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We report a comprehensive theory to describe exciton and biexciton valley dynamics in monolayer Mo1−xWxSe2 alloys. To probe the impact of different excitonic channels, including bright and dark excitons, intravalley biexcitons, intervalley scattering between bright excitons, as well as bright biexcitons, we have performed a systematic study from the simplest system to the most complex one. In contrast to the binary WSe2 monolayer with weak photoluminescence (PL) and high valley polarization at low temperatures and the MoSe2, that presents high PL intensity, but low valley polarization, our results demonstrate that it is possible to set up a ternary alloy with intermediate W-concentration that holds simultaneously a considerably robust light emission and an efficient optical orientation of the valley pseudospin. We find the critical value of W-concentration, xc, that turns alloys from bright to darkish. The dependence of the PL intensity on temperature shows three regimes: while bright monolayer alloys display a usual temperature dependence in which the intensity decreases with rising temperature, the darkish alloys exhibit the opposite behavior, and the alloys with x around xc show a non-monotonic temperature response. Remarkably, we observe that the biexciton enhances significantly the stability of the exciton emission against fluctuations of W-concentration for bright alloys. Our findings pave the way for developing high-performance valleytronic and photo-emitting devices.

The characterization of helical states can be performed by checking the existence of the re-entrant behaviour, which appears as a dip in the conductance probed in nanowires (NWs) with strong spin-orbit coupling (SOC) and under perpendicular magnetic field. On the other hand, the experiment described in Ref. 1 observed the re-entrant behaviour in the absence of a magnetic field, which was explained through spin-flipping two-particle backscattering. We theoretically show that the observation of the re-entrant behaviour is due to a multi-channel scattering mechanism, which causes a reduction of the transmission when an effective attractive potential and coupling between different channels are present. Both ingredients are provided by the SOC in the transport properties of NWs., 5 pages, 4 figures

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.

The identification of physical parameters that lead to magnetism in apparent nonmagnetic semiconductor systems has continuously challenged our community. In particular, for quantum-dot systems, they are key factors that contribute to their magneto-optical properties. We report, experimentally, optical evidences of induced nanomagnetism in nonmagnetic CdSe quantum dots and assess theoretically the role played by charged and uncharged vacancies. The analysis of these effects rests upon both the chemical and strain environments where the quantum dots are embedded. The interplay of spin-orbit interaction with built-in axial strains has been demonstrated to be a key factor for the magnetic moment alignment. This has been achieved in this paper by emulating the electronic structure at atomistic levels, considering various defect configurations and taking into account both the quantum dot composition and the influence of the host lattice. Postprint

We have studied the nature and character switching of relativistic bound states in quantum dot–ring structures produced by a set of circular concentric metallic gates on a graphene sheet placed over a substrate. The structure consists of an attractive core, a repulsive barrier and an attractive rim region where the resulting potential profiles and the interaction between the graphene layer and substrate are treated within a modified Dirac Hamiltonian describing the system. Our simulations allow a microscopic mapping of the character of electron and hole quasi-particle states and, in this environment, we study the effects of mixing between states in the dot–ring structure. Unusual electronic properties are reported by the emergence of localized states in the barrier region where electrons behave like holes in the inverted well potential and, as a direct consequence, the appearance of intertwined energy levels is envisaged which are tuned by bias voltages and the effective strength of the graphene–substrate interaction. The optical selection rules and the light absorption in effective gap regions between localized carrier states have been characterized and linked to the wavefunction engineering.

Owing to nonzero charge and spin degrees of freedom, trions offer unprecedented tunability and open new paths for applications in devices based on 2D semiconductors. However, in monolayer WSe2, the trion photoluminescence is commonly detected only at low temperatures and vanishes at room temperature, which undermines practical applications. To unveil how to overcome this obstacle, we have developed a comprehensive theory to probe the impact of different excitonic channels on the trion emission in WSe2 monolayers, which combines ab initio tight-binding formalism, Bethe-Salpeter equation and a set of coupled rate equations to describe valley dynamics of excitonic particles. Through a systematic study in which new scattering channels are progressively included, we found that, besides the low electron density, strong many-body correlations between bright and dark excitonic states quenches the trion emission in WSe2. Therefore, the reduction of scatterings from bright to dark states is required to achieve trion emission at room temperature for experimentally accessible carrier concentrations.

Electronic transport through a material depends on the response to local perturbations induced by defects or impurities in the material. The scattering processes can be described in terms of phase shifts and corresponding cross sections. The multiorbital nature of the spinor states in transition metal dichalcogenides would naturally suggest the consideration of a massive Dirac equation to describe the problem, while the parabolic dispersion of its conduction and valence bands would invite a simpler Schrodinger equation description. Here, we contrast the scattering of massive Dirac particles and Schrodinger electrons, in order to assess different asymptotic regimes (low and high Fermi energy) for each one of the electronic models and describe their regime of validity or transition. At low energies, where the dispersion is approximately parabolic, the scattering processes are dominated by low angular momentum channels, which results in nearly isotropic scattering amplitudes. On the other hand, the differential cross section at high Fermi energies exhibits clear signatures of the linear band dispersion, as the partial phase shifts approach a non-zero value. We analyze the electronic dynamics by presenting differential cross sections for both attractive and repulsive scattering centers. The dissimilar behavior between Dirac and Schrodinger carriers points to the limits and conditions over which different descriptions are required for the reliable treatment of scattering processes in these materials.

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.

Combining atomistic methods and effective mass approaches, the magnetic field response of Mn-doped deformed quantum dots has been characterized. The defect-induced asymmetry triggers unavoidable spin-orbit coupling effects that become more relevant the smaller the quantum confinement. Thus, the expected spin splitting modulation by the exchange interaction between the confined carriers and the Mn impurity is tuned by the intertwining of spatial geometry and magnetic field. Furthermore, ab initio calculations are presented for assessing the exchange interaction term and its response to confinement effects at the atomistic level.

The authors are grateful for financial support by the European Union (FPVII (2007-2013) under grant agreement no. 318287 LANDAUER), and the Brazilian Agencies FAPESP (grants 2012/13052 - 6, and 2012/51415 - 3), CNPq and CAPES. V. L.-R. acknowledges the support of FAPESP (grant: 2014/02112-3). Y.V.P. was supported by National Science Foundation grant ECCS-1202383. Electronic circuits composed of one or more elements with inherent memory - memristors, memcapacitors and meminductors - offer lower circuit complexity and enhanced functionality for certain computational tasks. Networks of these elements are proposed for novel computational paradigms that rely on information processing and storage on the same physical platform. We show a nanoscaled memdevice able to act as an electronic analogue of tipping buckets that allows reducing the dimensionality and complexity of a sensing problem by transforming it into a counting problem. The device offers a well adjustable, tunable and reliable periodic reset that is controlled by theamounts of transferred quantum dot charges per gate voltage sweep. When subjected to periodic voltage sweeps the quantum dot (bucket) may require up to several sweeps before a rapid full discharge occurs thus displaying period doubling, period tripling and so on between self-governing reset operations. Postprint Postprint

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.

The structural properties of twin-plane superlattices in InP nanowires are systematically analyzed. First, we employ molecular dynamics simulations to determine the strain fields in nanowires grown in the [111] direction. These fields are produced by the formation of twin-planes and by surface effects. By using the stress tensor obtained from molecular dynamics simulations, we are able to describe changes on the electronic structure of these nanowires. On the basis of the resulting electronic structure, we confirm that a one-dimensional superlattice is indeed formed. Furthermore, we describe the transport properties of both electrons and holes in the twin-plane superlattices. In contrast to the predicted transparency of Γ-electrons in heterolayered III-V semiconductor superlattices, we verify that surface effects in 1D systems open up possibilities of electronic structure engineering and the modulation of their transport and optical responses.

We have achieved conditions to obtain optical memory effects in semiconductor nanostructures. The system is based on strained InP quantum wires where the tuning of the heavy-light valence band splitting has allowed the existence of two independent optical channels with correlated and uncorrelated excitation and light-emission processes. The presence of an optical channel that preserves the excitation memory is unambiguously corroborated by photoluminescence measurements of free-standing quantum wires under different configurations of the incoming and outgoing light polarizations in various samples. High-resolution transmission electron microscopy and electron diffraction indicate the presence of strain effects in the optical response. By using this effect and under certain growth conditions, we have shown that the optical recombination is mediated by relaxation processes with different natures: one a Markov and another with a non-Markovian signature. Resonance intersubband light-heavy hole transitions assisted by optical phonons provide the desired mechanism for the correlated non-Markovian carrier relaxation process. A multiband calculation for strained InP quantum wires was developed to account for the description of the character of the valence band states and gives quantitative support for light hole-heavy hole transitions assisted by optical phonons.

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 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.

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 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 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 present a systematic theoretical study of the effective Zeeman spin- and non-linear-splitting of a single spherical quantum dot based on the Hamiltonian model. The effect of spin-hybridization on conduction band states is pointed out as the main source of the strong dependence of Lande factors and effective masses on external fields. The topology of the electronic orbitals is highly sensitive to the magnetic field tuning and to the spin polarization. The electron, hole and excitonic g-factors, as well as the diamagnetic coefficient are calculated for CdTe semiconductor quantum dots. Different systematic experimental methods are proposed in order to determine the behaviour of the electronic properties under analysis as a function of magnetic field and confinement geometry. Complementary optical transitions, in Faraday and Voigt configurations, can be used in the determination of electron, hole and exciton Lande factors, effective magnetic masses and diamagnetic coefficients.

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 present a systematic theoretical study, based on the Kane-Weiler 8 x 8 k p model, of the linear Zeeman splitting introduced by the interaction between the angular momentum and the magnetic field which can give a measure of the non-linear Zeeman effect associated with interband coupling and diamagnetic contributions. The conduction and valence bands g-factors are calculated for InSb spherical and semi-spherical quantum dots. The calculations of the g-factors showed an almost linear dependence, for the ground state, on the magnetic field. We have also found that the strong magnetic field dependence as well as the dependence on the dot size of the effective spin splitting can be unambiguously attributed to the strength of the inter-level mixing.

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.

We present a theoretical description of Bogolyubov-type excitations of exciton-polariton Bose-Einstein condensates (BECs) in semiconductor microcavities. For a typical two-dimensional (2D) BEC we focus on two limiting cases, the weak-and strong-coupling regimes, where a perturbation theory and the Thomas-Fermi approximation, respectively, are valid. We calculate integrated scattering intensity spectra for probing the collective excitations of the condensate in both considered limits. Moreover, in relation to recent experiments on optical modulation allowing localization of condensates in a trap with well-controlled shape and dimensions, we study the quasi-one-dimensional (1D) motion of the BEC inmicrowires and report the corresponding Bogolyubov excitation spectrum. We show that in the 1D case the characteristic polariton-polariton interaction constant is expressed as g(1) = 3 lambda N/(2L(y)) (lambda is the 2D polariton-polariton interaction parameter in the cavity, N the number of the particles, and L-y the wire cavity width). We reveal some interesting features for 2D and 1D Bogolyubov spectra for both repulsive (lambda > 0) and attractive (lambda < 0) interactions., Fundação para a Ciência e a Tecnologia (FCT) - Portuguese Foundation for Science and Technology Grants: PTDC-FIS-113199-2009, PEst-C/FIS/UI0607/2013, CNPq, FAPESP, Brazil, Russian Ministry of Education and Science 11.G34.31.0067

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.

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.

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.

Quantum interference patterns predicted by theory due to the finite structure of neutral excitons in InAs quantum rings are corroborated experimentally in the magneto-photoluminescence spectra of these nanostructures. The effects associated to built in electric fields and to the temperature on these Aharonov-Bohm-like oscillations are described and confirmed by complementary experimental procedures.

This work aims to investigate the effects of magnetic field strength and direction on the electronic properties and optical response of GaAs/AlGaAs-based heterostructures. An investigation of the excitonic spin-splitting of a disordered multiple quantum well embedded in a wide parabolic quantum well is presented. The results for polarization-resolved photoluminescence show that the magnetic field dependencies of the excitonic spin-splitting and photoluminescence linewidth are crucially sensitive to magnetic field orientation. Our experimental results are in good agreement with the calculated Zeeman splitting obtained by the Luttinger model, which predicts a hybridization of the spin character of states in the valence band under tilted magnetic fields.

We employ the circular-polarization-resolved magnetophotoluminescence technique to probe the spin character of electron and hole states in a GaAs/AlGaAs strongly coupled double-quantum-well system. The photoluminescence (PL) intensities of the lines associated with symmetric and antisymmetric electron states present clear out-of-phase oscillations between integer values of the filling factor $\ensuremath{\nu}$ and are caused by magnetic-field-induced changes in the population of occupied Landau levels near to the Fermi level of the system. Moreover, the degree of circular polarization of these emissions also exhibits the oscillatory behavior with increasing magnetic field. Both quantum oscillations observed in the PL intensities and in the degree of polarizations may be understood in terms of a simple single-particle approach model. The $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ method was used to calculate the photoluminescence peak energies and the degree of circular polarizations in the double-quantum-well structure as a function of the magnetic field. These calculations prove that the character of valence band states plays an important role in the determination of the degree of circular polarization and, thus, resulting in a magnetic-field-induced change of the polarization sign.

We present a systematic study of lead-salt nanocrystals (NCs) doped with Mn. We have developed a theoretical simulation of electronic and magneto-optical properties by using a multi-band calculation including intrinsic anisotropies and magnetic field effects in the diluted magnetic semiconductor regime. Theoretical findings regarding both broken symmetry and critical phenomena were studied by contrasting two different host materials (PbSe and PbTe) and changing the confinement geometry, dot size, and magnetic doping concentration. We also pointed out the relevance of optical absorption spectra modulated by the magnetic field that characterizes these NCs.

The photoluminescence of annealed Cd\(_x\)Zn\(_{1-x}\)Se quantum dots (QDs) under the influence of an external magnetic field has been studied in this paper. Post-growth annealing was performed for different annealing times. Above a critical annealing time, the QD luminescence shows a pronounced red-shift of the Zeeman split magnetic subcomponents. This observation is in contrast to the blue-shift caused by the diamagnetic behavior that is usually observed in non-magnetic QDs. We attribute our finding to the paramagnetism caused by the mixing of heavy and light hole states. Hence, post-growth thermal annealing treatment might be employed to render undoped epitaxial QDs intrinsically magnetic in a controlled manner. Two theoretical models were developed: a few-particle model to account for excitonic complex effects and a multiband calculation that describes the valence band hybridization. Contrasting the two models allowed us to unambiguously elucidate the nature of such an effect.

This work reports the investigation on the structural differences between InAs quantum rings and their precursor quantum dots species as well as on the presence of piezoelectric fields and asymmetries in these nanostructures. The experimental results show significant reduction in the ring dimensions when the sizes of capped and uncapped ring and dot samples are compared. The iso-lattice parameter mapped by grazing-incidence x-ray diffraction has revealed the lateral extent of strained regions in the buried rings. A comparison between strain and composition of dot and ring structures allows inferring on how the ring formation and its final configuration may affect optical response parameters. Based on the experimental observations, a discussion has been introduced on the effective potential profile to emulate theoretically the ring-shape confinement. The effects of confinement and strain field modulation on electron and hole band structures are simulated by a multiband k.p calculation.

We report a study of magnetic responses of neutral and charged excitons in quantum wells, which are very sensitive to the strong spin hybridization of holes. This effect can be used to engineer the spin character of excitonic complexes in two-dimensional systems tuned by the magnetic field strength. Conditions for spin flip for each kind of excitonic complex is detailed and the nature of the effect discussed. Differences in the effective Zeeman splitting between neutral and charged excitons are theoretically predicted and unambiguously confirmed experimentally. Circularly polarized resolved photoluminescence has been used to study these effects under applied magnetic fields. The intertwining of spin dynamics of excitons and trions is discussed.

L. Villegas-Lelovsky,1 Fanyao Qu,2 L. O. Massa,2 V. Lopez-Richard,3 and G. E. Marques3 1Instituto de Fisica, Universidade Federal de Uberlândia, 38.400-902, Uberlândia, Minas Gerais, Brazil 2Instituto de Fisica, Universidade de Brasilia, Brasilia-DF 70910-900, Brazil 3Departamento de Fisica, Universidade Federal de Sao Carlos, 13.565-905, Sao Carlos, Sao Paulo, Brazil (Received 24 March 2011; revised manuscript received 3 June 2011; published 10 August 2011)

We report a study of dynamic effects detected in the time-resolved emission from quantum dot ensembles. Experimental procedures were developed to search for common behaviors found in quantum dot systems independently of their composition: three quantum dot samples were experimentally characterized. Systems with contrasting interdot coupling are compared and their sensitivity to the excitation energy is analyzed. Our experimental results are compared and contrasted with other results available in literature. The optical recombination time dependence on system parameters is derived and compared to the experimental findings. We discuss the effects of occupation of the ground state in both valence and conduction bands of semiconductor quantum dots in the dynamics of the system relaxation as well as the nonlinear effects.

A method to determine the effects of the geometry and lateral ordering on the electronic properties of an array of one-dimensional self-assembled quantum dots is discussed. A model that takes into account the valence-band anisotropic effective masses and strain effects must be used to describe the behavior of the photoluminescence emission, proposed as a clean tool for the characterization of dot anisotropy and/or inter-dot coupling. Under special growth conditions, such as substrate temperature and Arsenic background, 1D chains of In0.4Ga0.6 As quantum dots were grown by molecular beam epitaxy. Grazing-incidence X-ray diffraction measurements directly evidence the strong strain anisotropy due to the formation of quantum dot chains, probed by polarization-resolved low-temperature photoluminescence. The results are in fair good agreement with the proposed model.

By breaking the spatial symmetry of two coupled quantum dots, we are able to manipulate the $p\text{\ensuremath{-}}d$-exchange interaction between a hole and a single-${\text{Mn}}^{2+}$ ion by means of an external electric field. It is found that the magnetic energy can be tuned in a wide range. Depending on the position of the Mn ion located within the system, effective Zeeman effects induced by the exchange interaction may be switched on/off and even exhibits peak maxima values which accounts for the interdot resonant coupling.

We report a systematic study of magneto-optical properties in CdZnSe/ZnSe semiconductor quantum dots QDs subjected to postgrowth thermal annealing. The theoretical and experimental characterizations are combined in order to understand the evolution of Zeeman splitting and the blueshift as well as redshift of the magnetic subcomponents with magnetic field strength of excitons confined in annealed QDs as a function of different annealing times. A combination of a multiband model as well as parameter interpolation supported by ab initio calculations is presented and points toward an inversion of the light-hole-heavy-hole states due to annealing. The band mixing of exciton states explains besides the variation in the Zeeman splitting for differently annealed QD excitons, the evolution of the diamagnetic blueshift into a paramagnetic redshift of the magnetic subcomponents. A discussion of the effects associated to the Coulomb interaction on the polarized excitonic recombinations modulated by magnetic field is given.

Multilayer In0.4Ga0.6As/GaAs quantum dot (QD) chain samples are investigated by means of cw and time-resolved photoluminescence (PL) spectroscopy in order to study the peculiarities of interdot coupling in such nanostructures. The temperature dependence of the PL has revealed details of the confinement. Non-thermal carrier distribution through in-chain, interdot wave function coupling is found. The peculiar dependences of the PL decay time on the excitation and detection energies are ascribed to the electronic interdot coupling and the long-range coupling through the radiation field. It is shown that the dependence of the PL decay time on the excitation wavelength is a result of the superradiance effect.

We examine the effects of the full Dresselhaus spin-orbit coupling on laterally confined quantum wire states. An analysis of the relative contributions due to linear, quadratic, and cubic Dresselhaus spin-orbit terms on the energy levels, spin splitting, and spin polarization has been carried out. The effects of wire cross-sectional geometry shapes on the electronic structure are explored. In particular we compared the results of semicylindrical and cylindrical confinements and have found important differences between the spin degeneracy of the ground-state level and the spin-polarization dependence on sign inversion of the free linear momentum quantum number along the wire axis. Different from other symmetries, in both cases here considered, the stronger spin-splitting effects come from the quadratic Dresselhaus term. We report ideal conditions for realization of spin-field filter devices based on symmetry properties of the spin splitting of the ground state in semicylindrical quantum wires.

C. Trallero-Giner,1 V. Lopez-Richard,2 S. E. Ulloa,3 and G. E. Marques2 1Department of Theoretical Physics, Havana University, Vedado 10400, Havana, Cuba 2Departamento de Fisica, Universidade Federal de Sao Carlos, 13560-905, Sao Carlos, SP, Brazil 3Department of Physics and Astronomy and Nanoscale and Quantum Phenomena Institute, Ohio University, Athens, Ohio 457501-2979, USA Received 17 November 2008; revised manuscript received 6 March 2009; published 3 April 2009

The one phonon Raman scattering efficiency for zincblende-type semiconductors in high magnetic fields has been obtained. Special emphasis has been given to the enhancement of the electronic lifetime broadening due to resonant coupling between different intraband states by the LO-phonons. The sharp resonances obtained by magneto-Raman scattering in GaAs further allow the identification of magnetopolaron effects.

The transport of electrons through a GaAs/AlAs double-barrier structure with $p$-type doped contacts was investigated using time-resolved photoluminescence spectroscopy. Under illumination, the current-voltage characteristics of the device present two additional features attributed, respectively, to resonant $\ensuremath{\Gamma}\text{\ensuremath{-}}\ensuremath{\Gamma}$ and $\ensuremath{\Gamma}\text{\ensuremath{-}}X$ electron tunneling. Optical measurements for biases where these two alternative transport mechanisms have competitive probabilities revealed an unusual carrier dynamics. The quantum well emission is strongly delayed and we observe a remarkable nonlinear effect where the emission intensity decreases at the arrival of a laser pulse. We propose a simple model that adequately describes our results where we assume that the indirect-transition rate depends on the density of electrons accumulated along the structure.

A negative magnetic dispersion of the degree of circular polarization is reported for the photoluminescence of self-assembled ZnCdSe quantum dots (QDs) after a postgrowth thermal annealing (TA) treatment. Multiband calculations in the framework of the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ model reveal that the phenomenon is explicable considering, apart from the changes in the confinement and composition of the QDs due to TA-induced Zn-Cd interdiffusion, the presence of extra charges of one kind within the structure.

We report theoretical studies of the spin splitting caused by inversion asymmetries in quantum ring-shaped self-assembled quantum dots with circular cross sections. We analyze the conduction band and the spinsplitting electronic energy dispersions of the system in the presence of both bulk Dresselhaus and structural Rashba inversion asymmetries as a function of quantum ring geometrical parameters. We have explored the interplay effects between the circular confinement symmetry and the magnitude of the spin-orbit constants for the dot material. We describe universal conditions to operate a spin-field-effect transistor based on quantum rings with identical linear Dresselhaus and Rashba spin-orbit coupling interactions. The results show to be sensitive to both the quantum ring radii and the dot thickness.

Considering the Gross-Pitaevskii integral equation we are able to formally obtain an analytical solution for the order parameter $\Phi (x)$ and for the chemical potential $\mu $ as a function of a unique dimensionless non-linear parameter $\Lambda $. We report solutions for different range of values for the repulsive and the attractive non-linear interactions in the condensate. Also, we study a bright soliton-like variational solution for the order parameter for positive and negative values of $\Lambda $. Introducing an accumulated error function we have performed a quantitative analysis with other well-established methods as: the perturbation theory, the Thomas-Fermi approximation, and the numerical solution. This study gives a very useful result establishing the universal range of the $\Lambda $-values where each solution can be easily implemented. In particular we showed that for $\Lambda, Comment: 8 figures

We report experimental evidence of excitonic spin-splitting, in addition to the conventional Zeeman effect, produced by a combination of the Rashba spin-orbit interaction, Stark shift and charge screening. The electric-field-induced modulation of the spin-splitting are studied during the charging and discharging processes of p-type GaAs/AlAs double barrier resonant tunneling diodes (RTD) under applied bias and magnetic field. The abrupt changes in the photoluminescence, with the applied bias, provide information of the charge accumulation effects on the device., 4 pages, 2 figures

We have investigated polarization-resolved photoluminescence under applied voltage in $p\text{\ensuremath{-}}i\text{\ensuremath{-}}p$ $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ double-barrier diodes. We have observed oscillations in the degree of polarization up to 36% at $B=15\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ with sign reversals occurring near to the hole subband resonances. At high voltages a polarization saturation up to 25% at $B=15\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ is observed. The data are interpreted by using simulations based on a simple theoretical model that considers spin conservation for tunneling and the relaxation processes for carriers at Zeeman states in the quantum well. Our work offers the prospect for the development of voltage-controlled spin filtering systems using standard nonmagnetic semiconductor heterostructures.