Dipolar excitons in a potential trap in a magnetic field

The conditions for observing the Zeeman spin splitting compensation in an exciton Bose gas have been investigated. The magnetoluminescence of spatially indirect, dipolar excitons in a 25-nm-wide GaAs/AlGaAs quantum well upon their accumulation in a lateral electrostatic trap has been studied in the Faraday geometry. The critical magnetic field B _cr below which the spin (paramagnetic) splitting of the luminescence line for a heavy-hole exciton at the trap center is almost completely compensated due to the exchange interaction in a dense Bose gas has been found to increase linearly with exciton concentration in qualitative agreement with the theory. Using a potential trap is fundamentally important. Incomplete compensation is observed in a homogeneous photoexcitation spot for dipolar excitons: the splitting is considerably smaller than that for a spatially direct exciton but differs noticeably from zero. The spin splitting compensation effect is observed only under neutral charge balance conditions—there is no Zeeman splitting suppression in a charged quantum well.     

Anisotropic exchange splitting of excitonic levels in small quantum systems

A theory of long-range exchange interaction between an electron and a hole in a zero-dimensional exciton has been developed taking into account the retardation effect. It has been shown that, for the anisotropic localizing potential in bulk semiconductors or semiconductor nanostructures, the long-range interaction leads to a splitting of radiative localized-exciton levels. Polarization of split sublevels is determined by the shape of the exciton envelope function. For excitons localized in quantum wells, typical splitting is expected to be tens of μ eV in agreement with recent experiments.     

Giant exciton Faraday rotation in Cd1−xMnxTe mixed crystals

New semiconductor compound Cd_1?xMn_xTe exhibits strong Faraday rotation in the interband region. It is shown by measurements of magneto-optical Kerr effect and reflection in magnetic field that unusually large exciton Zeeman splitting plays an essential role in the observed Faraday rotation. A possible explanation of the observed splitting (corresponding to a “g factor” value up to 100) by exchange interaction of excitons with manganese d states is suggested.     

Interwell exciton relaxation in semimagnetic asymmetric double quantum wells

The possibility of magnetic field control of the spectral and polarization characteristics of exciton recombination is examined in Cd(Mg, Mn) Te-based asymmetric double quantum wells. At low fields, the exciton transition in a semimagnetic well is higher in energy than that in a nonmagnetic well and the interwell exciton relaxation is fast. In contrast, when the energy order of the exciton transitions reverses at high fields, unexpectedly slow relaxation of σ⁻ polarized excitons from the nonmagnetic well to the σ⁺-polarized ground state in the semimagnetic well is observed. Strong dependence of the total circular polarization degree on the heavy-light hole splitting Δ _hh-lh in the nonmagnetic well is found and attributed to the spin dependent interwell tunneling controlled by exciton spin relaxation. Such a slowing down of the relaxation allows separation of oppositely spin-polarized excitons in adjacent wells. The text was submitted by the authors in English.     

Magnetic-field effects on exciton–polaron in a CdSe quantum disk

We study theoretically the interaction between excitons and longitudinal optical (LO) phonons in a cylindrical disk-like semiconductor quantum dot under an applied magnetic field. Due to the intensity of the interaction in the strong coupling regime, a composite quasi-particle called exciton–polaron is formed. We focus on the effect of the disk size and an external magnetic field on the exciton–phonon interaction energy and the exciton–polaron modes. The numerical computation for a CdSe quantum disk have shown that the exciton–phonon interaction energy is very significant and is even dominant when the disk height is small, which leads to a large Rabi splitting between the exciton–polaron modes. We investigate also the effect of the temperature on the integrated photoluminescence (PL) intensity, and show that at relatively high temperature the LO phonons have a noticeable effect on it. This physical parameter also shows a great dependence on quantum disk size and on magnetic field.     

Role of bright and dark excitons in the temperature-dependent photoluminescence of carbon nanotubes

We report studies of the temperature dependence of the photoluminescence efficiency of single walled carbon nanotubes which demonstrate the role of bright and dark excitons. This is determined by the energy splitting of the excitons combined with 1-D excitonic properties. The splitting of the bright and dark singlet exciton states is found to be only a few meV and is very strongly diameter dependent for diameters in the range 0.8-1.2 nm. The luminescence intensities are also found to be strongly enhanced by magnetic fields at low temperatures due to mixing of the exciton states.     

Photo-induced magnetic polarons in semiconductors

Exciton magnetic polarons observed in dilute magnetic semiconductors were investigated by steady-state and pico-second time-resolved photoluminescence measurements and have shown characteristic behavior of exciton localization processes in bulk-Cd_1-x Mn _x Te and also in the quantum structures composed of the dilute magnetic semiconductors. For the quantum structures spin-dependent coherent polarizations associated with excitons and biexcitons were studied by degenerate four-wave mixing experiment. Also investigated for different chalcogenide spinel ferromagnetic semiconductors was photo-induced enhancement of exchange interaction between magnetic ions by direct magnetic flux detection in the vicinity of the Curie temperatures.     

Fine structure of excitons in self-assembled In0.60Ga0.40As quantum dots: Zeeman-interaction and exchange energy enhancement

The fine structure of the ground state exciton has been studied by magnetophotoluminescence spectroscopy of self-assembled In_0.60Ga_0.40As single quantum dots. This was realized by using lithography for fabricating mesa structures which contain only a single dot. Due to a dot geometry-induced symmetry breaking we are able to observe the dark exciton states in magnetic field besides the bright excitons. From the spin-splitting data values for the corresponding exciton g-factors are obtained. In addition, the electron–hole exchange energies are determined, which are compared to detailed numerical calculations.     

Exchange splitting and Zeeman effect in direct excitons

The exchange splitting in direct excitons can be determined from observation of the Zeeman splitting of the exciton lines. The long-range exchange interaction gives rise to observable effects in the Voigt configuration. From the measurement of the Zeeman components separation, the gyromagnetic factors of the electron and the hole can be deduced and checked from the slope of the line split-off by the long-range exchange interaction.     

Magnetooptics and dynamics of a magnetic polaron in semimagnetic CdSe/ZnMnSe quantum dots

The magnetooptics and picosecond dynamics of the radiative recombination of excitons in self-assembled semimagnetic CdSe/ZnMnSe quantum dots is studied at low temperatures. The behavior of individual quantum dots in a magnetic field and with an increase in temperature is indicative of a strong exchange interaction of excitons and magnetic Mn ions giving rise to a quasi-zero-dimensional exciton magnetic polaron. When the exciton energy exceeds the intracenter transition energy in Mn, the energy is rapidly transferred from excitons to Mn ions (faster than 20 ps). In the opposite case, a substantial red shift (～15 meV) of the emission line maximum is observed during the exciton lifetime ～500 ps, presenting the dynamics of the formation of a magnetic polaron with a characteristic time of ～110 ps.     

On the possibility of a flare-up of discrete exciton absorption structure in n-CdxHg1−xTe crystals in a magnetic field

A discrete absorption peak with an exciton-like behavior was observed to flare up in Cd_xHg_1?xTe with x≈0.3 when a magnetic field H was applied. Its diamagnetic shift, which is replaced at high H by a close-to-linear relationship typical for diamagnetic exciton spectroscopy, and the splitting permit one to evaluate ?_g=0.2579±0.0001 eV and to calculate the band parameters of the material. Evaluation of the fluctuation potential well depth for the electrons and holes suggests, however, a high probability of exciton trapping at potential wells at H = 0 and x ≈ 0.3. Application of a magnetic field above 40 k0e can break the Coulomb bond between the electron and the hole, whereas at lower fields the existence of excitons is possible.     

The optical properties of Be,Mg and Zn-diffused gallium phosphide

Interest in the Ga-site acceptors Be and Mg was stimulated by the possibility that they might produce efficient luminescence on association with O, analogous to the well-known red Zn-O luminescence in GaP but at higher transition energy. Attention was directed to diffusion doping by Be and Mg of GaP O-doped during growth because the reactivity of Be and Mg with O renders double doping during crystal growth very difficult. Structured green donor-acceptor pair spectra were observed at 1.6°K from many Be-diffused crystals, yielding an accurate measure of (E_A)_BE, 50 ± 1 meV. Moderately efficient orange-red luminescence was also observed below ∼ 100 °K from these crystals, but the intensity of this luminescence decreased rapidly to negligible levels by ∼ 200°K. This luminescence also contains sharp structure at 1.6°K, of a form characteristic of the decay of excitons bound to complex centres. Many sharp phonon replicas occur, involving local modes as well as characteristic GaP modes. One set of no-phonon lines, at least, near 2.19 eV, shows zero-field splitting, luminescence decay times and behaviour in magnetic and external strain fields characteristic of exciton decay at a centre with <100>; or <111>-type symmetry axes, containing no extra electronic particles. The exciton state is split by 2.4 meV by J-J coupling, and the axial field of the centre splits the hole states by ∼ 1.0 meV. These bound excitons are specifically characteristics of diffused GaP and appear analogous to bound excitons observed below 2.12 eV in Zn-diffused GaP. It is probable that the relevant centres contain diffusion components such as Be or Zn interstitials and improbable that O_P is involved. By contrast, weak orange bound exciton luminescence observed in Mg-diffused GaP does involve O, presumably as O_P. No analysis of the magneto-optical behaviour of this Mg-related bound exciton was possible in our crystals, so its symmetry axis was not established. It is possible that this is the Mg_Ga-O_P bound exciton. If so, the two-fold reductions in the exciton localisation energy from ∼ 0.32 eV to ∼ 0.15 eV and in the mass of the Ga-site substituent has produced dramatic changes in the form of the phonon cooperation between the Zn-O and “Mg-O” excitons. The “Mg-O” exciton luminescence is not dominant in our crystals, even at low temperature. The exciton state is again split by a local crystal field as well as by J-J coupling, but here the former splitting is predominant; 2_∈0 = 3.9 meV, Δ = 0.60 meV.     

Spin relaxation of Mn ions in (CdMn)Te/(CdMg)Te quantum wells under picosecond optical pumping

Spin relaxation of Mn ions in a Cd_0.97Mn_0.03Te/Cd_0.75Mg_0.25Te quantum well with photogenerated quasi-two-dimensional electron-hole plasma at liquid helium temperatures in an external magnetic field has been investigated. Heating of Mn ions by photogenerated carriers due to spin and energy exchange between the hot electron-hole plasma and Mn ions through direct sd-interaction between electron and Mn spins has been detected. This process has a short characteristic time of about 4 ns, which leads to appreciable heating of the Mn spin subsystem in about 0.5 ns. Even under uniform excitation of a dense electron-hole plasma, the Mn heating is spatially nonuniform, and leads to formation of spin domains in the quantum well magnetic subsystem. The relaxation time of spin domains after pulsed excitation is measured to be about 70 ns. Energy relaxation of excitons in the random exchange potential due to spin domains results from exciton diffusion in magnetic field B=14 T with a characteristic time of 1 to 4 ns. The relaxation time decreases with decreasing optical pump power, which indicates smaller dimensions of spin domains. In weak magnetic fields (_B=2 T) a slow down in the exciton diffusion to 15 ns has been detected. This slow down is due to exciton binding to neutral donors (formation of bound excitons) and smaller spin domain amplitudes in low magnetic fields. The optically determined spin-lattice relaxation time of Mn ions in a magnetic field of 14 T is 270±10 and 16±7 ns for Mn concentrations of 3% and 12%, respectively. Zh. éksp. Teor. Fiz. 112, 1440–1463 (October 1997)     

Anisotropic exchange interaction in CdTe/Cd0.75Mn0.25Te quantum wires

We observed anisotropic large peak-energy shift of photoluminescence (PL) from CdTe/Cd_0.75Mn_0.25Te quantum wires (QWRs). The large PL-peak-energy shift is caused by the exchange interaction between excitons in the non-magnetic QWR and Mn ions in the DMS barrier. The exchange interaction is enhanced when magnetic field is perpendicular to the QWR.     

Dark-bright mixing of interband transitions in symmetric semiconductor quantum dots

In photoluminescence spectra of symmetric [111] grown GaAs/AlGaAs quantum dots in longitudinal magnetic fields applied along the growth axis, we observe in addition to the expected bright states also nominally dark transitions for both charged and neutral excitons. We uncover a strongly nonmonotonic, sign-changing field dependence of the bright neutral exciton splitting resulting from the interplay between exchange and Zeeman effects. Our theory shows quantitatively that these surprising experimental results are due to magnetic-field-induced ±3/2 heavy-hole mixing, an inherent property of systems with C(3v) point-group symmetry.     

Structural dependence of exciton in carbon nanotubes

The binding energies and sizes of excitons, and energy splitting of the bright-dark excitons in single-walled carbon nanotubes have been calculated using the nonorthogonal tight-binding model, supplemented by the long-range Coulomb interaction. It is found that the binding energies and the sizes of excitons not only depend on tube's diameter d, but also its chirality. However, the splitting of the bright-dark excitons mostly depends on 1/d². Our obtained results show that the curvature effect is very important for the exciton excitations in the SWNTs, especially in the smaller diameter ones.     

Longitudinal-transverse splitting of anisotropic excitons

The dipolar-interaction factors which determine the longitudinal—transverse splitting of excitons in anistropic materials are evaluated for hexagonal and tetragonal Bravais lattices. The results are applied to an analysis of the exciton lines in PbI₂.     

Optical orientation and alignment of excitons in quantum dots

Optical orientation and alignment of excitons in InAlAs quantum dots in the AlGaAs matrix have been studied both theoretically and experimentally. Experiments performed in a longitudinal magnetic field (Faraday geometry) reveal transformation of optical orientation to alignment and alignment to orientation, which is caused by exchange splitting of the dipole-active exciton doublet and allowed by the quantum-dot low symmetry. A comparison of theory with experiment made with inclusion of the anisotropy of exciton generation and recombination along the and [110] axes permits one to determine the character of dipole distribution in direction for resonant optical transitions in the self-organized quantum-dot ensemble studied. Fiz. Tverd. Tela (St. Petersburg) 40, 858–861 (May 1998)     

Magnetopolaritons in ZnTe

From an invariant expansion, we construct the exciton Hamiltonian for the Γ₆×Γ₈ excitons in theT _d -type material ZnTe represented by an 8×8 matrix including the influences of a finite wave vector and an external magnetic field. We diagonalize the Hamiltonian matrix to obtain the exciton states. Then the excitons are coupled to the electromagnetic radiation field thus giving the polariton states. The theoretical dispersion curves are fitted to the results of two-photon Raman scattering and reflection experiments in magnetic fields up to 22 T. From this fit we deduce precise values for the eigenergies, exciton masses,g-factors, and diamagnetic shifts.     

The 2s exciton in intermediate dimensionality structures

We report the observation of the 1s and 2s ground state excitons in small-offset superlattices with alternating layers of non-magnetic (ZnSe) and diluted magnetic (Zn_xMn_1 − xSe) semiconductors. Due to the large Zeeman splitting of the electron states in ZnMnSe, valence and conduction band offsets in these structures can be tuned by external magnetic field over the scale of tens of meV. Two series of superlattices were investigated, with Mn concentrations close to 10 and 15 atomic percent, respectively. Each series consisted of five superlattices with different layer thicknesses. The 2s exciton of the ground state is observed in all samples, and exhibits a diamagnetic shift that is consistent with earlier studies. The ability to determine the energy separation between the 1s and 2s exciton states allows us to analyze our results in the framework of thefractional dimension analysis , and to extract the values of exciton binding energies for the superlattices from the analysis.