Role Of Collective Excitations Research Articles

Excitons in Carbonic Nanostructures

Unexpectedly bright photoluminescence emission can be observed in materials incorporating inorganic carbon when their size is reduced from macro–micro to nano. At present, there is no consensus in its understanding, and many suggested explanations are not consistent with the broad range of experimental data. In this Review, I discuss the possible role of collective excitations (excitons) generated by resonance electronic interactions among the chromophore elements within these nanoparticles. The Förster-type resonance energy transfer (FRET) mechanism of energy migration within nanoparticles operates when the composing fluorophores are the localized electronic systems interacting at a distance. Meanwhile, the resonance interactions among closely located fluorophores may lead to delocalization of the excited states over many molecules resulting in Frenkel excitons. The H-aggregate-type quantum coherence originating from strong coupling among the transition dipoles of adjacent chromophores in a co-facial stacking arrangement and exciton transport to emissive traps are the basis of the presented model. It can explain most of the hitherto known experimental observations and must stimulate the progress towards their versatile applications.

Dynamics of electron impurity doped quantum dots in the presence of time-varying fields: Influence of impurity location

We explore the pattern of time evolution of eigenstates of a repulsive impurity doped quantum dot. The quantum dot is two dimensional and contains one electron which is harmonically confined. We have considered Gaussian impurity centers. A static transverse magnetic field is also present. For given strength of confining fields, intensity and oscillation frequency of external time-varying field, the system reveals a long time dynamics that is undulatory in nature. The location of impurity at on and off-center positions prominently affects the eigenstates of the unperturbed system and consequently the nature of the dynamics. Separations between various energy levels and corresponding transition moment values as a function of impurity position are invoked to explain the dynamical features. The Fourier transform of time evolution profiles indicates the role of collective excitations towards population build up in some particular states.

Instabilities of vortices in a binary mixture of trapped Bose-Einstein condensates: Role of collective excitations with positive and negative energies

Correspondence between frequency and energy spectra and biorthogonality conditions for the excitations of Bose-Einstein condensates described by Gross-Pitaevskii model have been derived selfconsistently revealing several novel aspects originating in nonselfadjoitness of the Bogoliubov operator. It has been demonstrated that frequency resonances of the excitations with positive and negative energies can lead to their mutual annihilation and appearance of the collective modes with complex frequencies and zero energies. Conditions for the avoided crossing of energy levels have also been discussed. General theory has been verified both numerically and analytically in the weak interaction limit considering an example of vortices in a binary mixture of condensates. Growth of excitations with complex frequencies leads to spiraling of the unit and double vortices out of the condensate center to its periphery and to splitting of the double and higher order vortices to the unit ones.

Molecular Dynamics Simulation of Transport in Diamond and GaN: Role of Collective Excitations

ABSTRACTIn this paper we consider the role of collective excitations in transport of charge carriers in the conduction band of diamond and GaN. The present molecular dynamics simulation uses a Monte Carlo algorithm to determine the time between collisions with scattering rates calculated quantum mechanically using the Fermi Golden Rule. For very thin films (L=0.01 μm) and for low fields (F<10 V/μm), the energy spectra of both diamond and GaN contain a series of peaks which are attributed to the absorption and emission of discrete plasmons. In diamond. the intervalley LA, LO and TO scattering becomes increasingly more important at higher fields while electron-plasmon interactions decrease. For thicker diamond films (L=0.1 μm), there is hot electron transport for F∼10 V/μm and quasi-ballistic transport for F-100 V/μm. In GaN with L=0.01 μm and fields F∼10 V/μm, there is a discrete series of peaks in the energy spectrum corresponding to excitations associated with polar optical scattering.