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In this work, carrier transport in a gold-seeded zinc diphosphide nanowire fabricated by vapor–liquid–solid and photolithography techniques is investigated in detail.

This paper discusses the combined effects of optical excitation power, interface roughness, lattice temperature, and applied magnetic fields on the spin coherence of excitonic states in GaAs/AlGaAs multiple quantum wells. For low optical powers, at lattice temperatures between 4 and 50 K, the scattering with acoustic phonons and short-range interactions appear as the main decoherence mechanisms. Statistical fluctuations of the band gap, however, become also relevant in this regime and we were able to deconvolute them from the decoherence contributions. The circularly polarized magneto-photoluminescence unveils a nonmonotonic tuning of the coherence for one of the spin components at low magnetic fields. This effect has been ascribed to the competition between short-range interactions and spin-flip scattering, modulated by the momentum relaxation time., This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Brasil (CAPES), Finance Code 001. The authors also acknowledge financial support from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Grants No. 2013/18719-1, No. 2014/19142-2, and No. 2018/01914-0, Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and National Science Foundation (NSF) Grant No. OIA-1457888. This work was also partially funded by Spanish MICINN under Grant No. PID2019-106088RB-C3. This work is also supported by European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 956548, project Quantimony.

The control of native defects in the ZnO material is strongly important for a wide range of technological applications. In this paper, native defects are tuned via the post-thermal treatment of ZnO films in a high vacuum atmosphere. The microstructure of the as-grown ZnO film shows columnar growth and strongly polar-oriented grains along the c-plane (002). Also, the obtained results indicate that the as-grown film contains a high amount of intrinsic defects and strong lattice distortions. After the thermal annealing, the ZnO films display significant structural changes, which are reflected in their electrical, vibrational, and optical properties. Our findings suggest that these changes were attributed to the selective cleanup effect of the native defects and the partial deoxidation process mainly on the exposed particle surface (at high temperatures) tuned up by the thermal annealing temperature. According to DFT calculations, oxygen vacancies (V O ) show lower energy, followed by zinc vacancies (V Zn ) and oxygen interstitials (O i ) indicating that V O defect is the most stable in ZnO. That sequence of stability could suggest the sequence of the annihilation of those defects, which is in line with our experimental findings and also in agreement with literature results.

We study the magneto-transport and magneto-electroluminescence properties of purely n-doped GaAs/Al$_{0.6}$Ga$_{0.4}$As resonant tunneling diodes with an In$_{0.15}$Ga$_{0.85}$As quantum well and emitter prewell. Before the resonant current condition, magneto-transport measurements reveal charge carrier densities comparable for diodes with and without the emitter prewell. The Landau level splitting is observed in the electroluminescence emission from the emitter prewell, enabling the determination of the charge carrier build-up. Our findings show that magneto-electroluminescence spectroscopy techniques provide useful insights on the charge carrier dynamics in resonant tunneling diodes and is a versatile tool to complement magneto-transport techniques. This approach will drive the way for developing potentially more efficient opto-electronic resonant tunneling devices, by e.g., monitoring voltage dependent charge accumulation for improving built-in fields and hence to maximize photodetector efficiency and/or minimize optical losses., 9 pages, 7 figures

A shape-selective preparation method was used to obtain highly crystalline rod-, needle-, nut-, and doughnut-like ZnO morphologies with distinct particle sizes and surface areas. We study the nucleation and growth mechanism of those structures and the influence of physical-chemical parameters, such as the solvent and the pH of the solution, on the morphology, as well as the structural and optical properties. A clear correlation between the growth rate along the c-axis and surface defects was established. Our results suggest that the needle- and rod-like morphologies are formed due to the crystal growth orientation along the c-axis and the occurrence of crystalline defects, such as oxygen vacancies and interstitial Zn2+ located at the surface, whereas nuts and doughnuts are formed due to growth along all crystalline planes except those related to growth along the c-axis. Based on the experimental results, growth mechanisms for the formation of ZnO structures were proposed. We believe this synthetic route will be of guidance to prepare several materials whose shapes will depend on the desired applications.

In this work, we present a coupled experimental and theoretical first-principles investigation on one of the more promising oxide-diluted magnetic semiconductors, the Sn

A comprehensive study of excitonic complexes in high-quality GaAs quantum wells has been performed. The polarization-resolved photoluminescence results show exciton, biexciton, and trion emissions tuned by temperature, excitation power, and by an external magnetic field. Excitons and biexcitons show important differences in their dependence on the external field. While the Zeeman splitting of biexcitons depends monotonically on the magnetic field, with a nearly constant g-factor, the behaviour of the exciton energy splitting is nonmonotonic and includes an inversion of sign as a function of the magnetic field. More importantly, we observe that a trion resonance emerges at a finite magnetic field, and only appears at low laser excitation power and for σ+ polarization emission. The non-trivial dependence of the energy levels on the external magnetic field, together with the polarization selective trion emission, leads to an intricate exciton and trion dynamics, which can be reproduced by a set of coupled rate equations. Based on this theoretical approach, we show that a fast spin-flip process can lead to the spin-asymmetric emission, which reveals a contrasting spin-dynamics of excitonic complexes ruled by their charge.

Made available in DSpace on 2020-12-10T19:52:01Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-02-14 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Brazilian Agency Fundacao de Amparo a Pesquisa do Distrito Federal (FAP/DF) In this work, we present a coupled experimental and theoretical first-principles investigation on one of the more promising oxide-diluted magnetic semiconductors, the Sn1-xCoxO2 nanoparticle system, in order to see the effect of cobalt doping on the physical and chemical properties. Our findings suggest that progressive surface enrichment with dopant ions plays an essential role in the monotonous quenching of the surface disorder modes. That weakening is associated with the passivation of the oxygen vacancies as the Co excess at the surface becomes larger. Room-temperature Sn-119 Mossbauer spectroscopy data analysis revealed the occurrence of a distribution of isomer shifts, related to the different non-equivalent surroundings of Sn4+ ions and the coexistence of Sn2+/Sn4+ at the particle surfaces provoked by the inhomogeneous distribution of Co ions, in agreement with the X-ray photoelectron spectroscopy measurements. Magnetic measurements revealed a paramagnetic behavior of the Co ions dispersed in the rutile-type matrix with antiferromagnetic correlations, which become stronger as the Co content is increased. Theoretical calculations show that a defect with two Co mediated by a nearby oxygen vacancy is the most likely defect. The predicted effects of this defect complex are in accordance with the experimental results. Univ Brasilia, Inst Fis, Nucleo Fis Aplicada, BR-70910900 Brasilia, DF, Brazil Univ Nacl San Agustin Arequipa, Lab Peliculas Delgadas, Escuela Profes Fis, Av Independencia S-N, Arequipa, Peru Univ Estadual Paulista, Dept Fis, IGCE, BR-13506900 Rio Claro, SP, Brazil Univ Fed Sao Carlos, Ctr Ciencias Exatas & Tecnol, Dept Fis, BR-13565905 Sao Carlos, SP, Brazil Univ Sao Paulo, Inst Fis, BR-05508090 Sao Paulo, Brazil CDTN, BR-3127901 Belo Horizonte, MG, Brazil Univ Estadual Paulista, Dept Fis, IGCE, BR-13506900 Rio Claro, SP, Brazil CNPq: 305.647/2015-6 FAPESP: 2016/21371-5 FAPESP: 2014/19142-2 FAPESP: 2017/02317-2 Brazilian Agency Fundacao de Amparo a Pesquisa do Distrito Federal (FAP/DF): 0193.001.345/2016 Brazilian Agency Fundacao de Amparo a Pesquisa do Distrito Federal (FAP/DF): 0193001.196/2016

GaAsBi/GaAs double quantum well (DQW) heterostructures are grown at low temperatures using molecular beam epitaxy without growth interruption and studied by means of high-resolution x-ray diffraction and continuous wave photoluminescence (PL). Line shape analysis of PL spectra measured at various excitation densities allows reveals QW coupling due to tunnelling. It is shown that an efficient PL of GaAs/GaAs 1−x Bi x /GaAs DQWs is significantly more thermally stable than that from a single quantum well (SQW) structure of similar composition. Such stabilization is described in terms of carrier redistribution between coupled quantum wells and is caused by carrier capture in the QW, thermal emission, and diffusion in the barrier.

A magnetophotoluminescence study of the carrier transfer with hybrid InAs/GaAs-quantum-dot-InGaAs-quantum-well structures is carried out where we observe an unusual dependence of the photoluminescence (PL) on the GaAs barrier thickness at strong magnetic field and excitation density. For the case of a thin barrier the quantum-well (QW) PL intensity is observed to increase at the expense of a decrease in the quantum-dot (QD) PL intensity. This is attributed to changes in the interplane carrier dynamics in the QW and the wetting layer (WL) resulting from increasing the magnetic field along with changes in the coupling between QD excited states and exciton states in the QW and the WL., The authors gratefully acknowledge the financial support of the following agencies: National Science Foundation of the U.S. (Grants No. DMR1008107 and No. DMR-1108285),CSIC grant ICOOP-2017-COOPB20320, MINEICO grant EUIN2017- 88844, FAPESP (grants # 2013/18719-1, 2014/07375- 2, 2014/19142-2, 2018/01914-0), CNPq(164765/2018-2) and CAPES (Finance Code 001).

Structural and electronic changes induced by laser irradiation are currently of interest owing to the possibility to tune the mechanical, optical, and transport properties of the irradiated materials. In this work, we investigate the effects of laser irradiation on indium phosphide, InP, by modifying the electronic temperature, T e, of the system within the density functional theory framework and performing molecular dynamics simulations to prove that the laser irradiation also provokes a local thermalization effect. We found that the process can be described by a two-stage mechanism. First, at low T e values (0–1.0 eV), the laser energy induces electronic transitions, while the InP lattice remains undisturbed and cool. In the second stage (with T e in the range of 1.0–4.0 eV), both electron-electron scattering and electron-phonon coupling processes are triggered, increasing the energy of the lattice so as to provoke a Coulomb explosion, which changes some physical chemical properties of InP. The close agreement between the simulations helps explain the formation of metallic In as it is observed in the transmission electron microscopy images.

We explore the nature of the electroluminescence (EL) emission of purely $n$-doped GaAs/AlGaAs resonant tunneling diodes (RTDs) and the EL evolution with voltage. A singular feature of such a device is unveiled when the electrical output current changes from higher to lower values and the EL on-off ratio is enhanced by two orders of magnitude compared to the current on-off ratio. By combining the EL and the current properties, we are able to identify two independent impact ionization channels associated with the coherent resonant tunneling current and the incoherent valley current. We also perform the same investigation with an associated series resistance, which induces a bistable electrical output in the system. By simulating a resistance variation for the current voltage and the EL, we are able to tune the EL on-off ratio by up to six orders of magnitude. We further observe that the EL on and off states can be either direct or inverted compared to the tunneling current of the on and off states. This electroluminescence, combined with the unique RTD properties, such as the negative differential resistance and high-frequency operation, enables the development of high-speed functional optoelectronic devices and optical switches.

We report the energy spectrum and the eigenstates of conduction and uncoupled valence bands of a quantum well under the influence of a tilted magnetic field. In the framework of the envelope approximation, we implement two analytical approaches to obtain the nontrivial solutions of the tilted magnetic field: (a) the Bubnov-Galerkin spectral method and b) the perturbation theory. We discuss the validity of each method for a broad range of magnetic field intensity and orientation as well as quantum well thickness. By estimating the accuracy of the perturbation method, we provide explicit analytical solutions for quantum wells in a tilted magnetic field configuration that can be employed to study several quantitative phenomena.

We investigate how the voltage control of the exciton lateral dipole moment induces a transition from singly to doubly connected topology in type-II InAs/GaAsxSb1−x quantum dots. The latter causes visible Aharonov-Bohm oscillations and a change of the exciton g factor, which are modulated by the applied bias. The results are explained in the frame of realistic →k⋅→p and effective Hamiltonian models and could open a venue for new spin quantum memories beyond the InAs/GaAs realm., The authors gratefully acknowledge financial support from EURAMET through EMPIR program 17FUN06-SIQUST, from Spanish MINEICO through Grants No. TEC2015-64189-C3-2-R, No. MAT2016-77491-C2-1-R, No. EUIN2017-88844, and No. RYC-2017-21995, from Comunidad de Madrid through Grant No. P2018/EMT-4308, and from CSIC through Grants No. I-COOP-2017-COOPB20320 and No. PTI-001. Support from Brazilian agencies is also acknowledged through FAPESP Grant No. 2014/02112-3 and CNPq Grant No. 306414/2015-5.

The study of the interaction of surface plasmon polaritons (SPPs) with quantum emitters has become very important in the last few years. The ability to design optical devices as well as investigate the physics of strongly interacting systems is some of its useful applications. In this paper, we will show some results on the decay of excitons confined in a InAs/GaAs quantum dot into SPP modes confined in a metallic thin film made of Au, an important step toward the investigation of the basic features of the SPP–exciton interaction. The lifetime of the ground state level was investigated and shown to decrease with the presence of the metallic film.

General expressions for the electron- and hole-acoustical-phonon deformation potential Hamiltonian (H_{E-DP}) are derived for the case of Ge/Si and Si/Ge core/shell nanowire structures (NWs) with circular cross section. Based on the short-range elastic continuum approach and on derived analytical results, the spatial confined effects on the vector phonon displacement, the phonon dispersion relation and the electron- and hole-phonon scattering amplitudes are analyzed. It is shown that the acoustical vector displacement, phonon frequencies and H_{E-DP} present mixed torsional, axial, and radial components depending on the angular momentum quantum number and phonon wavector under consideration. The treatment shows that bulk group velocities of the constituent materials are renormalized due to the spatial confinement and intrinsic strain at the interface. The role of insulating shell on the phonon dispersion and electron-phonon coupling in Ge/Si and Si/Ge NWs are discussed.

This work reports evidence of the induced migration of Mn 2+ ions in Cd (1− x ) Mn x S nanocrystals (NCs) by selecting a specific thermal treatment for each sample. The growth and characterization of these magnetic dots were investigated by atomic force microscopy (AFM), optical absorption (OA), and electronic paramagnetic resonance (EPR) techniques. The comparison of experimental and simulated EPR spectra confirms the incorporation of Mn 2+ ions both in the core and at the dot surface regions. The thermal treatment of a magnetic sample, via selected annealing temperature and/or time, affects the fine and hyperfine interaction constants which modify the shape and the intensity of the EPR transition spectrum. The identification of these changes has allowed tracing the magnetic ion migration from core to surface regions of a dot as well as inferring the local density of the magnetic impurity ions.

We calculate the spin relaxation rates in InAs and GaAs parabolic quantum dots due to the interaction of spin carriers with acoustical phonons. We consider a spin relaxation mechanism completely intrinsic to the system, since it is based on the modulation of the spin–orbit interaction by the acoustic phonon potential, which is independent of any structural properties of the confinement potential. The electron–phonon deformation potential and the piezoelectric interaction are described by the Pavlov–Firsov spin–phonon Hamiltonian. Our results demonstrate that, for narrow-gap semiconductors, the deformation potential interaction becomes dominant. This behavior is not observed for wide or intermediate gap semiconductors, where the piezoelectric coupling, in general, governs the relaxation processes. We also demonstrate that the spin relaxation rates are particularly sensitive to values of the Lande g -factor, which depend strongly on the spatial shape of the confinement.

We describe the effects of spin–orbit contribution in the presence of a uniform axial electric field on the electronic structure of single electron quantum rings in addition to the conventional Stark effect. The spin-split levels are obtained by considering the spatial asymmetries induced by the Rashba and the Dresselhaus spin–orbit interactions on the spinor states. The quantum-ring flat geometry allows us to study the spin-hybridization effects induced by spin–orbit coupling under a wide range of values of the axial electric field. The influence of the peculiar axial and perpendicular confinements in the structure on the spin orientation, the azimuthal component of the angular momentum and their coupling to the external fields is discussed in details for two materials.

We study the damping of the collective excitations of Bose-Einstein condensates in a harmonic trap potential loaded in an optical lattice. In the presence of a confining potential the system is non-homogeneous and the collective excitations are characterized by a set of discrete confined phonon-like excitations. We derive a general convenient analytical description for the damping rate, which takes into account, the trapping potential and the optical lattice, for the Landau and Beliaev processes at any temperature, $T$. At high temperature or weak spatial confinement, we show that both mechanisms display linear dependence on $T$. In the quantum limit, we found that the Landau damping is exponentially suppressed at low temperatures and the total damping is independent of $T$. Our theoretical predictions for the damping rate under thermal regime is in completely correspondence with the experimental values reported for 1D condensate of sodium atoms. We show that the laser intensity can tune the collision process, allowing a \textit{resonant effect} for the condensate lifetime. Also, we study the influence of the attractive or repulsive non-linear terms on the decay rate of the collective excitations. A general expression of the renormalized Goldstone frequency has been obtained as a function of the 1D non-linear self-interaction parameter, laser intensity and temperature.

We study the role of different orientations of an applied magnetic field as well as the interplay of structural asymmetries on the characteristics of eigenstates in a quantum ring system. We use a nearly analytical model description of the quantum ring, which allows for a thorough study of elliptical deformations and their influence on the spin content and Berry phase of different quantum states. The diamagnetic shift and Zeeman interaction compete with the Rashba spin-orbit interaction, induced by confinement asymmetries and external electric fields, to change spin textures of the different states. Smooth variations in the Berry phase are observed for symmetric quantum rings as function of applied magnetic fields. Interestingly, we find that asymmetries induce nontrivial Berry phases, suggesting that defects in realistic structures would facilitate the observation of geometric phases.

We settle a general expression for the Hamiltonian of the electron-zone-center optical phonon deformation potential (DP) interaction in the case of nonpolar core-shell cylindrical nanowires (NWs). On the basis of a long-range phenomenological continuum model for the optical modes and by taking into account the bulk phonon dispersions, we study the size dependence and strain-induced shift on the electron-phonon coupling strengths for Ge-Si and Si-Ge NWs. We derive analytically the DP electron-phonon Hamiltonian and report some numerical results for the frequency core modes and vibrational amplitudes. Our approach allows for the unambiguous identification of the strain and confinement effects on the optical phonons at the Γ point. We explore the dependence of mode frequencies, phonon amplitudes, and hole-DP scattering rate on the spatial symmetry and the structural parameters of these core-shell structures, which constitute a basic tool for the characterization and device applications of these novel nanosystems., This work was partially supported by Spanish MINECO through Grant No. FIS2012-33521. D.G.S.-P., C.T.-G., and G.E.M. acknowledge support from the Brazilian Agencies FAPESP and CNPq. R.P.-A. acknowledges CONACyT (México) support through Grant No. 208108 and hospitality at ICMM-CSIC, Madrid, Spain.

The interplay between structural properties and charge transfer in self-assembled quantum ring (QR) chains grown by molecular beam epitaxy on top of an InGaAs/GaAs quantum dot (QD) superlattice template is analyzed and characterized. The QDs and QRs are vertically stacked and laterally coupled as well as aligned within each layer due to the strain field distributions that governs the ordering. The strong interdot coupling influences the carrier transfer both along as well as between chains in the ring layer and dot template structures. A qualitative contrast between different dynamic models has been developed. By combining temperature and excitation intensity effects, the tuning of the photoluminescence gain for either the QR or the QD mode is attained. The information obtained here about relaxation parameters, energy scheme, interlayer and interdot coupling resulting in creation of 1D structures is very important for the usage of such specific QR–QD systems for applied purposes such as lasing, detection, and energy-harvesting technology of future solar panels.

We have calculated the spin-flip relaxation rates of electrons in a single CdTe spherical quantum dot due to the interaction with acoustical phonons via macroscopic deformation potential, also called the ripple mechanism. The relaxation of spin states is possible considering that the applied magnetic field and the coupling between valence and conduction bands produce admixture, both in the electrons as well as in the hole spin states. We have demonstrated that the inclusion of spin states mixture produces an appreciable probability of spin-preserving and spin-flip transitions. The electronic states are calculated within the multiband k·p approximation and the phonon modes are described as plane-waves. We have studied the dependence of the transition rates with the magnetic field and with the dot size. Our results show a fair compatibility with available experimental data.

The aim of this work is to study the dynamic formation and dissociation of trions and excitons in double barrier resonant tunneling diodes. We propose a system of rate equations that takes into account the formation, dissociation and annihilation of these complexes inside the quantum well. From the solutions of the coupled equations, we are able to study the modulation of excitons and trions formation in the device as a function of the applied bias. The results of our model agree qualitatively with the experiments showing the viability of these rate equations system to study the dynamics of complex systems.

We have analyzed how the symmetries and the anisotropies of energy dispersions and of the spinor states, within full spin-orbit interaction, form the two independent circular spin polarizations. We also compare how the effects produced by Rashba and Dresselhauss interaction terms act on the structure of the Hilbert space. These aspects are used to envisage a voltage controlled multichannel spin-filtering regime in nonmagnetic diode structures.

We have calculated the electronic structure and optical transitions in quantum islands of special geometrical shapes, using the finite element method. In this approach, the wavefunction of a carrier is constructed as a linear combination of a basis set formed by localized functions, built up as a product of three Cartesian cubic β -splines. The variational method is used to determine the best set of coefficients for each state. Three different geometries were used to describe quantum islands in the form of lenses and a pyramid. The first case is a simple cut on a spherical island. The second situation describes a deviation from sphericity, which involves cuts in the spheroidal oblate or prolate shapes. In third situation the carrier is confined inside a pyramidal quantum dot. Single-particle transitions for GaAs/Ga 1− x Al x As and InAs/InP heterostructures, as well as for GaAs and CdTe dots surrounded by a vacuum or embedded in glass matrices, were calculated using the simple four-band model to describe the electron and hole energy levels in a zinc-blende semiconductor structure near the Γ -point.

We discuss how symmetries and anisotropies of electron states, within full spin–orbit coupling, form independent circular polarization spinors. The aspects of a voltage controlled multichannel spin-filtering regime is discussed.

We calculate the interwire element of the impurity spectral function in coupled double quantum wires at finite temperatures. Simple anisotropy in the direction perpendicular to the wires gives rise to this element, which is responsible for tunneling-induced effects. We find interwire particle–hole (single-particle) excitations contributing to the total impurity spectral function for temperatures up to 20K.

We have studied effects of symmetry on spin dependent transport in CdTe-Cd 1-x Mn x Te magnetic tunneling diodes. The tunneling and shot-noise currents may give complementary information on charging processes of the resonant levels. Optimized sample design can explore the exchange induced Zeeman splitting to obtain efficient spin-filters.

The Zeeman splitting in a single CdTe quantum dot (QD) is theoretically analyzed in the frame of Kane-Weiler 8 × 8 k · p model. The conduction g-factor was calculated as a function of the QD size. We discuss the effects of the dot geometry on the magnitude and on sign of the g-factor, that opens new channels towards control and manipulation of magneto-optical properties and spin in different confinement regimes. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have studied carrier dynamics in quantum wells subjected to strong ac-electric fields. The ac-Stark shifts were calculated within the Kane model. The inherent symmetries and couplings in Kane Hamiltonian lead to the carrier effective masses depending on the k-vector. We have predicted a mapping between energy dispersion and ac-Stark effect. An anomalous behavior of light-hole states can be associated to the different dynamical responses of carrier to the driving ac-field.

The 8 x 8 k.p Kane-Weiler model has been extended to study narrow-gap semiconductor quantum wells in the presence of a magnetic field. It is shown that the study of the inherent symmetry in this Hamiltonian model permits a suitable separation of the solution into two orthogonal Hilbert sub-spaces. The electronic properties are evaluated as function of Kane-Luttinger parameters, magnetic field strength, and Landau level number. The selection rules and anomalies in the conduction Zeeman splitting observed in the interband magneto-optical absorption spectrum are analyzed for the Faraday and Voigt geometry configurations. Due to strong intra- and interband admixtures in narrow-gap materials, spin-flip-like transitions are predicted to occur in the magneto-optical spectrum of HgCdTe/CdTe quantum well solid solutions.

The Zeeman splitting in self-assembled quantum dots (QD) has been studied in the strong confinement regime. The QD is modeled by a cap of spherical symmetry of radius a, and Dirichlet's boundary condition is assumed. Explicit expressions for the wave functions and energy levels when a magnetic field is applied perpendicular to the dot base are derived. Analytical expressions up to the fourth order in the magnetic field (B 4 ) for the energy levels and to the second order (B 2 ) for the wave functions are derived. The magnetic fan plot as a function of a solely universal parameter a = (a/l B ) 2 , where l B is the Landau length, is given.

A comprehensive microscopy analysis has been undertaken to study three-dimensional quantum dot (QD) ordering in multilayered In 0.4 Ga 0.6 As/GaAs structures grown with an As 2 flux at different substrate temperatures. Atomic force microscopy, transmission electron microscopy, and photoluminescence measurements were employed to fully understand the formation of these extended dot structures. Changes in the lateral pattern of QD ordering are correlated with their vertical alignment. These correlations are analyzed in light of the inherent transformation of the wetting and spacer layers, as well as changes in the shape, strain, and composition of individual QDs. The experimental results are attributed to the anisotropy in the thermally activated surface mass transport and the relaxation of elastic stresses.

A multiband theory of the dielectric response in a quasi-two dimensional (Q2D) electron plasma of narrow-gap semiconductor structures in an external magnetic field has been developed. The strong interband mixing plays an essential role in the determination of the shape and behavior of the dielectric response in narrow-band Q2D systems. We have used a full 8×8 k⋅p Hamiltonian model to describe the Landau levels. A rigorous theoretical analysis shows, among other interesting features, that spin–flip-like intrasubband transitions are allowed and its effects on the related optical properties of Q2D systems are investigated. The influence of the Landau level filling factor on the dielectric response and the limit of integer and fractional filling factors cases are discussed. We found that the occupation of Landau levels modulates the polarizability strength of the plasma.

In the frame of a full 8×8 k.p Hamiltonian, an exhaustive study of magneto-optical properties of narrow-gap semiconductor heterostructures has been developed. The effects of spatial confinement on the conduction Landau ladders of Hg1−xCdxTe/CdTe quantum wells are described. We report anomalous spin crossings of the Landau fan plots which lead to a sign inversion of the effective Lande factor as the magnetic field strength increases. Our quantitative explanation shows that the analyzed effects appear to be very sensitive to the temperature and the Cd-concentration of the sample. We discuss how the complemental aspect of the absorption coefficient obtained within Faraday and Voigt configurations may allow a direct evaluation of the anomaly in the g-factor as a function of the field.

We report the inclusion of temperature effects on the Mahan-Nozieres-De Dominicis framework to study both many-body and temperature effects in photoluminescence spectra of doped semiconductors. The electronic part of the correlation function characterizes the photoluminescence spectra. We have treated the optical valence hole as a localized scattering potential center and studied effects of the electron-hole interaction enhancement on the photoluminescence spectra leading to the appearance of shake-up structures. We also have identified a term in the correlation function which represents the finite-temperature contribution to the intensities of the shake-up structures. The method is used to study the magnetophotoluminescence of modulation-doped quantum wells with a weak periodic lateral potential.

We present calculated and experimental interband and photoinduced intersubband absorption spectra for undoped HgCdTe multiple quantum wells. The results show that there is an enhancement of the interband absorption at energies close to the barrier energy for electrons. The calculated interband absorption, based on a full $8\ifmmode\times\else\texttimes\fi{}8 \mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Kane model to obtain the electronic structure, predicts that this absorption enhancement is caused by an increase in the electron-hole joint density of states due to band mixing for confined and continuous electron states. The photoinduced electron intersubband absorption presents an asymmetric line shape due to the nonparabolic bands. The calculated spectra reproduce well the experimental results.

Theoretical research on electronic properties in mesoscopic condensed matter systems has focused primarily on the electron charge freedom degrees, while its corresponding spin freedom degrees have not yet received the same attention. Nevertheless nowadays there has been an increment in the number of electron spin-related experiments showing unique possibilities for finding novel mechanisms of information processing and transmission, opening ample fields of opportunities in the theoretical developed of new models. In this spirit we have calculated the resonant tunneling characteristics curves in double-barrier heterostructures of GaAs-Ga1-x AlxAs under external stress and considering two charges with spin half. The resonant tunneling study has been carried out by means of the diagrammatic techniques for non equilibrium processes following the model proposed by Keldysh also a simple one-band tight-binding Hamiltonian is adopted in the theoretical framework. We have compared our results of the spin-tunneling with previous ones reported in literature., 9 pages, 2 figures

The Raman profiles are calculated as a function of magnetic field ${B}_{0}$ and the laser frequency ${\ensuremath{\omega}}_{L}$ for the Fr\"ohlich type of electron-phonon interaction in parallel scattering configurations with circularly polarized light $[\overline{z}({\ensuremath{\sigma}}^{\ifmmode\pm\else\textpm\fi{}},{\ensuremath{\sigma}}^{\ifmmode\pm\else\textpm\fi{}})z]$ spread in z along the [001] crystal direction. A $4\ifmmode\times\else\texttimes\fi{}4$ Luttinger Hamiltonian has been used in order to take the valence-band admixture into account. Explicit expressions for the Raman scattering efficiency as a function of ${\ensuremath{\omega}}_{L},$ ${B}_{0},$ and the Luttinger parameters are given. Different effects due to the band nonparabolicity and the mixing of valence-band states are discussed. For a realistic picture of the resonance profiles an explicit calculation of the lifetime broadening of the intermediate electronic states assisted by a LO phonon has been carried out including the dependence on the laser incident energy, magnetic field, and Landau quantum number. Results are applied to explain recent experimental data for GaAs.

A theoretical and experimental study of the spatial confinement of phonons in ternary CdSxSe1−x nanocrystals embedded in a glass matrix is reported. We discuss the morphological characteristics of the nanocrystals by analyzing the dependence of surface phonon modes on the geometrical parameters. The calculated frequencies are compared with experimental Raman spectra. A good theoretical and experimental correlation for CdS-like and CdSe-like surface optical modes is observed. Raman selection rules and their connection with the nature of the surface optical phonons is discussed. In this work, we explore theoretically and experimentally the connection between phonon surface modes (SOM) and the geometry of a semiconductor quantum dot (QD). By analyzing the behavior of SOM in ternary alloy based QD’s, we may determine the composition fluctuation and the geometrical parameter r, that measures the sphericity degree of deviation. To do this, we have extended the Comas et al. [1] model for binary spheroidal QDs to study ternary QD alloys of A 1−x A 2 B, and calculated the dependence of the A 1 B-like and A 2 B-like SO-modes as a function of x and r. Samples were prepared over SiO2-Na2CO3-B2O3-Al2O3 doped with CdO, S and Se. The mixture was melted in an aluminum crucible at 1200 ◦ C, for 2 h. Then, it was quickly cooled down to room temperature. In order to enhance the diffusion of the Cd 2+ ,S 2− , and Se 2− ions, we have performed 600 ◦ C thermal treatments on each samples using different annealing times of ta=3, 5, 6, 10, 12, 14, 16, 18, 20, and 22 h, respectively. As a result of these thermal treatments, the QD’s of CdSxSe1−x are formed over the glass matrix. The QD Raman spectra were recorded at room temperature. An argon-ion laser operating at 100 mW with the line λ = 514 nm was collected in back scattering Raman geometry. From a theoretical point of view, the quantities involved in the description of polar-optical phonons are derived from e(ω)∇ 2 ϕ =0 , where ϕ is the electrostatic phonon potential. For ternary alloys, e(ω) can be calculated as [2] e(ω )= e∞ + X1/(ω 2

Departament of Theoretical Physics, Havana University,Vedado 10400, CubaReceived30July 2006, accepted 18 August 2006Published online 7 February 2007PACS 75.50.Pp, 78.67.Hc, 85.70.SqWe have exploredanexchange-inducedpropertyoftheelectronicstructureofdilutedmagneticsemiconduc-tor quantum dots in order to propose an externally controlled optical switching device. Various confinementshapes, dot sizes and host material compositions are used in order to optimize the device operation. Thisexcitonic optical switching can be established within well defined temperature and magnetic compositionranges. We are proposing amethod for the implementation of this optical device that can be switchedbetween twowell-defined regimes.

We have studied theoretically the influence of crossed electric and magnetic fields on the resonant tunnelling of photogenerated electrons and holes in an asymmetric double quantum well system. The electronic structure of this system has been calculated using the k · p multi-band model within a six-component envelope-function formalism. For a given value of applied electric field we determine the magnetic field necessary to bring into resonance the bottom (top) of the electron (hole) ground state of the narrow quantum well with a state in the wide well. We show that the obtained electric versus resonant magnetic field curves can be used to map out the dispersion relation of the wide quantum well.

The one-phonon Raman scattering efficiency for III-V and II-VI zinc-blende-type semiconductors in high magnetic fields including the valence-band admixture has been obtained. The coupling of the heavy-hole and light-hole valence bands has been taken into account through the Luttinger-Kohn Hamiltonian model. The Raman profiles are calculated as a function of magnetic field ${B}_{0}$ and laser frequency ${\ensuremath{\omega}}_{L}$ with a deformation-potential type of electron-phonon interaction. The influence on the Raman tensor of the nonparabolicity as well as the anisotropic behavior of Landau ladders in terms of magnetic field and the $z$ component of the hole wave vector are analyzed. Explicit expressions for the Raman scattering efficiency as a function of ${\ensuremath{\omega}}_{L},$ ${B}_{0}$, and the Luttinger-Kohn parameters are given. The dependence of the lifetime broadening on the laser incident energy, magnetic field, and Landau quantum number of the intermediate electronic states has been considered in the Raman scattering efficiency. Based on these grounds, the essential features of recent magneto-Raman experiments in bulk GaAs, using the crossed polarization geometries $\overline{z}({\ensuremath{\sigma}}^{\ifmmode\pm\else\textpm\fi{}},{\ensuremath{\sigma}}^{\ensuremath{\mp}})z$ can be explained.