correlation between the interface characteristics and the physical behavior of the magnetic excitations propagating in the system.     

Tunable coupled plasmon modes via nanoshell particle chains

Coupled plasmon modes have been studied theoretically in periodic chains of nanoshell particles embedded in a graded dielectric host. These chains not only sustain a variety of localized modes as unshelled nanoparticle chains, but also offer precise control of the localization-delocalization transition among these modes by varying the permittivity contrast and/or core-shell radius ratio. By optimizing these parameters, the upper band can be tuned into higher frequencies while the lower band can be tuned into the optical communication frequencies for practical application. We also discuss the Ohmic loss effects in the metallic component of the nanoshells.     

Infrared reflectivity and coupled modes in Gd-doped EuO

The reflectivity of EuO single crystals, doped with various amounts of Gd, has been measured between 250μ and 12 eV photon energy. A Kramers-Kronig analysis of the data reveals LO and TO phonon modes, plasmons and LO phonon-plasmon coupled modes.