Effect of disorder on the trapping of Frenkel excitons in three-dimensional systems

A numerical study is made of the effect of disorder on the trapping of Frenkel excitons in three-dimensional systems atT=0 K. A Gaussian distribution of optical transition frequencies is assumed. The disorder enhances the decay of ak = 0 exciton created by pulsed optical excitation, but reduces the overall exciton trapping rate. An interpretation of the results in terms of increased exciton scattering and reduced exciton mobility is outlined.     

Evolution of Frenkel pairs from excited states of self-trapped excitons in alkali halides

We find that the transition to the first and second electron excited states from the lowest self-trapped excitons in KCl and RbCl are π- and σ-polarized, respectively. For KCl the orientation of the H centers evolved from the self-trapped exciton by exciting to these states are shown to be in parallel to the orientation of the self-trapped excitons.     

Diffusion equation for the quasi-incoherent motion of Frenkel excitons in molecular crystals

In a recent paper a master equation describing the quasi-incoherent motion of Frenkel excitons in molecular crystals has been derived within the Haken-Strobl model for the coupled coherent and incoherent motion of Frenkel excitons. Starting from this master equation and using only the translational symmetry of the crystal lattice, for crystals with one molecule in the unit cell a diffusion equation is derived. For crystals with several molecules in the unit cell instead of a diffusion equation one obtains a set of diffusion-like equations. These equations are solved explicitly for the case of two molecules in the unit cell and asymptotic expressions are discussed. It is shown that this asymptotic behaviour is again described by a diffusion equation.     

Luminescence kinetics of dipolar excitons in circular traps

A detailed investigation of multiband superconductivity and Leggett’s mode in the Mg_1?x Al_xB₂ (0 ≤ x ≤ 0.45) system was carried out using tunneling and Andreev spectroscopy. Temperature dependences of superconducting gaps Δ_σ and Δ_π and their variation upon the degree of disorder and the Al concentration were studied. The dependence of the Leggett’s mode energy ε₀ upon the values of the gaps Δ_σ and Δ_π has been derived.     

The role of Frenkel excitons in low-energy-electron diffraction theory for molecular crystals

In this paper a low-energy-electron diffraction theory for molecular crystals is established. The possibility of creating Frenkel excitons by the incident low-energy electrons is taken into account. The whole formalism is based on a set of diagram rules, which facilitates the evaluation of the differential scattering cross section. The influence of the bulk and surface excitons on the LEED intensities will be discussed in a series of subsequent papers.     

Exact asymptotics for one-dimensional diffusion with mobile traps

We consider a diffusing particle, with diffusion constant D', moving in one dimension in an infinite sea of noninteracting mobile traps with diffusion constant D and density rho. We show that the asymptotic behavior of the survival probability, P(t), satisfies lim([-ln(P(t)]/sqrt[rho2Dt]=4/sqrt[pi], independent of D'. The result comes from obtaining upper and lower bounds on P(t), and showing that they coincide asymptotically. We also obtain exact results for P(t) to first order in D(')/D for an arbitrary finite number of traps.     

Localized self-trapping in the two-dimensional discrete molecular lattice with the interaction between Frenkel excitons and phonons

We investigate the interactions of lattice phonons with Frenkel exciton, which has a small radius in a two-dimensional discrete molecular lattice, by the virtue of the quasi-discreteness approximation and the method of multiple-scale, and obtain that the self-trapping can also appear in the two-dimensional discrete molecular lattice with harmonic and nonlinear potential. The excitons' effect on the molecular lattice does not distort it but only causes it to localize which enables it to react again through phonon coupling to trap the energy and prevent its dispersion.     

Converting Wannier into Frenkel excitons in an inorganic/organic hybrid semiconductor nanostructure

Electronic coupling between Wannier and Frenkel excitons in an inorganic/organic semiconductor hybrid structure is experimentally observed. Time-resolved photoluminescence and excitation spectroscopy directly demonstrate that electronic excitation energy can be transferred with an efficiency of up to 50% from an inorganic ZnO quantum well to an organic [2,2-p-phenylenebis-(5-phenyloxazol), alpha-sexithiophene] overlayer. The coupling is mediated via dipole-dipole-interaction analog to the F?rster transfer in donor-acceptor systems.     

Lasing threshold in traps for Bose-condensation of dipolar excitons

We consider exciton recombination lasing in heterostructure traps for Bose–Einstein condensation of dipolar excitons. We show that such structures suit well for class D lasers where cavity decay strongly exceeds polarization decay. We evaluate lasing threshold taking into account specific inhomogeneous broadening of the exciton spectral line owing to Bose–Einstein condensation phenomenon under quasi-equilibrium conditions.It is found that narrowing of the exciton momentum distribution just before the condensation onset considerably lowers lasing threshold. At the same time, it is pointed out that a subsequent formation of condensate itself does not help lasing much. We conclude that it is possible to achieve lasing on polariton modes in nowadays experiments aimed on Bose–Einstein condensation of excitons.     

Observation of excitons in one-dimensional metallic single-walled carbon nanotubes

Excitons are generally believed not to exist in metals because of strong screening by free carriers. Here we demonstrate that excitonic states can in fact be produced in metallic systems of a one-dimensional character. Using metallic single-walled carbon nanotubes as a model system, we show both experimentally and theoretically that electron-hole pairs form tightly bound excitons. The exciton binding energy of 50 meV, deduced from optical absorption spectra of individual metallic nanotubes, significantly exceeds that of excitons in most bulk semiconductors and agrees well with ab initio theoretical predictions.     

Raman scattering from one-dimensional carbon systems

Although the Raman effect was discovered nearly 80 years ago, it is only recently that the special characteristics of Raman scattering for one-dimensional systems have been seriously considered. This review focuses on the special interest of the Raman effect for one-dimensional systems that is of particular relevance to carbon nanostructures. Two examples of Raman scattering in one-dimensional systems are given. The first illustrates the use of Raman spectroscopy to reveal the remarkable structure and properties of carbon nanotubes arising from their one-dimensionality. Some of the recent advances in using Raman spectroscopy to study doping and intercalation to modify nanotube properties are reviewed, in the context of a one-dimensional system. The second example is the Raman spectra of a linear chain of carbon atoms and the special properties of this interesting system. New approaches toward applying Raman spectroscopy to carbon nanostructures are also emphasized.     

Diffusion with random traps: Transient one-dimensional kinetics in a linear potential

The problem of one-dimensional diffusion with random traps is solved without and with a constant field of force. Using an eigenvalue expansion for long times and the method of images for short times we give an exact, straightforward solution for the time dependence of the mean survival probability and the mean probability density for returning to the origin. Using the backward equation approach, we determine the mean survival time and the mean residence time density at the origin. We comment on the relation between these solutions and those for one-dimensional diffusion with random reflectors.