Shape dependent electronic structure and exciton dynamics in small In(Ga)As quantum dots

We present a study of the primary optical transitions and recombination dynamics in InGaAs self-assembled quantum nanostructures with different shape. Starting from the same quantum dot seeding layer, and depending on the overgrowth conditions, these new nanostructures can be tailored in shape and are characterized by heights lower than 2 nm and base lengths around 100 nm. The geometrical shape strongly influences the electronic and optical properties of these nanostructuctures. We measure for them ground state optical transitions in the range 1.25–1.35 eV and varying energy splitting between their excited states. The temperature dependence of the exciton recombination dynamics is reported focusing on the intermediate temperature regime (before thermal escape begins to be important). In this range, an important increase of the effective photoluminescence decay time is observed and attributed to the state filling and exciton thermalization between excited and ground states. A rate equation model is also developed reproducing quite well the observed exciton dynamics.     

Superconducting phase transitions in quantum dots

In NdN and SdS nanostructures, anomalous ground states with a non-integer average number of electrons in the quantum dots d occur. These states correspond to pair or single-electron superconductivity and are separated from states with a definite (integer) number of electrons by, as a rule, second-order phase transitions. The characteristic features of the pair and single-electron superconducting Josephson current in SdS are discussed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 9, 618–623 (10 November 1996)     

Mobility edges and reentrant localization in one-dimensional dimerized non-Hermitian quasiperiodic lattice

The mobility edges and reentrant localization transitions are studied in one-dimensional dimerized lattice with nonHermitian either uniform or staggered quasiperiodic potentials.We find that the non-Hermitian uniform quasiperiodic disorder can induce an intermediate phase where the extended states coexist with the localized ones,which implies that the system has mobility edges.The localization transition is accompanied by the PT symmetry breaking transition.While if the non-Hermitian quasiperiodic disorder is staggered,we demonstrate the existence of multiple intermediate phases and multiple reentrant localization transitions based on the finite size scaling analysis.Interestingly,some already localized states will become extended states and can also be localized again for certain non-Hermitian parameters.The reentrant localization transitions are associated with the intermediate phases hosting mobility edges.Besides,we also find that the non-Hermiticity can break the reentrant localization transition where only one intermediate phase survives.More detailed information about the mobility edges and reentrant localization transitions are presented by analyzing the eigenenergy spectrum,inverse participation ratio,and normalized participation ratio.     

Controlling double vortex states in low-dimensional dipolar systems

The reversal process of the chirality of each opposite vortex belonging to a double vortex state in ferromagnetic hysterons, via the application of in-plane magnetic fields, is reported. Simulations reveal that such a process involves the formation of four intermediate states, including original ones. Hysteresis loops can occur only in a counterclockwise fashion because of one of these intermediate states. Double vortex states can also be controlled by electric fields in ferroelectric nanostructures of different shapes, but with some key differences with respect to the ferromagnetic case.     

Soft X-ray transition matrix elements: The role of approximation of the valence states

The coordinate, momentum and potential forms of the interband transitions matrix elements are equivalent when the initial and the final states are exactly known. For the soft X-ray transitions we assume exact core states, and approximate valence states. Then the three different forms of the matrix elements are no longer equivalent and are related by new, more general expressions. From these the qualitative conclusions regarding the appropriate form of the approximated valence states are derived. It turns out, that the orthogonalization of the approximate valence wave function to the core states is very important for the correct behaviour of the photon energy dependence of the soft X-ray emission. These results are illustrated by simple numerical examples.     

Nonlinear absorption of femtosecond light pulses under conditions of multiphoton resonances in solids

A theory of nonlinear absorption of femtosecond light pulses by bulk crystals and nanostructures of differing dimensionality is developed. The pulse width is assumed to be small compared to the relaxation times of the electron and hole momenta. Expressions for the absorbed energy under conditions of multiphoton resonance are derived for transitions between discrete or band-related electron states and between sub-band states of size quantization in quantum wells and quantum wires. The dependence of the absorbed energy on the multiphoton resonance detuning and pulse width is analyzed.     

Effect of collisional transitions on the population distributions of excited atoms in a plasma

The effect of collisional transitions on the population distribution of excited atoms has been studied by considering a plasma in which the electron density varies with time. It has been found that for highly excited states, for which the ionization rate is much larger than that of electron-density variations, the collisional transitions are not important compared with the collisional ionization and recombination processes. However, for intermediate and lower excited states, they play an important role in making the population distribution approach Saha equilibrium with the electrons.     

Laser control of low-temperature reaction <Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> + O<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack>

A theory is developed to describe dissociative recombination of a slow electron with a ground-state molecular ion of oxygen driven by a strong monochromatic electromagnetic field. Mathematically, the theory is based on the time-independent formalism of radiative scattering matrix whose poles correspond to dressed intermediate states of the complex. The analysis embraces both transitions to dissociative states and laserinduced nonadiabatic transitions to intermediate bound states of valence configurations. Key reaction mechanisms are considered, and a classification is given of all allowed transitions to dissociative states. Partial and total reaction cross sections are calculated by taking into account the contributions from Rydberg, valence, and dissociative states of O₂^**. A detailed discussion of results is presented, and feasibility of laser control of the reaction is demonstrated.     

Localized bifurcations and defect instabilities in the convection of a nematic liquid crystal

The stationary and the time-dependent homogeneous ordered states in convection may both become unstable against localized perturbations. Defects are then created and they may contribute to the disorganization of the homogeneous state. We present an experimental study of defects in some homogeneous stationary structures as well as in the traveling-wave states of convection of a nematic liquid crystal. We show that the core of the defects is a germ of the unstable state and it can become unstable under the external stress. Then, either fully homogeneous states with the symmetry of the core, or complex disordered states can develop from the local instability of defects in processes quite similar to displacive transitions in solids. Some of the main features are qualitatively similar to numerical simulations of an appropriate Landau-Ginzburg equation.     

Saturation of optical transitions in quantum dots and wires of porous silicon

The bleaching bands have been observed in the time-resolved nonlinear transmission spectra of porous silicon. The increase of transmission at discrete frequencies has been attributed to a saturation of optical transitions between the energy levels of electrons and holes spatially confined within quasi-zero-dimensional (quantum dots) and quasi-one-dimensional (quantum wires) nanostructures. The results of independent measurements using transmission electron microscopy have confirmed the existence of quantum dots and wires of corresponding size. The slowed-down energy relaxation from upper to lower levels of size quantization compared with intraband relaxation in the bulk have been observed in the cooled (80K) platelets of porous silicon.     

An exact limit of a local mixed valence model

Recently, Khomskii and Kocharjan have considered a local mixed valence model and, in a generalized linear Hartree-Fock approximation, found solutions which, when generalized to the many site problem, displayed continuous and discontinuous transitions from ground states of integral to intermediate valence. The transitions were due to an energy dependence of the virtual bound state width and persisted in the limit of zero hybridization. We examine the model of Khomskii and Kocharjan and demonstrate that in the limit of zero hybridization an exact solution may be found for the valence behaviour. The generalization of this result to the case of a small but finite concentration of localized level sites, exhibits intermediate valence only as a consequence of the pinning of the Fermi level to the narrow localized levels and the transitions between the ground states of integral and intermediate valence are continuous.     

Retardation of the relaxation over quantum-well energy levels in CdSe/ZnS quantum dots with increasing number of excited carriers

The differential transmission spectra of CdSe/ZnS quantum dots are investigated. It is revealed that the differential transmission spectra measured upon resonant excitation of electrons into the first excited state 1P(e) exhibit a number of specific features, such as a decrease in transmission at the pump frequency, bleaching in the course of the pump pulse at frequencies corresponding to the fundamental optical transition 1S _3/2(h)-1S(e) and transitions between excited hole states and the 1S(e) electron ground state, and retardation of this process with an increase in the energy of the pump pulse. The observed specific features can be explained by the following factors: (i) the absence of a “phonon bottleneck” for electrons due to the energy transfer from hot electrons to rapidly relaxing holes, (ii) relaxation through intermediate quantum-well energy levels of holes, and (iii) retardation of relaxation with increasing number of excited charge carriers in a quantum dot.     

Nonlinear absorption of light pulses under conditions of two-photon resonance in bulk crystals and nanostructures in the femtosecond pump-probe spectroscopy mode

A theory of nonlinear absorption of femtosecond light pulses by bulk crystals and nanostructures of different dimensions in the pump-probe spectroscopy mode has been developed. Expressions for the energy absorbed from the probe pulse under conditions of two-photon resonances are derived for transitions between the discrete or band electronic states and between the size-quantization subband states in quantum wells and quantum wires. The dependences of the energy absorbed from the probe pulse on the two-photon resonance detuning and on the time delay between the pulses are analyzed. The dependence of the power absorbed from the probe pulse on the time is also obtained.     

碱土金属ns(n-1)d1D-ns21S电四极跃迁的能级和振子强度

利用全相对论性多组态Dirac-Fock方法系统地计算了碱土金属激发态1D-1S电四极光谱跃迁的能级间隔,跃迁几率和振子强度,计算中考虑了重要的核的有限体积效应,Breit修正和QED修正,所得结果和最近的实验数据及理论计算值进行了比较,并探讨了采用电四极矩作为中间共振,获得碱土金属元素的高里德伯态,特别是自电离态的可能性.     

Editorial

This review contains up-to-date information about experimental and theoretical investigations of phase transitions and specific features of properties of mixed crystals involving the incipient ferroelectric KTaO₃ with the off-center Li⁺, Nb⁵⁺, Na⁺ impurities. Main attention is paid to the results of the study of the mixed systems by radiospectroscopy methods (ESR and NMR).

Theoretical criteria and experimental evidence of ferroelectric and ferroelastic phase transitions as well as of dipole and quadrupolar glass states induced by the off-center impurities are discussed. It is shown that at high enough concentrations of the off-center impurities phases with long-range order appear, but at low concentrations dipole glass states occur. At intermediate concentrations of the off-center impurities new phases appear in which the coexistence of long-range properties and those of dipole glasses is possible. These phases could be named ferroelectric or ferroelastic glasses to stress their similarity with the ferroglass phases which were discovered earlier in mixed magnetic systems. The usefulness of radiospectroscopy methods for the study of phase transitions induced by off-center impurities in mixed systems, which can be considered as model disordered systems, is underlined.

In conclusion, some questions on the physics of phase transitions in disordered systems which should be solved in the future are discussed.     

Influence of solvent and intramolecular relaxation on dynamics of luminescence spectra in donor–acceptor complexes

The oscillator strengths for Rydberg states of a NaHe molecule are calculated using a semianalytic procedure with the l-coupling effect taken into account (due to the dipole potential of a core). This effect gives rise to nonzero oscillator strengths for transitions forbidden in the atomic model of molecular Rydberg states. The difference between calculations in terms of the atomic model and calculations with consideration of the dipole moment of a core is shown for allowed transitions.     

Two-state double-continuum Fano resonance at the Si(100) surface

Fano resonances are well-known manifestations of the interference between a direct and an indirect ionization process. Here we report on a more complicated interference pattern observed in two-photon photoemission at the Si(100) surface. This two-dimensional Fano profile involves two discrete surface resonances which couple as initial and intermediate states to the silicon valence and conduction band, respectively. Tuning the photon energy across the surface resonance reveals asymmetric line profiles with pronounced destructive interference in the two-photon photoelectron intensities of both initial and intermediate states. The interference pattern is explained by an analytic extension of Fano's model to describe the coupling of two discrete states with two continua. This coupling strongly modifies the photoabsorption and is of general importance for light conversion in nanostructures and light-harvesting devices.     

苝四甲酸二酐薄膜电子结构的同步辐射共振光电子能谱研究

利用基于同步辐射的近边X射线吸收精细结构谱(NEXAFS)和共振光电子谱(RPES)研究了苝四甲酸二酐分子(PTCDA)薄膜的电子结构.碳K边NEXAFS谱中能量小于290 eV的四个峰对应于PTCDA分子不同化学环境碳原子1s电子到未占据分子轨道的共振跃迁.RPES谱中观察到共振光电子发射和共振俄歇电子发射导致的共振峰结构,以及二次谐波激发的碳1s信号.根据电子动能对入射光能量的依赖性分别对三类峰结构进行了归属.同时,发现PTCDA分子轨道共振光电子峰的强度具有光子能量依赖性.这种能量选择性共振增强效应是由于PTCDA分子轨道空间分布差异导致的.共振俄歇峰主要源于高结合能(4.1 eV)分子轨道能级电子参与的退激发过程.明确RPES实验谱图中各个峰结构的起源有助于准确利用基于RPES的芯能级空穴时钟谱技术定量估算有机分子/电极异质界面处电子从分子未占据轨道到电极导带的超快转移时间.     

Oscillator Strengths for Rydberg States in the Polar Molecule NeH

We calculate oscillator strengths for Rydberg electron transitions in polar molecules with a simple analytical procedure of taking into account the effectsof nonspherical symmetry of molecular core field due to its dipole moment. Sucheffects result in nonzero oscillator strengths values for some transitions which are forbidden in widely used atom-like models of molecular Rydberg states. For the allowed transitions we also report the difference between the atom-like calculations and the calculations that account for the dipole moment of the molecular core.