Superfluidity of “dirty” excitons

The effect of a random field due to impurities, boundary irregularities, and so on, on the superfluidity of a three-dimensional system of excitons and a quasi-two-dimensional system of direct or spatially indirect excitons is studied. The influence of a random field on the density of the superfluid component in the indicated excitonic systems at low temperatures T is investigated. The interaction between excitons is taken into account in the ladder approximation. For quasi-two-dimensional excitonic systems in a random field the Kosterlitz-Thouless temperature in the superfluid state is calculated.     

Interwell excitons in GaAs superlattices

The formation of spatially indirect excitons in superlattices with narrow minibands is investigated experimentally. The interwell exciton is similar to the first Wannier–Stark localized exciton of an electrically biased superlattice. However, in the present case the localization is mediated by the Coulomb interaction of the electron and the hole without external fields.     

Excitons and magnetoexcitons in coupled quantum nanostructures: the role of a dirty environment

The effect of a random field caused by impurities, interface roughness and so on, on the optical properties and superfluidity of a quasi-two-dimensional system of excitons is studied. The influence of a random field on the density of the superfluid component of excitonic systems at low temperatures is investigated. For quasi-two-dimensional excitonic systems in a random field the Kosterlitz–Thouless temperature in the superfluid state is calculated. The superfluidity and Bose–Einstein condensation of indirect excitons in coupled quantum dots are studied. Magnetoexciton light absorption in the disordered quantum wells is considered. The two-particle problem of the magnetoexciton motion in the external field depending on the external magnetic field is reduced to the one-particle motion with effective magnetic mass in some effective field. The energy and optical absorption of the magnetoexciton in a single and coupled quantum dots are studied using the effective-magnetic-mass Hamiltonian. In the coherent potential approximation the coefficient of magnetoexciton optical absorption in single and coupled quantum wells is calculated. In the strong magnetic fields the exciton peak decreases with magnetic field increasing in accordance with the experimental data. The localization of direct and indirect magnetoexcitons is investigated. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000     

Charged magneto-exciton states in semiconductor quantum dots

By embedding a layer of self-assembled quantum dots into a field-effect structure, we are able to control the exciton charge in a single dot. We present the results of photoluminescence experiments as a function of both charge and magnetic field. The results demonstrate a hierarchy of energy scales determined by quantization, the direct Coulomb interaction, the electron–electron exchange interaction, and the electron–hole exchange interaction. For excitons up to the triply charged exciton, the behavior can be understood from a model assuming discrete levels within the quantum dot. For the triply charged exciton, this is no longer the case. In a magnetic field, we discover a coherent interaction with the continuum states, the Landau levels associated with the wetting layer.     

Ground state energy of N Frenkel excitons

By using the composite many-body theory for Frenkel excitons we have recently developed, we here derive the ground state energy of N Frenkel excitons in the Born approximation through the Hamiltonian mean value in a state made of N identical Q = 0 excitons. While this quantity reads as a density expansion in the case of Wannier excitons, due to many-body effects induced by fermion exchanges between N composite particles, we show that the Hamiltonian mean value for N Frenkel excitons only contains a first order term in density, just as for elementary bosons. Such a simple result comes from a subtle balance, difficult to guess a priori, between fermion exchanges for two or more Frenkel excitons appearing in Coulomb term and the ones appearing in the N exciton normalization factor – the cancellation being exact within terms in 1/N_s where N_s is the number of atomic sites in the sample. This result could make us naively believe that, due to the tight binding approximation on which Frenkel excitons are based, these excitons are just bare elementary bosons while their composite nature definitely appears at various stages in the precise calculation of the Hamiltonian mean value.     

Polaronic shift of the Wannier-Mott exciton binding energy in a quantum wire with allowance for the phonon confinement effect

Within the framework of the Li-Low-Pines model the interaction of a Wannier-Mott exciton with polar optical phonons in a cylindrical semiconductor wire is studied, taking into account the phonon confinement effect. An analytical expression for the exciton binding energy with allowance for the polaronic effect is obtained. Numerical calculations of the binding energy are carried out for AlAs/GaAs/AlAs and ZnSe/CdSe/ZnSe wires with a various degree of polarity of quantum wire materials. The polaronic shift of the binding energy of light and heavy hole excitons is calculated.     

Exciton absorption of light in a smooth random field

A study is made of interband absorption of light in a smooth random field that varies slowly in space. When the exciton energy in the random field is much less than the exciton binding energy, the influence of the random field on the discrete spectrum of the exciton is expressed through the appearance of an exponential “tail” to every exciton line. But if the exciton energy in the random field is much greater than the exciton binding energy, the exciton effects are unimportant because the random field breaks up the excitons.     

Rigorous Asymptotic Study of the Screened Electrostatic Potential in a Thin Dielectric Slab

The screened Coulomb potential plays a crucial role in the binding energies of excitons in a thin dielectric slab. The asymptotic behavior of this potential is studied when the thickness of the slab is very small as compared to the exciton Bohr radius. A regularized expression is given and the exact effective 2D potential is derived. These expressions may be useful for the computation of the exciton binding energy in 2D or quasi‐2D materials.     

Influence of acoustic phonons on dephasing of free excitons in quantum wells

A theory of the dephasing rate of quasi-2D free excitons due to acoustic phonon interaction at low exciton densities is presented. Both deformation potential and piezoelectric couplings are considered for the exciton–phonon interaction in quantum wells. Using the derived interaction Hamiltonian obtained recently by us, exciton linewidth and dephasing rate are calculated as a function of the exciton density, exciton temperature, exciton momentum and lattice temperature.     

Properties of Excitons Bound to Neutral Donors in GaAs-AlxGa1-xAs Quantum-Well Wires

In the effective-mass approximation, using a simple two-parameter wave function and a one-dimensional (1D) equivalent potential model, we calculate variationally the binding energy of an exciton bound to a neutral donor (D⁰,X) in finite GaAs-Al_xGa_1-xAs quantum well wires (QWWs). At the wire width of 25 Å, the binding energy has a peak value, which is also at the position of the peak of the exciton binding energy, and the center-of-mass wave functions of excitons reaches the most centralized distribution. In addition, the changing tendency of the average interparticle distance as the wire width is reverse to that of the binding energy.     

Theoretical study of indirect excitons’ lifetime in coupled AlGaN/GaN quantum wells in the presence of an electrostatic trap

The lifetime of electrostatically trapped indirect excitons in a field-effect structure based on coupled AlGaN/GaN quantum wells has been theoretically studied. Within the plane of a double quantum well, indirect excitons are trapped between the surfaces of the AlGaN/GaN heterostructures and a semitransparent metallic top gate. The trapping mechanism has been assumed to be a combination of the quantum confined Stark effect and local field enhancement. In order to study the trapped exciton lifetime, the binding energy of indirect excitons in coupled quantum wells is calculated by finite difference method in the presence of an electric field. Thus, the lifetime of trapped excitons is computed as a function of well width, AlGaN barrier width, the position of double quantum well in the device and applied voltage.     

On the type of a phase transition in the system of exciton polaritons in an optical microcavity

The type of a phase transition in the quasi-equilibrium system of exciton polaritons in a two-dimensional optical microcavity has been analyzed. It has been shown that, although the system contains two types of bosons undergoing mutual transformations into each other, only one phase transition to the superfluid state with the quasilong-range order occurs in the two-dimensional system. This phase transition is a Kosterlitz-Thouless phase transition. A new physical implementation—excitons in a photon crystal—has been proposed for the Bose condensation of exciton polaritons. The superfluid properties of the ordered phase are discussed, and the superfluid density and Kosterlitz-Thouless transition temperature have been calculated in the low-density approximation.     

Radiative life time of an exciton confined in a strained GaN/Ga1-xAlxN cylindrical dot: built-in electric field effects

The binding energy of an exciton in a wurtzite GaN/GaAlN strained cylindrical quantum dot is investigated theoretically.The strong built-in electric field due to the spontaneous and piezoelectric polarizations of a GaN/GaAlN quantum dot is included.Numerical calculations are performed using a variational procedure within the single band effective mass approximation.Valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions.The exciton oscillator strength and the exciton lifetime for radiative recombination each as a function of dot radius have been computed.The result elucidates that the strong built-in electric field influences the oscillator strength and the recombination life time of the exciton.It is observed that the ground state exciton binding energy and the interband emission energy increase when the cylindrical quantum dot height or radius is decreased,and that the exciton binding energy,the oscillator strength and the radiative lifetime each as a function of structural parameters (height and radius) sensitively depend on the strong built-in electric field.The obtained results are useful for the design of some opto-photoelectronic devices.     

垂直耦合自组织InAs双量子点中激子能的计算

在有效质量近似条件下研究了垂直耦合的自组织InAs/GaAs量子点的激子态.在绝热近似条件下，采用传递矩阵方法计算了电子和空穴的能谱.通过哈密顿量矩阵的对角化，对电子和空穴间的库仑相互作用进行了精确处理.讨论了两量子点间的垂直距离对激子基态能的影响.从基态波函数概率分布的角度，讨论了激子的束缚能.计算了重空穴和轻空穴激子的基态能随外部垂直磁场变化的函数关系.计算了量子点大小（量子点半径）对激子能的影响. 关键词： 量子点 激子 对角化     

Stark effect of negatively and positively charged excitons in semiconductor quantum wells

We study the effect of an electric field applied normal to the layers on the binding energy of charged excitons (or trions) in GaAs quantum wells. We find that, in contrast to the neutral exciton, their binding energy is sharply reduced by modest electric fields. The effect is stronger for the positively charged exciton than the negatively charged one. The ionisation of the excess carrier is explained by the field-induced polarisation of the electron and hole subband wave functions.     

EFFECT OF DIELECTRIC CONSTANT ON THE EXCITON GROUND STATE ENERGY OF CdSe QUANTUM DOTS

The B-spline technique is used in the calculation of the exciton ground state energy based on the effective mass approximation (EMA) model. The exciton is confined in CdSe microspherical crystallites with a finite-height potential wall (dots). In this approach, (a) the wave function is allowed to penetrate to the outside of the dots; (b) the dielectric constants of the quantum dot and the surrounding material are considered to be different; and (c) the dielectric constant of the dots are size-dependent. The exciton energies as functions of radii of the dots in the range 0.5-3.5 nm are calculated and compared with experimental and previous theoretical data. The results show that: (1) The exciton energy is convergent as the radius of the dot becomes very small. (2) A good agreement with the experimental data better than other theoretical results is achieved. (3) The penetration (or leaking) of the wave function and the difference of the dielectric constants in different regions are necessary for correcting the Coulomb interaction energy and reproducing experimental data. (4) The EMA model with B-spline technique can describe the status of excition confined in quantum dot very well.     

Enhancement of the Coulomb correlations in type-II quantum dots

We report on calculation of binding energies of excitons as well as positively and negatively charged excitons and biexcitons in type-II quantum dots. The shape of the GaSb/GaAs quantum dot is assumed lens-like and the energies are calculated within the Hartree–Fock approximation. A large enhancement of the binding energies has been estimated in comparison with the type-I quantum dots (InAs/GaAs) which is in good agreement with the recent experimental findings.     

Phase transition in a two-dimensional dipole-oriented exciton system

A dipole-oriented two-dimensional exciton system in electrically biased GaAs/AlGaAs coupled quantum wells has been studied through photoluminescence. The system has a sample-dependent built-in random potential which traps excitons at low temperature. The average photoluminescence photon energy shows a sudden reduction when the excitation intensity exceeds a critical value at low temperatures. This suggests a phase transition from a Bose glass to superfluid phase.     

The neutron scattering and exciton spectrum due to impurities in the molecular crystals with the resonance interaction

The differential cross-section of neutron scattering by excitons and the frequency equation for exciton chain with point impurity are obtained by using the exact solution, received in our previous papers [1, 2] for the case of ideal exciton chain problem. All results are compared with those given by the known approximate solution of the same problem [4] based on the approximation of Pauli's operators of creation and annihilation of excitons by Bose's ones.     

Low-temperature photoluminescence of MBE-grown GaAs subject to an electric field

Low-temperature photoluminescence of GaAs has been investigated in MBE-grown Al _x Ga_1–x As-GaAs single heterojunctions subject to an electric field. No peak energy shift is observed in the emission lines due to free excitons and excitons bound to isolated centers when the electric field is applied. In contrast, the excitonic lines arising from the previously described defect-induced bound exciton (DIBX) transitions exhibit a prominent low-energy shift when the electric field is increased. We attribute these lines to excitons bound to acceptor pairs. The excitons bound to distant pairs have smaller binding energies than those bound to closer pairs. They are, therefore, easily dissociated in a weak electric field. The electrons and holes thus dissociated may again be trapped by closer pairs, which results in a low-energy shift of the overall spectrum. The photocurrent measured as a function of the electric field supports Dingle's rule for the valence bandedge discontinuity.