Confined Wave Functions Research Articles

The second-harmonic generation of anisotropic quantum disks in the presence of a tilted magnetic field

We theoretically investigate the second-harmonic generation (SHG) coefficient of anisotropic quantum disks in the presence of a tilted magnetic field. Within the framework of the effective-mass approximation, we obtain the confined wave functions and energies of electrons of anisotropic quantum disks. We also obtain the SHG coefficient by the compact-density-matrix approach and the iterative method. The numerical results for the typical GaAs/AlGaAs anisotropic quantum disks show that the SHG coefficient depends strongly on the anisotropy and the magnetic field direction θ. And we find that the maximum value of SHG coefficient is occurring at maximum η.

Energy dependence of electron effective mass and effect of wave function confinement in a nanoscale In0.53Ga0.47As/In0.52Al0.48As quantum well

The effective mass in the conduction band was analyzed as a function of the kinetic energy in a 5–20 nm-thick In0.53Ga0.47As/In0.52Al0.48As quantum well (QW). An increase in the effective mass caused by wave function confinement in the QW, which was previously proposed theoretically, was not found to be present in this material under the framework of the energy effective mass. In the direction normal to the QW plane, the mass determined by the interband optical transition at 100-300 K fitted well with the calculated result based on Kane's bulk band theory. In a direction parallel to the QW plane, the cyclotron resonance energy at less than 70 T and the magneto photoluminescence energy at less than 13 T fitted with the calculated result to within an error range of ±2 meV. In the analysis of the magneto-photoluminescence at 1.4 K, the bandgap renormalization was determined and large new peaks appeared above 8 T, possibly because of the interaction of the magneto-exciton states with the ground-state zero-dimensional Landau level.

Second-harmonic generation in asymmetric quantum dots in the presence of a static magnetic field

The second-harmonic generation (SHG) coefficient in an asymmetric quantum dot (QD) with a static magnetic field is theoretically investigated. Within the framework of the effective-mass approximation, we obtain the confined wave functions and energies of electrons in the QD. We also obtain the SHG coefficient by the compact-density-matrix approach and the iterative method. The numerical results for the typical GaAs/AlGaAs QD show that the SHG coefficient depends strongly on the magnitude of magnetic field, parameters of the asymmetric potential and the radius of the QD. The resonant peak shifts with the magnetic field or the radius of the QD changing.

Formation and Characterization of Filamentary Current Paths in $\hbox{HfO}_{2}$-Based Resistive Switching Structures

In this letter, the progressive nature of the forming process step in HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based resistive switching structures is investigated. Contrary to what happens with ramped or pulsed voltage stresses, current-driven degradation experiments shed light on the formation dynamics of the filamentary path across the oxide layer. The resulting voltage-current characteristics are interpreted in terms of electron transport through a mesoscopic constriction with adiabatic shape. The voltage decrease during the forming process is ascribed to a relaxation of the electron wavefunction confinement effect. The role of the compliance level on the leakage current magnitude is also discussed within this framework.

Nonequilibrium electron leakage in terahertz quantum cascade structures

The nonequilibrium absorption of longitudinal-optical phonons by hot electrons are studied in terahertz quantum cascade structures. We present a method for including electron leakage to the continuum that takes into account the mobility of the electrons. This is incorporated into a density matrix Monte Carlo method that includes the optical field within the resonant cavity. The effects of electron leakage to the continuum as a function of lattice temperature are discussed. Results are compared with experiment and found to be consistent. It is shown that using only confined wave functions and thereby neglecting the leakage via tunneling is inadequate for describing the electron transport.

Intersubband optical refractive index changes in an asymmetric quantum dot underlying an external static magnetic field

We theoretically investigate the refractive index (RI) changes in an asymmetric quantum dot (QD) underlying an external static magnetic field. We obtain the confined wave functions and energies of an electron in QD by the effective-mass approximation. Using the compact-density-matrix approach and iterative method, we obtain the analytical expressions of linear, nonlinear and total RI changes. The results of numerical calculations for the typical GaAs/AlGaAs QD show that the RI changes are sensitive to the parameters of the asymmetric potential and incident optical intensity. Moreover, the resonance peaks of the RI changes shift with the value of magnetic field B or the radius of the QD changing.

Effects of composition and compositional distribution on the electronic structure of ZnSe1−xTex ternary quantum dots

We report results of first-principles density functional theory (DFT) calculations for the electronic structure of ZnSe1−xTex ternary quantum dots (TQDs) and the effects of composition and compositional distribution on the electron density distribution, electronic density of states, and band gap. We analyze the electronic structure of five types of nanocrystal configurations, namely, pristine ZnSe and ZnTe quantum dots, as well as ZnSe/ZnTe core/shell, ZnTe/ZnSe reverse core/shell, and randomly alloyed ZnSe1−xTex TQDs. We find that the band gaps for ZnSe/ZnTe core/shell TQDs are nonlinearly dependent on the number of Te atoms in the shell, whereas presence of Te in the core of alloyed ZnSe1−xTex TQDs modifies the electronic energy levels abruptly and significantly in the limits of x → 0 and x → 1. Our results imply that distribution of Te atoms in the TQD in the form of a ZnSe/ZnTe core/shell configuration allows for optimum tunability of the band gap and wave function confinement in TQDs.

Single-Electron Charging and Discharging Analyses in Ge-Nanocrystal Memories

The transient charging/discharging of electrons in Ge-nanocrystal (NC) memories are measured by a pump-and-probe method that allows keeping track of the number of electrons per NC. The experiments are simulated with a quantum kinetic mechanical model based on the density-functional theory, which can describe the NCs' charging state. In the transient charging, electrons are captured faster than predicted by simulations. This was attributed to the presence of defects in the NC surface, action of which is twofold: 1) The incoming electrons are captured by NC states and are quickly thermalized down to the surface traps. 2) Those traps enlarge the spatial distribution of the confined wave functions, increasing their penetration in the tunneling oxide and the incoming transition rates. As for the discharging, the calculations and experiments agree until there are only few electrons left per NC. Then, the out tunneling becomes slower than predicted by calculations. The remaining electrons are confined in trap states with energies located in the NC bandgap, and they have to be thermally excited to NC states and to tunnel out to the substrate.

Role of Symmetry Breaking on the Optical Transitions in Lead-Salt Quantum Dots

The influence of quantum confinement on the one- and two-photon absorption spectra (1PA and 2PA) of PbS and PbSe semiconductor quantum dots (QDs) is investigated. The results show 2PA peaks at energies where only 1PA transitions are predicted and 1PA peaks where only 2PA transitions are predicted by the often used isotropic k x p four-band envelope function formalism. The first experimentally identified two-photon absorption peak coincides with the energy of the first one photon allowed transition. This first two-photon peak cannot be explained by band anisotropy, verifying that the inversion symmetry of the wave functions is broken and relaxation of the parity selection rules has to be taken into account to explain optical transitions in lead-salt QDs. Thus, while the band anisotropy of the bulk semiconductor plays a role in the absorption spectra, especially for the more anisotropic PbSe QDs, a complete model of the absorption spectra, for both 1PA and 2PA, must also include symmetry breaking of the quantum confined wave functions. These studies clarify the controversy of the origin of spectral features in lead-salt QDs.

Baryon wave functions and free neutron decay in the scalar strong interaction hadron theory (SSI)

From the equations of motion for baryons in the scalar strong interaction hadron theory (SSI), two coupled third order radial wave equations for baryon doublets have been derived and published in 1994. These equations are solved numerically here, using quark masses obtained from meson spectra and the masses of the neutron, ?0 and ?0 as input. Confined wave functions dependent upon the quark-diquark distance as well as the values of the four integration constants entering the quark-diquark interaction potential are found approximately. These approximative, zeroth order results are employed in a first order perturbational treatment of the equations of motion for baryons in SSI for free neutron decay. The predicted magnitude of neutron’s half life agrees with data. If the only free parameter is adjusted to produce the known A asymmetry coefficient, the predicted B asymmetry agrees well with data and vice versa. It is pointed out that angular momentum is not conserved in free neutron decay and that the weak coupling constant is detached from the much stronger fine structure constant of electromagnetic coupling.

Baryon wave functions and free neutron decay in the scalar strong interaction hadron theory (SSI)

From the equations of motion for baryons in the scalar strong interaction hadron theory (SSI), two coupled third order radial wave equations for baryon doublets have been derived and published in 1994. These equations are solved numerically here, using quark masses obtained from meson spectra and the masses of the neutron, ?0 and ?0 as input. Confined wave functions dependent upon the quark-diquark distance as well as the values of the four integration constants entering the quark-diquark interaction potential are found approximately. These approximative, zeroth order results are employed in a first order perturbational treatment of the equations of motion for baryons in SSI for free neutron decay. The predicted magnitude of neutron’s half life agrees with data. If the only free parameter is adjusted to produce the known A asymmetry coefficient, the predicted B asymmetry agrees well with data and vice versa. It is pointed out that angular momentum is not conserved in free neutron decay and that the weak coupling constant is detached from the much stronger fine structure constant of electromagnetic coupling.

Gate-inducedg-factor control and dimensional transition for donors in multivalley semiconductors

The dependence of the g-factors of semiconductor donors on applied electric and magnetic fields is of immense importance in spin based quantum computation and in semiconductor spintronics. The donor g-factor Stark shift is sensitive to the orientation of the electric and magnetic fields and strongly influenced by the band-structure and spin-orbit interactions of the host. Using a multimillion atom tight-binding framework the spin-orbit Stark parameters are computed for donors in multi-valley semiconductors, silicon and germanium. Comparison with limited experimental data shows good agreement for a donor in silicon. Results for gate induced transition from 3D to 2D wave function confinement show that the corresponding g-factor shift in Si is experimentally observable.

Third-harmonic generation in cubical quantum dots

Third-harmonic generation (THG) for cubical quantum dots (CQDs) with an applied electric field is theoretically investigated in the framework of the compact-density-matrix approach and an iterative method. The confined wave functions and energies of electrons in the CQDs are calculated in the effective-mass approximation. Numerical calculations are presented for typical GaAs/AlAs CQDs. The results demonstrate that the THG strongly depends on the length of the CQDs and the magnitude of the electric field. Also, the peaks shift towards the higher energy region with increasing electric field.

Exciton-polariton condensation in a natural two-dimensional trap

Bose-Einstein condensation of exciton polaritons has recently been reported in homogeneous structures only affected by random in-plane fluctuations. We have taken advantage of the ubiquitous defects in semiconductor microcavities to reveal the spontaneous dynamical condensation of polaritons in the quantized levels of a trap. We observe condensation in several quantized states, taking their snapshots in real and reciprocal space. We also show the effect of particle interactions for high occupation numbers, revealed by a change in the confined wave function toward the Thomas-Fermi profile.

Nonlinear optical rectification in cubical quantum dots

Abstract The optical rectification (OR) coefficient for cubical quantum dots (CQDs), with an applied electric field is theoretically investigated in the framework of the compact-density-matrix approach and an iterative method. The confined wave functions and energies of electrons in the CQDs are calculated in the effective-mass approximation. Numerical calculations are presented for typical GaAs/AlAs CQDs. The results show that the calculation for OR coefficient in the CQDs system can reach a magnitude of 10 - 4 m / V , two orders higher than that in the spherical quantum dots system. The OR coefficient strongly depends on the length of CQDs and the magnitude of electric field. And the peak shifts to the aspect of high energy when considering the electric field.

Nonlinear optical rectification in parabolic quantum dots in the presence of electric and magnetic fields

The optical rectification (OR) coefficient in a parabolic quantum dots (QDs) subject to applied electric and magnetic fields is theoretically investigated in the framework of the compact-density-matrix approach and an iterative method. The confined wave functions and energies of electrons in the QDs are calculated in the effective-mass approximation. Numerical results are presented for typical GaAs/AlGaAs parabolic QDs. These results show that the OR coefficient strongly depends on the radius of QDs and the magnitude of electric and magnetic fields. And the peak shifts to the aspect of high energy when considering the influence of electric and magnetic fields.

Structural modifications in InP nanostructures prepared by Ar+-ion irradiation

The evolution of nanopatterned InP surfaces by low-energy Ar+-ion irradiation and their dependence on incidence angle were investigated by field emission scanning electron microscopy, atomic force microscopy, and Raman scattering. Ordered surface nanodots of high aspect ratio were created. At large ion incidence angle the dot density decreases and the size varies in the range of 65–130nm with height of around 25–30nm. Rapid thermal annealing of the patterned surface shows cluster formation at annealing temperatures of 400°C and above, with some micro-cracks at ion incidence angle of 45°. With increasing ion incidence angle, the optical phonon Raman modes display systematic downward shift and large asymmetric broadening associated with increased contribution of disorder activated LO and TO modes from the patterned surface. The lowering of phonon frequencies, induced by the phonon wave function confinement, signifies the presence of embedded nanocrystallites in the large sized nanodot patterned surface. The surface damage recovery is achieved by rapid thermal annealing of the samples as reflected in the increased optical phonon frequencies and reduced line shape broadening with annealing temperature. For large ion incident angle, the strain relaxation in the irradiated surface region leads to micro-crack formation in the patterned surface and further hardening of the phonon frequencies.

Characteristics of quadratic electro-optic effects and electro-absorption process in CdSe parabolic quantum dots

The nonlinear susceptibilities have been calculated theoretically for CdSe disk-like parabolic quantum dots by using a two-energy-level model in the strong-confinement regime. The confined wave functions and eigenenergies of excitons in parabolic quantum dots have been studied by solving the electron–hole effective-mass Schrodinger equation. The third-order susceptibilities of the quantum dots for quadratic electro-optic effects (QEOE) and electro-absorption (EA) process as a function of quantum dot size, relaxation time, parabolic confinement frequency, and pump photon energy have been also analyzed.

Energy dependence of the electron‐hole in‐plane anisotropy in InAs/GaAs quantum dots

AbstractDue to a regrettable technical error some Greek and special letters in the paper were not reproduced. Please, find here the fully corrected article.In pump‐probe experiments, we measured the anisotropic exchange energy splitting of the low‐lying electron‐hole pair states in an ensemble of as‐grown InAs/GaAs self‐assembled quantum dots. This splitting shows a monotonic decrease with the energy of interband emission. We discuss this result considering different parameters: shape anisotropy, wavefunction confinement, size of the quantum dots, and piezoelectric effect. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Energy dependence of the electron‐hole in‐plane anisotropy in InAs/GaAs quantum dots

AbstractIn pump‐probe experiments, we measured the anisotropic exchange energy splitting of the low‐lying electron‐hole pair states in an ensemble of as‐grown InAs/GaAs self‐assembled quantum dots. This splitting shows a monotonic decrease with the energy of interband emission. We discuss this result considering different parameters: shape anisotropy, wavefunction confinement, size of the quantum dots, and piezoelectric effect. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)