Charge transfer excitons: dynamic theory of vibronic spectra

The vibronic spectra of charge transfer excitons (CTE) in a molecular one-component or alternatingly ordered two-component chain are treated in the framework of a dynamic approach (neglecting thermal excitations of the intramolecular vibrations). The model introduces two mechanisms of coupling between CTEs and vibrational quanta: (1) shift of the equilibrium positions of the nuclei in the ionized donor or acceptor; (2) change of the vibrational frequency in the ionized molecule. This model allows to generalize the simple CTE Hamiltonian and the vibronic Hamiltonian of Frenkel excitons. The linear optical susceptibility is calculated in the vibronic region (one CTE and one vibrational quantum). The double splitting of vibronics of CTEs was analyzed: (1) the splitting connected with the location of the intramolecular vibration on the donors or on the acceptors; (2) the splitting connected with the symmetry of the vibronic spectra (in the degenerate case). The general structure of the vibronic spectra of CTEs is established. It contains structureless absorption lines, which correspond to two-particle bands (the phonon is excited on a neutral molecule neighboring the donor or the acceptor) and Lorentz-type lines of one-particle states, which correspond to the bound propagation of the CTE and the phonon.     

New polaronic excitons in ferroelectric oxides: phenomenological theory

It is shown that three types of new polaronic excitons exist in ferroelectric oxides-charge transfer vibronic excitons (CTVEs). The electron-hole bipolaron pairs are common in all these cases, the difference having to do with the origin of charge transfer-lattice instabilities. These are the cases of the charge transfer and lattice distortions and their vibronic interactions induced in the harmonic (i) and anharmonic (ii) regimes both in the electronic excited state on the one hand, and the (iii) case of the CTVE excited vibronic state induced in the anharmonic regime but related with electronic ground state on the other. All these CTVEs are characterized by strong coupling with good self-localization conditions in separate orientation wells of their multi-well potentials. The relevant phenomenological theory is developed. While the (i) type CTVE is confirmed by luminescence experiments, the (iii) type CTVE with low excitation energy and with metastable intrinsic defect behaviour can be a candidate for the explanation of an order-disorder effect in the classical ferroelectric oxides discovered recently [Phys. Rev. Lett. 90 (2003) 037601].     

Energy transfer by singlet and triplet excitons in carbazole-containing polymers

Quantum yields of pyrene fluorescence and bis[2-(2′-benzothienyl)-pyridinato-N,C^3′](acetylacetonate)iridium [Btp₂Ir(acac)] phosphorescence upon excitation via a matrix of poly-N-epoxypropylcarbazole (PEPC) or poly-N-epoxypropyl-3,6-dibromocarbazole (DBrPEPC), respectively, were found to be lower than those for the compounds directly excited in a polystyrene (PS) matrix. It was established that the energy in PEPC was transferred to an acceptor by both singlet excitons (by migration and long-range dipole–dipole interaction) and triplet excitons (through migration and short-range exchange electron interaction); however, only by triplet excitons in DBrPEPC, which did not show any fluorescence. The energy-transfer efficiency in PEPC by singlet excitons was higher than by triplet excitons. The observed effects were explained by the fact that energy transfer to the acceptor competed with such processes as localization of the excitons in the “tail” energy states, dissociation of singlet excitons into geminal electron–hole pairs (EHP), and capture of triplet excitons by polymer oxidation products.     

Energy of K-momentum dark excitons in carbon nanotubes by optical spectroscopy

Phonon sideband optical spectroscopy determines the energy of the dark K-momentum exciton for (6,5) carbon nanotubes. One-phonon sidebands appear in absorption and emission, split by two zone-boundary (K-point) phonons. Their average energy locates the E11 K-momentum exciton 36 meV above the E11 bright level, higher than available theoretical estimates. A model for exciton-phonon coupling shows the absorbance sideband depends sensitively on the K-momentum exciton effective mass and has minimal contributions from zone-center phonons, which dominate the Raman spectra of carbon nanotubes.     

Toward a theory of light absorption by excitons in quantum-well structures

A theory of light absorption by excitons in periodic structures with an arbitrary number of quantum wells is constructed. It is shown that the temperature dependence of the frequency-integrated absorption characteristics is due to a competition between the processes of dissipative decay of quasi-two-dimensional excitons and the light-exciton interaction. Fiz. Tverd. Tela (St. Petersburg) 40, 824–826 (May 1998)     

On an application of the causality principle to the theory of ion transport processes

We suggest an approach for ion transport processes that is motivated by analogies with the S-matrix theory. By using causality-based analyticity assumptions, conductivity frequency pre-factors are expressed in terms of the transfer amplitude. This provides a means of dealing with a unified scheme that includes ionic oscillations in wells and classical-like escaping. As an application, we use this technique to reproduce results obtained in terms of the Kramers problem without invoking the Kramers’ Fokker-Planck equation or details of the transfer mechanism in solid electrolytes.     

Energy relaxation and transport of indirect excitons in AlAs/GaAs coupled quantum wells in magnetic field

The evolution of indirect exciton luminescence in AlAs/GaAs coupled quantum wells after excitation by pulsed laser radiation has been studied in strong magnetic fields (B⩽12 T) at low temperatures (T⩾1.3 K), both in the normal regime and under conditions of anomalously fast exciton transport, which is an indication of the onset of exciton superfluidity. The energy relaxation rate of indirect excitons measured in the range of relaxation times between several and several hundreds of nanoseconds is found to be controlled by the properties of the exciton transport, specifically, this parameter increases with the coefficient of excitonic diffusion. This behavior is qualitatively explained in terms of migration of excitons between local minima of the random potential in the plane of the quantum well. Zh. éksp. Teor. Fiz. 114, 1115–1120 (September 1998)     

Magnetic excitons in near-surface quantum wells: Experiment and theory

We have measured the photoluminescence spectra and photoluminescence excitation spectra of magnetic excitons in InGaAs/GaAs near-surface quantum wells in a magnetic field. We have quantitatively investigated the effect of dielectric enhancement of excitons in quantum wells brought about by decreasing the thickness of the barrier layer, both in a magnetic field and without. Fiz. Tverd. Tela (St. Petersburg) 40, 806–808 (May 1998)     

Anisotropy and interference in wave transport: an analytic theory

A theory is presented which incorporates the effect of dielectric anisotropy in random multiple scattering media. It predicts anisotropic diffusion, and a deflection of the diffuse energy flow in anisotropic slabs in the direction parallel to the slab. The transmittance integrated over all incoming and outgoing directions scales with the transport mean free path along the surface normal. The escape function in anisotropic dielectrics is no longer bell shaped. In this model anisotropy facilitates Anderson localization.     

Bound excitons in time-dependent density-functional theory: optical and energy-loss spectra

A robust and efficient frequency dependent and nonlocal exchange correlation f(xc)(r,r(');omega) is derived by imposing time-dependent density-functional theory (TDDFT) to reproduce the many-body diagrammatic expansion of the Bethe-Salpeter polarization function. As an illustration, we compute the optical spectra of LiF, SiO2, and diamond and the finite momentum transfer energy-loss spectrum of LiF. The TDDFT results reproduce extremely well the excitonic effects embodied in the Bethe-Salpeter approach, both for strongly bound and resonant excitons. We provide a working expression for f(xc) that is fast to evaluate and easy to implement.     

Scattering of excitons by excitons in semiconducting quantum well structures

We have theoretically investigated the scattering of excitons by excitons in a two-dimensional semiconducting quantum well system. The scattering cross sections have been calculated using the Born approximation for both the elastic and inelastic scattering of the excitons by excitons. The threshold for inelastic scattering is increased over the value in a bulk semiconductor because of the enhancement of the exciton binding energy by its confinement. The behavior of the scattering cross section as a function of the energy of relative motion of the excitons is different than in the bulk and the cross section is a more sensitive function of the ratio of the electron and hole masses than in the bulk.     

Energy transport induced by an external alternating field in strongly disordered media.

The delocalization of excitations in an ensemble of two-level systems with a strong disorder due to an external alternating acoustic or electric field is considered. The propagating modes are shown to appear if a ratio of the field amplitude to the frequency is large enough. Two complementary approaches, the static one similar to that of Anderson and the dynamic one related to Landau-Zener, are developed. It is shown that the field-induced relaxation mechanism can have a strong influence on the kinetics. The internal friction is argued to be vastly affected by the relaxation mechanism proposed.     

Variational tight-binding theory of excitons in compositionally modified semiconductor superlattices

We present results for the binding energy of an exciton formed when an electron–hole pair is photoexcited within a single, compositionally modified layer of a semiconductor superlattice, for example by adding a small percentage of In atoms to a single GaAs layer of a GaAs/AlGaAs system. Such a system could serve as the basis for spatially-selective photoexcitation, a process whereby a laser pulse would create electron–heavy-hole pairs exclusively in the modified layer. We first derive an effective one-dimensional (1D) Hamiltonian for an electron, by averaging the 3D electron–hole Hamiltonian using a one-parameter trial wavefunction, which is dependent on the in-plane relative coordinates, as well as a normalized Wannier orbital for a single hole. The exciton binding energy is then obtained by computing the lowest bound-state energy of the effective 1D electron Hamiltonian in the nearest-neighbor tight-binding approximation. As a demonstration of the effectiveness of our approach, we find that for periodic superlattices our results for the exciton binding energy are in very good agreement both with experiment and the results of other theoretical calculations.     

Landauer-type transport theory for interacting quantum wires: application to carbon nanotube y junctions

We propose a Landauerlike theory for nonlinear transport in networks of one-dimensional interacting quantum wires (Luttinger liquids). A concrete example of current experimental focus is given by carbon nanotube Y junctions. Our theory has three basic ingredients that allow one to explicitly solve this transport problem: (i) radiative boundary conditions to describe the coupling to external leads, (ii) the Kirchhoff node rule describing charge conservation, and (iii) density matching conditions at every node.     

Energy transfer due to excitons and holes under host-sensitization in KBr:I

Energy transfer due to excitons and holes to iodine ions in KBr:I crystals has been investigated. It is confirmed that, when the exciting photon energy is increased beyond the Γ-band edges of KBr, probability of the energy transfer due to exciton diffusion rapidly decreases with the complementary enhancement of that due to hole migration followed by electron capture.     

半导体量子阱中激子波函数及其 Fourier系数的计算和应用

利用准玻色子方法发展的激子动力学方程是研究半导体纳米结构中激子超快动力学的有效理论手段. 为了将这种方法应用于半导体量子阱, 需要知道量子阱中的激子波函数及其在动量空间的表示, 从而得到激子动力学方程中所必须的系数. 详细讨论了理想和实际量子阱中的激子波函数, 特别是其在动量空间的表示, 并进一步讨论了激子动力学方程中所必须系数的计算方法. 通过求解这些系数, 对量子阱中因激子密度变化而引起的太赫兹脉冲作用下激子能级间跃迁过程中的非线性效应进行了理论预测, 得到了与实验符合很好的结果.