Classical and quantum hall effect measurements in GaInNAs/GaAs quantum wells

We have performed magneto-transport experiments in modulation-doped Ga_0.7In_0.3N_yAs_1−y/GaAs quantum wells with nitrogen mole fractions 0.4%, 1.0% and 1.5%. Classical magnetotransport (resistivity and low-field Hall effect) measurements have been performed in the temperatures between 1.8 and 275 K, while quantum Hall effect measurements in the temperatures between 1.8 and 47 K and magnetic fields up to 11 T.The variations of Hall mobility and Hall carrier density with nitrogen mole fractions and temperature have been obtained from the classical magnetotransport measurements. The results are used to investigate the scattering mechanisms of electrons in the modulation-doped Ga_0.7In_0.3N_yAs_1−y/GaAs quantum wells. It is shown that the alloy disorder scattering is the major scattering mechanism at investigated temperatures.The quantum oscillations in Hall resistance have been used to determine the carrier density, effective mass, transport mobility, quantum mobility and Fermi energy of two-dimensional (2D) electrons in the modulation-doped Ga_0.7In_0.3N_yAs_1−y/GaAs quantum wells. The carrier density, in-plane effective mass and Fermi energy of the 2D electrons increases when the nitrogen mole fraction is increased from y=0.004 to 0.015. The results found for these parameters are in good agreement with those determined from the Shubnikov-de Haas effect in magnetoresistance.     

Field effect as a method for controlling the quality of i-InP-based heteronanostructures with two-dimensional electron gases

The field effect in i-InP-based heteronanostructures has been studied in InP and In_0.52Al_0.48As layers with an In _x Ga_1?x As quantum well with a high Hall electron mobility in the frequency range of (20?2) × 10⁶ Hz and temperature range of 77?C450 K. It is shown that mobility in the field effect depends on the location of Si ??-doped areas with respect to the quantum well and on the doping level. The ??-doping level and the configurations of nanostructures have been determined for the case in which the values of the electron mobility in the field and Hall effects are close to each other and correspond to high values of mobility in the quantum well.     

Si/Si1−xGex/Si-based quantum wells infrared photodetector operating at 1.55 μm

A quantum well infrared photodetector consisting of self-assembled type II SiGe/Si based quantum wells operating around 1.55 μm at room temperature has been investigated. The Si_1−yGe_y/Si/Si_1−xGe_x/Si/Si_1−yGe_y stack results in a ‘W’ like profiles of the conduction and valence bands strain-compensated in the two low absorption windows of silica fibers infrared photodetectors have been proposed. Such computations have been used for the study of the p-i-n infrared photodetectors operating, around (1.3–1.55 μm) at room temperature. The quantum transport properties of electrons and holes were approved with Schrödinger and kinetic equations resolved self-consistently with the Poisson equation. The theoretical performances of the photodetector were carried out such as the dark current mechanisms, the temperature dependence of normalized dark current and the zero-bias resistance area product (R₀A).     

Band edge offsets in strained (InGa)As-(AlGa)As heterostructures

The excitonic transitions between the ground electron and hole quantum well sublevels in strained In_xGa_1-xAs-Al_yGa_1-yAs multiple quantum well structures (x = 0.12−0.35 and y = 0.2−0.35) have been investigated by means of photoluminescence and photoconductivity measurements. The molecular beam epitaxy grown structures contained an Al_yGa_1-yAs matrix with one unstrained GaAs and three strained In_xGa_1-xAs quantum wells one of which was in the GaAs cladding layers. The ratio of the conduction band edhe line up to the band gap offset for the strained In_xGa_1-xAs-unstrained Al_yGa_1-yAs interface has been found to be 0.67 ± 0.08 for the studied regions of x and y.     

Impurity magneto-optical absorption with the participation of resonance states of D 2 − centers in quantum wells

The dependence of the average binding energy of the resonance g-state of a D ₂ ^? center on the induction of an external magnetic field in a quantum well with a parabolic confining potential is studied using the zero-range potential method. It has been shown that with an increasing exchange interaction, the character of the dependence of the average binding energy of the resonance g-state of the D ₂ ^? center on the induction of the external magnetic field changes. It has been assumed that in GaAs/AlGaAs quantum wells alloyed with small Si donors, resonance D ₂ ^? states can exist under conditions of exchange interaction. It has been found that in spectra of impurity magneto-optical absorption in multiwall quantum structures, exchange interaction manifests itself as oscillations of interference origin.     

Characteristics of degenerated four wave mixing in ZnS/CdSe/ZnS quantum dot quantum well with multi-shell structure

The wave functions and eigenenergies of electrons in ZnS/CdSe/ZnS cylindrical quantum dot quantum well (QDQW) have been calculated by solving a three-dimensional nonlinear Schrödinger equation, in the framework of the effective-mass envelope-function theory. The third-order susceptibilities of the degenerated four waves mixing (DFWM) have been calculated theoretically by means of compact density matrix. The third-order susceptibilities as the function of the shell radius R₂, R₃ have been analyzed. The results show that the magnitude of nonlinear susceptibility is increased with the increasing of well radius. The resonance frequency of the photon have a shift when R₂ or R₃ is increasing and the relation between nonlinear susceptibility and relaxation time has also been studied.     

Resonant Raman scattering in nanostructures with InGaAs/AlAs quantum dots

Raman scattering by optical phonons in In_xGa_{1 ? x} As/AlAs nanostructures with quantum dots has been studied experimentally for compositions corresponding to x = 0.3?1 under out-resonance conditions. Features due to scattering by GaAs-and InAs-like optical phonons in quantum dots have been detected, and the phonon frequencies have been determined as a function of the dot composition. With increasing excitation energy, a red shift is observed in the frequency of the GaAs-like phonon in quantum dots, which testifies to Raman scattering selective by the size of quantum dots. Under resonant conditions, multiphonon light scattering by optical and interface phonons is observed up to the third order, including overtones of the first-order phonons of InGaAs and AlAs materials and their combinations.     

Role of dipole image forces in molecular adsorption

The transverse-field XY model in one dimension is a well-known spin model for which the ground state properties and excitation spectrum are known exactly. The model has an interesting phase diagram describing quantum phase transitions (QPTs) belonging to two different universality classes. These are the transverse-field Ising model and the XX model universality classes with both the models being special cases of the transverse-field XY model. In recent years, quantities related to quantum information theoretic measures like entanglement, quantum discord (QD) and fidelity have been shown to provide signatures of QPTs. Another interesting issue is that of decoherence to which a quantum system is subjected due to its interaction, represented by a quantum channel, with an environment. In this paper, we determine the dynamics of different types of correlations present in a quantum system, namely, the mutual information I(?? _AB ), the classical correlations C(?? _AB ) and the quantum correlations Q(?? _AB ), as measured by the quantum discord, in a two-qubit state. The density matrix of this state is given by the nearest-neighbour reduced density matrix obtained from the ground state of the transverse-field XY model in 1d. We assume Markovian dynamics for the time-evolution due to system-environment interactions. The quantum channels considered include the bit-flip, bit-phase-flip and phase-flip channels. Two different types of dynamics are identified for the channels in one of which the quantum correlations are greater in magnitude than the classical correlations in a finite time interval. The origins of the different types of dynamics are further explained. For the different channels, appropriate quantities associated with the dynamics of the correlations are identified which provide signatures of QPTs. We also report results for further-neighbour two-qubit states and finite temperatures.     

Exciton capture and thermal escape in InAs dot-in-a-well laser structures

The photoluminescence (PL), its temperature and power dependences have been studied in InAs quantum dots (QDs) embedded in asymmetric In_xGa_1?y As/GaAs quantum wells (QWs) with variable In_xGa_1?x As compositions in the capping layer. Three stages for thermally activated decay of QD PL intensity have been revealed. A set of rate equations for exciton dynamics (relaxation into QWs and QDs, and thermal escape) are solved to analyze the mechanism of PL thermal decay. The variety of PL intensities and peak positions, as well as the activation energies of PL intensity decay in DWELL structures with different compositions of a capping layer are discussed.     

Impurity effects on the magnetization and magnetic susceptibility of an electron confined in a quantum ring under the presence of an external magnetic field

The magnetization and the magnetic susceptibility of a single electron confined in a two-dimensional (2D) parabolic quantum ring under the effect of external uniform magnetic field and in the presence of an acceptor impurity have been studied. The shifted 1/N expansion method was used to solve the Hamiltonian quantum ring within the effective mass approximation. The computed energy spectra, the magnetization and magnetic susceptibility have been displayed as a function of the quantum ring parameters: confinement strength ω₀, magnetic field strength (ω_c), and temperature (T). The obtained energy results show level-crossings, in the absence and presence of acceptor impurity, which are manifested as oscillations in the magnetization and magnetic susceptibility curves.     

Diamagnetic susceptibility of hydrogenic donor impurity in low-dimensional semiconducting systems

The binding energies of a hydrogenic donor both in the parabolic and non-parabolic conduction band model within the effective mass approximation have been computed for the low-dimensional semiconducting systems (LDSS) like quantum well, quantum well wire and quantum dot taking GaAs/Al_xGa_1−xAs systems as an example. It is observed that the effect of non-parabolicity is not effective when the system goes to lower dimensionality. The diamagnetic susceptibility of a hydrogenic donor impurity has also been computed in these LDSS in the infinite barrier model. Since no theoretical or experimental works on the diamagnetic susceptibility of LDSS are available in the literature, as a realistic case the diamagnetic susceptibility has been computed in the finite barrier model (x=0.3) for a quantum well and the results are discussed in the light of semiconductor-metal transition.     

Microwave photoresistance of a double quantum well at high filling factors

The effect of millimeter wave radiation on the electronic transport in a GaAs double quantum well at a temperature of 4.2 K in a magnetic field of up to 2 T has been studied. Resistance (conductance) oscillations have been shown to appear in the two-dimensional electronic system under investigation at high filling factors. The magnetic field positions of the oscillation maxima are determined by the condition Δ_SAS/? = lω_c, where Δ_SAS = (E ₂ ? E ₁) is the size quantization sublevel splitting in the quantum well, ω_c is the cyclotron frequency, and l is a positive integer. It has been found that the microwave field substantially modifies the oscillations in the double quantum well, which results in alternating two-frequency oscillations of photoresistance with the inverse magnetic field.     

Subband transitions in AlxGa1−xN /GaN/ AlxGa1−xN and AlxGa1−xN /InN/ AlxGa1−xN single quantum wells

We have calculated the spectral regime of subband transitions in Al_xGa_1−xN/GaN and Al_xGa_1−xN/InN single quantum wells. We used a simplified model to account for the internal electric fields, which modify the shape of the quantum well. Some of the parameters for these materials have not yet been firmly established. Therefore, we carried out the analysis for the extremes of the reported values of conduction band discontinuities and band gaps (in the case of InN). This analysis shows that the spectral regime of interband transitions for 1–4 nm thick wells has wavelengths above 0.5 μm for AlGaN/InN and above 0.8 μm for AlGaN/GaN and both heterostructures cover several μm wavelengths. The spectral variation with alloy composition is less pronounced in the Al_xGa_1−xN/InN single quantum wells due to the higher electric field present across the InN quantum well as compared to GaN. The results of these calculations are in good agreement with more rigorous theoretical approaches and available experimental values for Al_xGa_1−xN/GaN.     

Confinement of polar optical phonons in quasi-one-dimensional Wurtzite GaN-based quantum well wires: propagating and half-space phonon modes

With the aid of the macroscopic dielectric continuum and Loudon’s uniaxial crystal models, the propagating (PR) and half-space (HS) optical phonon modes and corresponding Fröhlich-like electron-phonon interaction Hamiltonians in a quasi-one-dimensionality (Q1D) wurtzite quantum well wire (QWW) structure are derived and studied. Numerical calculations on a wurtzite GaN/A_l0.15Ga_0.85N QWW are performed, and discussion is focused mainly on the dependence of the frequency dispersions of PR and HS modes on the free wave-number k _z in the z-direction and on the azimuthal quantum number m. The calculated results show that, for given k _z and m, there usually exist infinite branches of PR and HS modes in the high-frequency range, and only finite branches of HS modes in the low-frequency range in wurtzite QWW systems. The reducing behaviors of the PR modes to HS modes, and of the HS mode to interface phonon mode have been observed clearly in Q1D wurtzite heterostructures. Moreover, the dispersive properties of the PR and HS modes in Q1D QWWs have been compared with those in Q2D quantum well structures. The underlying physical reasons for these features have also been analyzed in depth.     

Influence of quantum confinement on the photoemission from superlattices of optoelectronic materials

We study the photoemission from quantum wire and quantum dot superlattices with graded interfaces of optoelectronic materials on the basis of newly formulated electron dispersion relations in the presence of external photo-excitation. Besides, the influence of a magnetic field on the photoemission from the aforementioned superlattices together with quantum well superlattices in the presence of a quantizing magnetic field has also been studied in this context. It has been observed taking into account HgTe/Hg_1−xCd_xTe and In_xGa_1−xAs/InP that the photoemission from these nanostructures increases with increasing photon energy in quantized steps and exhibits oscillatory dependences with the increase in carrier concentration. Besides, the photoemission decreases with increasing light intensity and wavelength, together with the fact that said emission decreases with increasing thickness exhibiting oscillatory spikes. The strong dependences of the photoemission on the light intensity reflects the direct signature of light waves on the carrier energy spectra. The content of this paper finds six applications in the fields of low dimensional systems in general.     

A scheme of quantum repeaters with single atom and cavity-QED

To implement long-distance quantum communication, quantum repeaters have been proposed. The distribution and storage of quantum entanglement are essential to implement quantum repeaters. Here, we propose a new quantum repeaters protocol which is based on single atom-cavity QED. We use simple long-life two-level atoms to store quantum entanglement unlike three-level atoms which are commonly used in other quantum repeaters proposals. The property of long life-time (T₁) and transverse decay time (T₂) between excited level and ground level, such as rare-earth atoms, may store quantum entanglement as long as possible. Modulations of cavity mode and rate of coupling between cavity mode and output mode are also key steps to our scheme. And the efficiency of our protocol is analyzed by quantum trajectory theory.     

Temperature dependent dynamic of excitons in Zn1 − xCdxSe/ZnSe multiple quantum wells

The exciton dynamics in Zn_1 − xCd_xSe/ZnSe multiple quantum wells have been investigated by temperature-dependent time-resolved photoluminescence experiments. A polariton effect (leading to a linearT-dependence of the decay time τ_PL) and thermal escape of carriers from the quantum well at relatively high temperature (resulting in a decrease of τ_PLwith the temperature) are observed in samples of different stoichiometry and well.     

Energy levels of single nonabrupt GaAs/AlxGa1-xAs quantum wells

Energy levels of electrons in nonabrupt GaAs/Al_xGa_1-xAs single quantum wells are calculated with and beyond the constant interfacial effective mass approximation (CIEMA), and compared with those of abrupt GaAs/Al_xGa_1-xAs quantum wells. For a given interface width, the energy levels calculated with the CIEMA are higher than those calculated beyond it, but both are higher than those of the abrupt semiconductor quantum well. The shifts of the energy levels increase with the interfacial width of the nonabrupt quantum well, as well as with the degree of interfacial asymmetry.     

A comparison between optically active CdZnSe/ZnSe and CdZnSe/ZnBeSe self-assembled quantum dots: effect of beryllium

The composition and size of optically active Cd_xZn_1−xSe/ZnSe quantum dots are estimated with a previously developed method. The results are then compared with those obtained for Cd_xZn_1−xSe/Zn_0.97Be_0.03Se QDs. We show that introducing Be into the barrier material enhances both Cd composition and quantum size effect of optically active quantum dots.     

Dynamics of spins in semiconductor quantum wells under drift

The dynamics of spins in semiconductor quantum wells under applied electric bias has been investigated by photoluminescence (PL) spectroscopy. The bias-dependent polarization of PL (P_PL) was measured at different temperatures. The P_PL was found to decay with an enhancement of increasing the strength of the negative bias, with an exception occurred for a low value of the negative bias. The P_PL was also found to depend on the temperature. The P_PL in the presence of a transverse magnetic field was also studied. The results showed that P_PL in the magnetic field oscillates under an applied bias, demonstrating that the dephasing of electron spin occurs during the drift transport in semiconductor quantum wells.