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Abstract: The possible existence of the bound states of the interacting two-dimensional (2D) magnetoexcitons in the lowest Landau levels (LLLs) approximation was investigated using the Landau gauge description. The magnetoexcitons taking part in the formation of the bound state with resultant wave vector have opposite in-plane wave vectors and and look as two electric dipoles with the arms oriented in-plane perpendicularly to the corresponding wave vectors. The bound state of two antiparallel dipoles moving with equal probability in any direction of the plane with equal but antiparallel wave vectors is characterized by the variational wave function of the relative motion depending on the modulus . The spins of two electrons and the effective spins of two holes forming the bound states were combined separately in the symmetric or in the antisymmetric forms with the same parameter for electrons and holes. In the case of the variational wave function the maximum density of the magnetoexcitons in the momentum space representation is concentrated on the in-plane ring with the radius The stable bound states of the bimagnetoexciton molecule do not exist for both spin orientations. Instead of them, a deep metastable bound state with the activation barrier comparable with the ionization potential of the magnetoexciton with was revealed in the case and . In the case and only a shallow metastable bound state can appear. [ABSTRACT FROM AUTHOR]

The spontaneous breaking of the continuous symmetries of a two-dimensional electron-hole system in a strong magnetic field perpendicular to the plane leads to the formation of new ground states and determines the energy spectrum of collective elementary excitations that appear above these new ground states. In this review, the main attention is paid to the electron-hole system formed from coplanar magnetoexcitons under conditions of Bose-Einstein condensation in the ground state with the wave vector k = 0 taking into account the influence of excited Landau levels, when exciton-type elementary excitations coexist with plasmon-type oscillations. At the same time, the properties of a two-component system consisting of a two-dimensional electron gas and a two-dimensional hole gas spatially separated in a double quantum well under conditions of the fractional quantum Hall effect are of great interest, because these properties can affect the quantum states of magnetic excitons that are formed when the distance between the layers tends to zero. Bilayer electron systems are also considered under conditions of the fractional quantum Hall effect with the one-half filling factor for each layer and the total filling factor equal to unity for both layers. The coherence between the electron states in the two layers is equivalent to the formation of excitons in a macroscopic coherent state. This makes it possible to compare the energy spectrum of collective elementary excitations of Bose-Einstein condensed excitons under conditions of the quantum Hall effect and coplanar magnetoexcitons. The breaking of the global gauge symmetry or of the continuous rotational symmetry leads to the formation of a gapless spectrum of the Nambu-Goldstone type, whereas the breaking of the local gauge symmetry is accompanied by the appearance of a gap in the energy spectrum (Higgs phenomenon). These phenomena are equivalent to the formation of massless and massive particles in the relativistic physics. The application of the Nielsen-Chadha theorem, which determines the number of Nambu-Goldstone modes as a function of the number of broken symmetry operators, is demonstrated using the example of Bose-Einstein condensation of spinor atomic gases in an optical trap. This example is presented for a better understanding of the results obtained in the case of a Bose-Einstein condensation of coplanar magnetoexcitons. The Higgs phenomenon leads to the formation of composite particles under conditions of the fractional quantum Hall effect. Their description is given in terms of the Ginzburg-Landau theory. The possibilities for the appearance of spontaneous coherence in a system of indirect excitons in structures with a double quantum well are analyzed. The experimental attempts to create these conditions, the main results obtained in this field, and the accumulated knowledge are reviewed. The basic properties of the energy spectrum of magnetoexciton polaritons in a microcavity are formulated. A hypothesis is put forward about the possibility of forming two-dimensional magnetoexcitons and two-dimensional magnetoexciton polaritons of high density with attached point quantum vortices, i.e., about the possibility of forming new composite particles. [ABSTRACT FROM AUTHOR]

Abstract: The Landau quantization of the two-dimensional (2D) heavy holes, its influence on the energy spectrum of 2D magnetoexcitons, as well as their optical orientation are studied. The Hamiltonian of the heavy holes is written in two-band model taking into account the Rashba spin–orbit coupling (RSOC) with two spin projections, but with nonparabolic dispersion law and third-order chirality terms. The most Landau levels, except three with , are characterized by two quantum numbers and m for for two spin projections correspondingly. The difference between them is determined by the third-order chirality. Four lowest Landau levels (LLLs) for heavy holes were combined with two LLLs for conduction electron, which were taken the same as they were deduced by Rashba in his theory of spin–orbit coupling (SOC) based on the initial parabolic dispersion law and first-order chirality terms. As a result of these combinations eight 2D magnetoexciton states were formed. Their energy spectrum and the selection rules for the quantum transitions from the ground state of the crystal to exciton states were determined. On this base such optical orientation effects as spin polarization and magnetoexciton alignment are discussed. The continuous transformation of the shake-up (SU) into the shake-down (SD) recombination lines is explained on the base of nonmonotonous dependence of the heavy hole Landau quantization levels as a function of applied magnetic field. [Copyright &y& Elsevier]

Abstract: The collective elementary excitations of two-dimensional magnetoexcitons in a Bose–Einstein condensate (BEC) with wave vector were investigated in the framework of the Bogoliubov theory of quasi-averages. The Hamiltonian of the electrons and holes lying in the lowest Landau levels (LLLs) contains supplementary interactions due to virtual quantum transitions of the particles to the excited Landau levels (ELLs) and back. As a result, the interaction between the magnetoexcitons with does not vanish and their BEC becomes stable. The energy spectrum contains only one gapless, true Nambu–Goldstone (NG) mode of the second kind with dependence at small values k describing the optical-plasmon-type oscillations. There are two exciton-type branches corresponding to normal and abnormal Green''s functions. Both modes are gapped with roton-type segments at intermediary values of the wave vectors and can be named as quasi-NG modes. The fourth branch is the acoustical plasmon-type mode with absolute instability in the region of small and intermediary values of the wave vectors. All branches have a saturation-type dependencies at great values of the wave vectors. The number and the kind of the true NG modes are in accordance with the number of the broken symmetry operators. [Copyright &y& Elsevier]

The dynamics of the exciton and the biexciton related emission from a single InGaN quantum dot (QD) have been measured by time-resolved microphotoluminescence spectroscopy. An exciton-biexciton pair of the same QD was identified by the combination of power dependence and polarization-resolved spectroscopy. Moreover, the spectral temperature evolution was utilized in order to distinguish the biexciton from a trion. Both the exciton and the biexciton related emission reveal mono-exponential decays corresponding to time constants of ∼900 and ∼500 ps, respectively. The obtained lifetime ratio of ∼1.8 indicates that the QD is small, with a size comparable to the exciton Bohr radius. [ABSTRACT FROM AUTHOR]

The article consists of two parts describing two associated properties of two-dimensional magnetoexcitons. In the first part, the theory of combined exciton-cyclotron resonance in the quantum well structures is developed for a strong magnetic field. In the first part, we calculate the absorption band structure for optical quantum transitions with circularly polarized radiation creating a two-dimensional exciton and simultaneously exciting one of the resident electrons from the lowest to the first excited Landau level. In the second part, we discuss the collective elementary excitations of the system of two-dimensional magnetoexcitons in the ground state of Bose–Einstein condensation on the arbitrary wave vector $\mathop K\limits^ \to$ ≠ 0 in the Hartree–Fock approximation. The breaking of the gauge symmetry of the initial Hamiltonian was introduced following the idea proposed by Bogoliubov in his theory of quasi-averages. The energy spectrum of the collective elementary excitations is characterized by the interconnection of the exciton and plasmon branches. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

The states of electron–hole pairs in spherical silicon nanocrystals are theoretically studied using the “multiband” effective-mass approximation in the limit of an infinitely high potential barrier at the boundary. The degeneracy of the states at the top of the valence band is taken into account in the spherical approximation, and the ellipsoidal character of the electronic spectrum in the conduction band is allowed for. Coulomb interaction–induced corrections to the energy of an electron–hole pair are found. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

A study is reported on the low-temperature (T≤30 K) photoluminescence in the spatially indirect exciton line in GaAs/Al[sub 0.33]Ga[sub 0.67]As double quantum wells as a function of optical pump power and applied electric field. We have revealed a giant (threefold) burst of luminescence intensity in a part of the spectral profile of the indirect-exciton line occurring at certain values of the external electric field, temperature, and optical pumping. We have also observed that this part of the indirect-exciton line profile varies in intensity (fluctuates) with time with a characteristic period of tens of seconds. The results obtained are discussed within a model of the Bose–Einstein condensation of a system of two-dimensional bosons that have, besides the free, a discrete energy spectrum lying below the bottom of the free-state band. © 2000 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

Low-temperature (T=1.8, 4.2 K) luminescence of GaAs/Al[sub 0.3]Ga[sub 0.7]As double-coupled asymmetric quantum wells (DQW) is observed under variation of an electric field V[sub dc] applied along the normal to the DQW plane. The FWHM of the indirect exciton (IX) emission line was found to undergo, within a certain interval of V[sub dc], a strong (up to a factor 3.5) narrowing, accompanied by anomalously large IX intensity fluctuations in time within the V[sub dc] region where a strong decrease (or increase) of the FWHM is observed to occur. The dependence of the FWHM on the pumping level I[sub p] also exhibits a substantial decrease of the FWHM within a certain I[sub p] interval. The results obtained are discussed in the frame of an assumption which relates the observed phenomena to the onset of a condensed state in the interacting ensemble of spatially indirect excitons in DQWs. [ABSTRACT FROM AUTHOR]

We report the first studies of exciton luminescence spectra from asymmetric double quantum wells (DQWs) of very similar width. The DQWs were of GaAs/AlGaAs and the differences in widths of the coupled wells were one or two monolayers. The coupled direct and indirect exciton states anticross with a resonance splitting of 1.33 meV. An additional luminescence line appearing at low temperatures is identified as a localized indirect exciton. [ABSTRACT FROM AUTHOR]

We examine the absorption and amplification bands of a weak probe signal in the presence of Bose-Einstein condensation of excitons that emerges in nonequilibrium conditions in the field of coherent laser radiation with a wave vector k[sub 0]. We assume that the detuning &Δtilde; from resonance between the energy hω[sub ex](k[sub 0]) + L[sub 0] of the exciton level, which is shifted because of exciton-exciton interaction, and the laser photon energy hω[sub L], is generally nonzero. The elementary excitation spectrum consisting of the quasiexcitonic and quasienergy branches determines the optical properties of the system. When there is real induced Bose-Einstein condensation, at &Δtilde; = 0 the two branches touch, as they do in spontaneous Bose-Einstein condensation. In virtual induced Bose-Einstein condensation, when &Δtilde; < 0, instabilities emerge in the spectrum in certain regions of the k-space. These instabilities are caused by a real transformation of two laser photons into two extracondensate particles. Nonequilibrium extracondensate excitons strongly affect the absorption and amplification of the probe light signal. We show that light absorption is due to the quantum transition from the ground state of the crystal to the quasiexcitonic branch of the spectrum. On the other hand, amplification of the signal is caused by the transition from the quasienergy branch to the ground state of the crystal. The same transition can be explained by a real transformation of two laser photons into a vacuum photon of frequency hcq and a crystal exciton with a wave vector 2k[sub 0] - q. Finally, we show that the excitonic absorption and light-amplification bands are essentially anisotropic at &Δtilde; ≈ 0 and depend on the orientation of the vectors q and k[sub 0]. [ABSTRACT FROM AUTHOR]

Circular polarization studies of photoluminescence from the neutral (X0) and the positively charged (X+) excitons are reported for individual InAs/GaAs quantum dots (QDs). High polarization degrees, 60% for X0 and 73% for X+, were recorded without any external magnetic field applied. These studies show that the QD polarization and population dynamics are controllable either by varying the photoexcitation intensity or by using a second IR laser excitation. [ABSTRACT FROM AUTHOR]

It is demonstrated that the photoluminescence spectra of single self-assembled InAs/GaAs quantum dots are very sensitive to excitation energy and crystal temperature. This is qualitatively explained in terms of the effective diffusivity of photogenerated particles, which affects the capture probability of the quantum dot. As a consequence, this opens the possibility of controlling the average number of excess electrons in the quantum dot by optical means. This technique may be used as a simple tool to create and study charged exciton complexes without any specially fabricated samples. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The absorption and circular dichroism spectra of the B800-850 complex from Chromatium minutissimum before and after the Triton X-100 treatment were simulated by means of standard exciton theory, taking into account inhomogeneous broadening. To explain the spectral changes of the B800-850 complex treated with Triton X-100, we have assumed that all bacteriochlorophyll pigments absorbing at 850 nm exhibit the same additional rotation of 20° around the axis perpendicular to the membrane plane. This has been sufficient to fit the transformation in absorption and circular dichroism spectra induced by detergent treatment of the B800-850 complex. [ABSTRACT FROM AUTHOR]

The absorption and gain bands of a weak probe signal in the presence of the Bose-Einstein condensation of excitons, which appears under nonequilibrium conditions in a field of coherent laser radiation, are considered. It is shown that the absorption of light is caused by a quantum transition from the ground state of the crystal to the quasiexciton branch of the spectrum. Amplification of the signal occurs as a result of transitions from the quasiexciton branch of the spectrum to the ground state of the crystal. [ABSTRACT FROM AUTHOR]

The emission of a charged exciton in an InGaN quantum dot (QD) on top of a GaN pyramid is identified experimentally. The intensity of the charged exciton exhibits the expected competition with that of the single exciton, as observed in temperature-dependent micro-photoluminescence measurements, performed with different excitation energies. The non-zero charge state of this complex is further supported by time resolved micro-photoluminescence measurements, which excludes neutral alternatives of biexciton. The potential fluctuations in the vicinity of the QD that localizes the charge carriers are proposed to be responsible for the unequal supply of electrons and holes into the QD. [ABSTRACT FROM AUTHOR]

We study interaction of the two-dimensional magnetoexcitons with in-plane wave vector k → ∥ = 0 , taking into account the influence of the excited Landau levels (ELLs) and of the external electric field perpendicular to the surface of the quantum well and parallel to the external magnetic field. It is shown that the account of the ELLs gives rise to the repulsion between the spinless magnetoexcitons with k → ∥ = 0 in the Fock approximation, with the interaction constant g decreasing inverse proportional to the magnetic field strength B ( g ( 0 ) ∼ 1 / B ) . In the presence of the perpendicular electric field the Rashba spin-orbit coupling (RSOC), Zeeman splitting (ZS) and nonparabolicity of the heavy-hole dispersion law affect the Landau quantization of the electrons and holes. They move along the new cyclotron orbits, change their Coulomb interactions and cause the interaction between 2D magnetoexcitons with k → ∥ = 0 . The changes of the Coulomb interactions caused by the electrons and by the holes moving with new cyclotron orbits are characterized by some coefficients, which in the absence of the electric field turn to be unity. The differences between these coefficients of the electron-hole pairs forming the magnetoexcitons determine their affinities to the interactions. The interactions between the homogeneous, semihomogeneous and heterogeneous magnetoexcitons forming the symmetric states with the same signs of their affinities are attractive whereas in the case of different sign affinities are repulsive. In the heterogeneous asymmetric states the interactions have opposite signs in comparison with the symmetric states. In all these cases the interaction constant g have the dependence g ( 0 ) ~ 1 / B . [ABSTRACT FROM AUTHOR]

A high degree ([?]55%) of circular polarization has been observed for the neutral exciton in InAs/GaAs quantum dots (QDs). The possibility to record non-zero polarization of the neutral exciton is explained in terms of different capture times of the light electron compared with the heavier holes into the QDs from the wetting layer. This interpretation is supported by the progressive reduction of the polarization degree with increasing QD density, and also with increasing temperature. [ABSTRACT FROM AUTHOR]

Fluctuations in time of the intensity of indirect exciton photoluminescence in GaAs/Al0.33Ga0.67As double quantum wells have been studied. An analysis of the behavior of the intensity fluctuations under variation in external control parameters, such as the temperature and the voltage applied perpendicular to the structure, has revealed that they directly reflect the manifestation of Bose condensation. It has been demonstrated that the current flowing perpendicular to the structure also undergoes pronounced fluctuations simultaneously with those of the intensity. A study of possible reasons that could account for the onset of such fluctuations suggests the need for taking into account tunneling transitions in the exciton Bose condensate system. [ABSTRACT FROM AUTHOR]

Studies of the photoluminescence spectra of spatially indirect excitons in coupled quantum wells revealed that the emission has a directional pattern dependent on the external electric field and pumping power. The experimentally detected correlation between the spectral emission parameters of spatially indirect excitons, namely, the concentration (phase state) of such excitons, the line half-width, the degree of linear polarization, and the existence of a directional pattern, permits a suggestion that the spontaneous photoluminescence of spatially indirect excitons in the condensed state is of a coherent nature. [ABSTRACT FROM AUTHOR]

A study is reported of exciton luminescence in GaAs double quantum wells produced in electric and magnetic fields. It has been found that the indirect-exciton line (IX) behaves anomalously, namely, one observes a magnetic-field-induced low-energy shift of the IX line, and the onset of periodic (≈5 s) fluctuations in the IX-line intensity. [ABSTRACT FROM AUTHOR]

Abstract: Optical characterization of single quantum dots (QDs) by means of micro-photoluminescence (μPL) will be reviewed. Both QDs formed in the Stranski–Krastanov mode as well as dots in the apex of pyramidal structures will be presented. For InGaAs/GaAs dots, several excitonic features with different charge states will be demonstrated. By varying the magnitude of an external electric or magnetic field and/or the temperature, it has been demonstrated that the transportation of carriers is affected and accordingly the charge state of a single QD can be tuned. In addition, we have shown that the charge state of the QD can be controlled also by pure optical means, i.e. by altering the photo excitation conditions. Based on the experience of the developed InAs/GaAs QD system, similar methods have been applied on the InGaN/GaN QD system. The coupling of LO phonons to the QD emission is experimentally examined for both charged and neutral excitons in single InGaAs/GaAs QDs in the apex of pyramidal structures. It is shown that the positively charged exciton exhibits a significantly weaker LO phonon coupling in the μPL spectra than the neutral and negatively charged species, a fact, which is in consistency with model simulations performed. [Copyright &y& Elsevier]