G. E. Marques

Aharonov-Bohm interference in neutral excitons: effects of built-in electric fields

We report a comprehensive discussion of quantum interference effects due to the finite structure of neutral excitons in quantum rings and their first experimental corroboration observed in the optical recombinations. The signatures of built-in electric fields and temperature on quantum interference are demonstrated by theoretical models that describe the modulation of the interference pattern and confirmed by complementary experimental procedures.

Morphology in semimagnetic Pb1−xMnxSe nanocrystals: Thermal annealing effects

Samples of Pb1−xMnxSe nanocrystals were synthesized by fusion method and characterized by optical absorption, atomic force microscopy, x-ray diffraction, and electron paramagnetic resonance (EPR) techniques. Effects of Mn2+ ion incorporation into PbSe nanocrystals are manifested by well resolved optical spectra for different concentrations of Mn. The EPR spectra of as grown and thermal annealed Pb1−xMnxSe samples show that the magnetic properties of these Mn-doped nanocrystals can be tuned by thermal processes using different annealing times.

Interface optical phonons in spheroidal quantum dots

Interface optical phonons are studied in the case of a quantum dot (QD) with prolate and oblate spheroidal geometries within the dielectric continuum approach. We considered CdSe or CdS QDs imbedded in a host material which is modelled as an infinite medium. The surface optical phonon modes, the corresponding frequencies, and the electron-phonon interaction Hamiltonian are reported. Comparison is made with previous works which only considered strictly spherical dots. We conclude that deviations from the perfect spherical shape could be responsible for observable physical effects in Raman spectra.

Magneto-optical properties of Cd1−xMnxS nanoparticles: influences of magnetic doping, Mn2+ ions localization, and quantum confinement

Cd(1-x)Mn(x)S nanoparticles (NPs) were successfully grown in a glass matrix and investigated by optical absorption (OA), magnetic circularly polarized photoluminescence (MCPL) measurements, and magnetic force microscopy (MFM). The room temperature OA spectra have revealed the formation of two groups of Cd(1-x)Mn(x)S NPs with different sizes: bulk-like nanocrystals (NCs) and quantum dots (QDs). The MCPL spectra were recorded at 2.0 K with several magnetic fields up to 15 T, allowing a detailed comparison between the degrees of circular polarization of the two groups of NPs. The different behaviours of magneto-optical properties of bulk-like NCs and QDs were explained by taking into account a considerable alteration of exchange interaction between the carrier spins and the substitutional doping magnetic ions incorporated into the NPs. As a main result, we have demonstrated that self-purification is the dominant mechanism that controls the doping in semiconductor QDs grown by the melting-nucleation synthesis approach due to the relatively high temperature that was used in thermal annealing of samples.

Radiative versus nonradiative optical processes in PbS nanocrystals

The Stokes shift detected in PbS nanocrystals is analyzed by optical absorption (OA), photoluminescence (PL) and atomic force microscopy (AFM). The samples were synthesized by fusion method over two different glass substrates, and identical size PbS dot structures with radius 4.1 nm grown showed PL and OA peaks separated by 0.15 and 0.10 eV. The origin of these large Stokes shifts are analyzed whether they are attributed to the nonradiative recombination associated to impurity states in prepared doped samples or to radiative transitions between exciton states induced by electron-hole (e-h) exchange Coulomb interaction.

Deformed cyclotronic orbits for shallow impurities in cylindrical quantum well wires

The purpose this work is to study the behavior of the binding energy for donor shallow impurities in cylindrical quantum well wires at different radial positions. The model considers a finite confinement potential and the presence of a magnetic field applied parallel to the wire axis. Our results show that cyclotronic orbits are deformed as the impurity moves along the radial position, due to a competition between geometrical and magnetic confinement. As a result of this broken symmetry, we observe an induced change, from diamagnetic to paramagnetic, in the character of the ground-state of the system. These quantum mechanical calculations have been performed by using a variational method within the effective-mass theory.

Markovian and non-Markovian light-emission channels in strained quantum wires.

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.

Mechanisms of interdot coupling in (In,Ga)As/GaAs quantum dot arrays

Interdot coupling in (In,Ga)As/GaAs quantum dot arrays is studied by means of steady state and time-resolved photoluminescence (PL). A peculiar dependence of the PL decay time on the excitation and detection energy is revealed and ascribed to the peculiarities of the carrier and energy relaxation caused by both immediate electronic interdot coupling and long-range coupling through the radiation field.

Tailoring electronic transparency of twin-plane 1D superlattices.

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.

Circular polarization from a nonmagnetic p-i-n resonant tunneling diode

The authors investigate the circular polarization of the electro- and photoluminescence emissions from the quantum well and contact layers of a nonmagnetic GaAs–AlAs p-i-n resonant tunneling diode under an external magnetic field. The contact emission evidences the formation of a spin polarized two-dimensional electron gas at the n-accumulation layer. The quantum well electroluminescence presents a strong σ− degree of polarization, even for null Zeeman splitting energies, and a slight bias dependence. The observed circular polarization is mainly attributed to the spin polarization of the electrons injected into the quantum well from the two-dimensional electron gas.