Theory of excitons, charged excitons, exciton fine-structure and entangled excitons in self-assembled semiconductor quantum dots

We show how the atomistic pseudopotential many-body theory of InGaAs/GaAs addresses some important effects, including (i) the fine-structure splittings (originating from interband spin exchange), (ii) the optical spectra of charged quantum dots and (iii) the degree of entanglement in a quantum dot molecule.     

GaAs量子阱半导体微腔中腔极化激元的动态行为

GaAs量子阱半导体微腔中, 光子同时与重空穴激子、轻空穴激子耦合形成腔极化激元. 本文采用三谐振子耦合模型, 计算了腔极化激元的三支的色散关系、线宽、有效质量及其群速度; 结果表明, 由于腔极化激元的三支中光子、重空穴激子、轻空穴激子所占的权重随着平面波矢（或入射角度）变化, 腔极化激元三支的线宽、有效质量及其群速度呈现出不同的动态行为.     

Phonon heating of two-dimensional exciton gases in GaAs/AlGaAs quantum wells

We report the first measurements of the interaction of non-equilibrium phonons with two-dimensional exciton gases (2DExGs). The rise in the effective temperature of the 2DExG produced by the phonons depends on the width of the quantum well and the exciton sheet density and hence on the ratio τ^?1 (ex-ph)/τ^?1 (ex-ex). The dependence of the effective temperature rise on this ratio is attributed to the non-equilibrium frequency distribution of the phonons incident on the 2DExG.     

Polaron effects on excitons in parabolic quantum wells: fractional-dimension variational approach

Polaron effects on excitons in parabolic quantum wells are studied theoretically by using a variational approach with the so-called fractional dimension model. The numerical results for the exciton binding energies and longitudinal-optical phonon contributions in GaAs/Al_0.3Ga_0.7As parabolic quantum well structures are obtained as functions of the well width. It is shown that the exciton binding energies are obviously reduced by the electron (hole)-phonon interaction and the polaron effects are un-negligible. The results demonstrate that the fractional-dimension variational theory is effectual in the investigations of excitonic polaron problems in parabolic quantum wells.     

Study of optical absorption and EPR spectra of Mn<Superscript>2+</Superscript> ion in cadmium maleate dihydrate

The optical absorption and EPR spectra of Mn²⁺ ion doped in cadmium maleate dihydrate have been theoretically investigated by diagonalizing the complete energy matrices for a d⁵ configuration ion in a trigonal ligand-field. According to the suggestion of the optical absorption studies, we assume that the Mn²⁺ ion enters the host lattice interstitially and the distorted octahedral symmetry for the impurity ion is trigonal. Moreover, the local lattice structure parameters of the system are determined. The results show that the six oxygen ions around the Mn²⁺ ion are at the same distance R=2.115 ?, and there are three Mn-O bonds forming an angle θ₁ of 66.26° with the C₃-axis and three others forming an angle θ₂ of 43.40°.     

A super coupling of radiation field and excitons confined in spatially periodic structure

By means of the microscopic nonlocal response theory, we analyze the mode structure of the coupled system of radiation and periodically confined excitons in multiple thin films. It is clarified how the spatially separated excitons which are coupled with each other via radiation share the radiative shift and width depending on the period and thickness of the excitonic active layers, and on their quantum number of confined levels. These results show the possibility that the very large radiative width can be realized by setting up the period and thickness of the excitonic active layers at appropriate values, which leads to very fast radiative decay beyond the saturated value of the giant oscillator strength effect.     

Carrier-induced changes in the phase resolved reflection of GaAs quantum wells

The influence of thermalized non-coherent carriers on the dielectric function of GaAs/AlAs quantum wells is investigated by reflection spectroscopy. Experiments are performed using the method of spectral interferometry, where both amplitude and phase of reflected pulses can be determined. For low excitation density the complex coefficient of reflection can be described using as dielectric function a broadened Elliot formula. With increasing carrier density pronounced nonlinearities appear in both amplitude and phase due to many-body effects between excited carriers. The nonlinear behavior fits very well to the results of a many-body theory based on the Semiconductor Bloch equations including memory effects in the scattering processes between carriers and the polarization induced by the probe pulse. Received 29 May 2002 / Received in final form 23 September 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: manzke@physik3.uni-rostock.de     

Optical investigations of GaAs/Ga1−x Al x As Quantum wells grown by molecular-beam epitaxy

Summary Measurements of photoluminescence, excitation photoluminescence and reflectance are performed at various temperatures on a series of GaAs/Ga_1−x Al _x As quantum well structures grown by molecularbeam epitaxy. The selective photoluminescence data of the GaAs buffer layers are analysed in order to correlate the optical properties with the growth conditions. The Stokes shift of the excitation emission line from quantum wells is investigated under various excitation conditions. A considerable decrease of the Stokes shift is observed in the case of nonresonant and intense excitations. Also the extrinsic photoluminescence, as well as its temperature dependence, are interpreted. In addition, the temperature effects on both the bulk and quantum well spectra are shown to clarify the excitation features and the contribution of the interband transitions. To speed up publication, the authors of this paper has agreed to not receive the proofs for correction.     

Quantum Monte Carlo simulation of exciton–exciton scattering in a GaAs/AlGaAs quantum well

We present a computer simulation of exciton–exciton scattering in a quantum well. Specifically, we use quantum Monte Carlo techniques to study the bound and continuum states of two excitons in a 10 nm wide GaAs/Al_0.3Ga_0.7As quantum well. From these bound and continuum states we extract the momentum-dependent phase shifts for s-wave scattering. A surprising finding of this work is that a commonly studied effective-mass model for excitons in a 10 nm quantum well actually supports two bound biexciton states. The second, weakly bound state may dramatically enhance exciton–exciton interactions. We also fit our results to a hard-disk model and indicate directions for future work.     

Band-mixing and strain effects in InAs/GaAs quantum rings

We analyze for the first time the coupled influence of band mixing, strain, and piezoelectricity on electronic structure, eigenstates, and optical transition strengths for InAs/GaAs quantum-ring structures. It is shown that band mixing and strain alter the level energies and optical absorption coefficients significantly.     

First-principles study of the local structure and crystal field of Yb2+ in sodium and potassium halides

The local coordination structures around the doping Yb²⁺ ions in sodium and potassium halides were calculated by using the first-principles supercell model. Both the cases with and without the charge compensation vacancy in the local environment of the doping Yb²⁺ were calculated to study the effect of the doping on the local coordination structures of Yb²⁺. Using the calculated local structures, we obtained the crystal-field parameters for the Yb²⁺ ions doped in sodium and potassium halides by a method based on the combination of the quantum-chemical calculations and the effective Hamiltonian method. The calculated crystal-field parameters were analyzed and compared with the fitted results.     

Lateral carrier tunnelling in stacked In(Ga)As/GaAs quantum rings

Exciton recombination dynamics in vertical stacks of InGaAs quantum rings have been studied by means of continuous wave and time resolved photoluminescence under low excitation density conditions. We have paid special attention to the effect of the carrier coupling on the exciton radiative lifetime: weak (14 nm spacer sample), intermediate (4.5 nm spacer sample), where the size filtering effects (towards small rings) compensate partially that arising from carrier coupling (towards lower energies), and strong electron and hole coupling (1.5 nm spacer sample) between layers. Experimental decay times in the latter two cases have been compared to the times simulated with a multi-quantum well based model, which accounts for the observed change of carrier coupling regime. The most important effect is observed when the hole wave function overlap along the growth direction becomes important (1.5 nm spacer sample). This situation makes important the lateral tunneling of excitons between rings, given their large lateral size, which is characterized by times around 5 ns at the emission peak energy (rings with the most probable size of the distribution).     

Magnetic structure of L{i_{1 - x}}N{i_{1 + x}}{o_2}

We study the magnetic structure of layered Li_1-xNi_1+xO₂ and propose a new scheme: the AF interaction between the excess Ni²⁺ in the Li layers and the Ni³⁺ ions in the Ni planes, gives rise to the formation of ferrimagnetic clusters, which control the physics of these systems. The values of the different interactions are estimated from a mean field calculation in the high temperature limit. For the small x samples studied here the method does not yield an accurate value of , but it is very sensitive to the intralayer interactions, allowing to conclude that they are ferromagnetic. The recent proposal of a quantum spin-orbital liquid in this system is discussed and the comparison with Jahn-Teller distorted NaNiO₂ is made. Received 8 December 1999     

X-ray determination of charge transfer in solar grade GaAs

The magnitude and direction of charge transfer in GaAs at various temperatures have been analyzed using our precise and extensive single-crystal X-ray diffraction data. The charge transfer parameters were obtained by employing a quadratic equation method and precise X-ray structure factors collected at 170, 200, 250 and 300 K. A transfer of charge from gallium atom to arsenide atom is evidenced at all the above temperatures.     

Dependence of the ferromagnetic resonance modes on the coupling strength in exchange-coupled trilayer structures

The dependence of the dispersion relation and the magnetization on the exchange coupling strength was calculated for a system consisting of two ferromagnetic layers exchange-coupled through a nonmagnetic spacer layer. The magnetic layers are characterized by both uniaxial and cubic magnetocrystalline anisotropies. A minimization procedure was developed which allows the resonance modes to be obtained for any magnetic field orientation and strength, as well as for any exchange coupling strength. If the antiparallel coupled film is unsaturated at resonance, the dispersion relation for both acoustic and optic modes could be rather complex, especially when the field is applied in the plane of the film.     

Optical spectra and energy band structure of AgAsS2 crystals

Ground and excited states of three exciton series are observed in the region of fundamental absorption edge of AgAsS₂ crystals. The contours of exciton reflection spectra are calculated and the main parameters of excitons and energy bands are determined in the center of Brillouin zone. The optical reflection spectra are investigated at 30 K in E∥c and E⊥c polarizations in AgAsS₂ crystals in the region of 2-6 eV. The optical functions are calculated from the reflection spectra and a scheme of electronic transitions responsible for peculiarities of reflection spectra deep into the absorption band is proposed.     

Effects of an intense, high-frequency laser field on the binding energy of excitons confined in a GaInNAs/GaAs quantum well

The effects of an intense, high-frequency laser field linearly polarized along the growth direction on the binding energy of excitons confined in a GaInNAs/GaAs quantum well is computed for different nitrogen and indium mole fractions by means of a variational technique within the effective-mass approximation. Our results show that such laser field creates an additional geometric confinement on the electronic and exciton states in the quantum well and the exciton binding energy depends on both the nitrogen and indium concentrations.     

Longitudinal-optical phonon effects on the binding energy of a hydrogenic impurity in a quantum well structure

Summary The polaron effect on the binding energy of a hydrogenic impurity in a semiconductor quantum well is calculated by a variational approach taking into account the quasi-two-dimensional and the realist structures of systems. For the finite potential barrier, a correction is obtained as a function of the well thickness and presents a maximum; its evolution is analogous to the binding energy.     

Magnetic properties of doped GdI2

Motivated by the recent experimental studies on layered ferromagnetic metallic system GdI₂  and its doped variant GdI₂H_x we develop a model to understand their ground state magnetic phase diagram. Based on first principle electronic structure calculations we write down a phenomenological model and solve it under certain approximations to obtain the ground state energy. In the process we work out the phase diagram of the correlated double exchange model on a triangular lattice for the specific band structure at hand.