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The effects of a growth-direction applied magnetic field on the cyclotron effective masses and electron effective Lande [g.sub.II] factor in semiconductor GaAs-[Ga.sub.1-x][Al.sub.x]As quantum wells (QWs) are theoretically evaluated. The calculations are performed for different widths of the QWs and as functions of the applied magnetic field, with results in good agreement with experimental measurements of the electron cyclotron effective mass and [g.sub.II] factor.

Using the transfer matrix method, we study the transmittance of 1D photonic crystals made of alternated layers of a semiconductor (GaAs) and a high-Tc superconductor (HgBa2Ca2Cu3O8+δ) under the effects of temperature, applied hydrostatic pressure, and thickness of the layers. The frequency-dependent dispersion formula according to the two-fluid model was adopted to describe the optical response of the superconducting system. We found that increasing the superconductor (semiconductor) layer thickness results in a shift to higher (lower) values of the transmittance cutoff frequency. Additionally, this cutoff frequency is shifted to lower values with the temperature increase. Furthermore, we found that the width of the photonic bandgaps varies with the applied pressure. The most notorious variation is presented near the 17 THz region, where a new gap appears with the increase in pressure. We hope this work may be taken into consideration for the development of new perspectives in the design of new optical devices. [ABSTRACT FROM AUTHOR]

Parallelepiped-shaped GaAs-(Ga,AI)As quantum dots were investigated to determine the effects of uniaxial stress on binding energies of shallow donor impurities.

The influence of electric fields on the states of a donor impurity in rectangular cross-section vacuum/GaAs/vacuum quantum-well wires was investigated using effective mass approximations within a variational approach. The results revealed that applied electric fields decompose the degeneracy of the states for impurities symmetrically positioned within the structure. Moreover, the electric field and geometric configuration were indicators of the presence of bound excited states.

Binding energy calculation with effective mass-approximation of a shallow-donor hydrogenic impurity in a rectangular cross section of GaAs quantum well wire indicated break downs in state degeneracy for impurities within the structure due to the electric field. Impurity binding energy is conditioned by quantum confinement, the applied electric field and impurity distribution inside the wires.

The binding energies of shallow impurities in spherical GaAs-(Ga,Al)As quantum dots are calculated by a variational procedure. This procedure is based on the effect of spatially dependent screening and is within the range necessary for effective-mass approximation. Binding energies are increased by the gamma-dependent dielectric response. When dimensionality is reduced, the spatially-dependent screening increases quantum confinement.

Ferrofluids based on magnetic Co0.25Zn0.75Fe2O4 ferrite nanoparticles were prepared by co-precipitation method from aqueous salt solutions of Co (II), ZnSO4, and Fe (III) in an alkaline medium. Ferrofluids placed in an external magnetic field show properties that make them interesting as magneto-controllable soft photonic crystals. Morphological and structural characterizations of the samples were obtained from Scanning Electron Microscopy and Transmission Electron Microscopy studies. Magnetic properties were investigated with the aid of a vibrating sample magnetometer at room temperature. Herein, the Co0.25Zn0.75Fe2O4 samples showed superparamagnetic behavior, according to hysteresis loop results. Taking in mind that the Co-Zn ferrite hysteresis loop is very small, our magnetic nanoparticles can be considered soft magnetic material with interesting technological applications. In addition, by using the plane-wave expansion method, we studied the photonic band structure of 2D photonic crystals made of ferrofluids with the same nanoparticles. Previous experimental results show that a magnetic field applied perpendicular to the ferrofluid plane agglomerates the magnetic nanoparticles in parallel rods to form a hexagonal 2D photonic crystal. We calculated the photonic band structure of photonic crystals by means of the effective refractive index of the magnetic fluid, basing the study on the Maxwell-Garnett theory, finding that the photonic band structure does not present any band gaps under the action of applied magnetic field strengths used in our experimental conditions. [ABSTRACT FROM AUTHOR]

A theoretical investigation of the photoluminescence spectra linked with free-carrier to acceptor-impurity recombination in quantum-size GaAs wire crystals, has been undertaken. The actual GaAs quantum wires were modeled via low-dimensional systems having finite cylindrical shape and hard-wall boundaries. The photoluminescence spectra were calculated for a homogeneous distribution of acceptor impurities, and the photoluminescence line shape exhibited three structures linked with impurities.

We have studied the quantum confinement, applied hydrostatic pressure, and temperature dependence of the binding energy of a magnetoexciton bound to a ionized-donor impurity in GaAs/Ga1-xAlxAs quantum wells, taking into account the spin-orbit coupling between the (Γ7v,Γ8v) and (Γ7c,Γ8c) multiplets, including the Al concentration, temperature, and applied hydrostatic pressure dependence on the electron effective-mass me(P,T,x) and the Landé ge(P,T,x) factor by using the well known five-level k · p theory. We have found that the binding energy Eb increases with the strong geometrical confinement, as well as with the growth-direction applied magnetic field. The presence of the ionized-donor impurity clearly increases the heavy-hole exciton binding energy. The quantum confinement, in part determined by the height of the barrier potential-well, i.e., by the Al concentration and the hydrostatic pressure, contributes to enhance the binding energy. Also, we found that the exciton binding energy increases with temperature due to the different temperature band-gap dependence of the well and barrier regions, which conduces to a net increasing of the potential barrier. Also, we have obtained a good agreement with previous theoretical and experimental findings. We hope the present work must be taken into account for the understanding of experimental reports and for the design of optoelectronic devices with multiple technological purposes. [ABSTRACT FROM AUTHOR]

We have performed a theoretical study of the hydrostatic pressure effects on the conduction-electron Landé g∥ factor in GaAs–Ga1-xAlxAs quantum heterostructures (QHs) under the influence of applied magnetic fields. Numerical calculations are performed by using the Ogg–McCombe effective Hamiltonian, which include nonparabolicity and anisotropy effects for the conduction-band electrons. The QHs is assumed to consist of a finite-length cylinder of GaAs surrounded by Ga1-xAlxAs barrier. Theoretical results are given as functions of the radii, lengths, hydrostatic pressure, and applied magnetic fields. We have studied the competition between the geometrical and magnetic confinement versus hydrostatic pressure effects, finding that the geometrical confinement commands the behavior of the g∥ factor. Present theoretical results are in very good agreement with previous experimental and theoretical reports in GaAs–Ga1-xAlxAs heterostructures. [ABSTRACT FROM AUTHOR]

Here we address a theoretical analysis of the effects of applied hydrostatic pressure on electron states in concentric GaAs–(Ga,Al)As double quantum rings, under axial magnetic fields. Emphasis is put on the dependence of such effects on the system geometry confinement described within a hard potential model and following an effective-mass approximation. The energy of the ground and excited electronic states were found to decrease with the applied hydrostatic pressure, due mainly to an effective reduction in the barrier potential confinement. Also, while the increase in the magnetic field opens the electron states degeneracy with different angular momenta, the increase in the applied hydrostatic pressure does not alter significantly the energy of these states. For both symmetric and asymmetric double quantum rings, one found that the electron-heavy hole transition energies augment with the applied hydrostatic pressure, mainly due to the increase in the GaAs gap. [ABSTRACT FROM AUTHOR]

A standard plane-wave expansion technique is used to investigate the evolution of the photonic band structure of a two-dimensional honeycomb lattice composed by cylindrical shell rods with dielectric permittivities [variant_greek_epsilon]1 and [variant_greek_epsilon]2, and embedded in a background with permittivity [variant_greek_epsilon]3. We have considered the effect of dispersive dielectric responses as well as the influence of an externally applied magnetic field aiming to obtain efficient tunable bandgaps. Present results suggest that a combination of a doped semiconductor constituent with an anisotropic geometry, which breaks symmetry and unfolds degeneracies, provides an efficient realization of photonic systems with tunable bandgaps. [ABSTRACT FROM AUTHOR]

Photoluminescence and photoreflectance responses at 12 K of Ga1-xInxAsySb1-y films grown over GaSb single-crystal substrates by liquid-phase epitaxy do not exhibit the same energy values. The photoluminescence peak shifts to 20 meV in relation to the photoreflectance response, indicating a possible electronic confinement in some parts of the growth heterostructure. The possibility of low-dimensional structures is investigated herein. They are possibly formed during the growth process and are responsible for this energy shift. The optical emission spectra associated with transitions between the first conduction and valence-band levels in spherical Ga1-xInxAsySb1-y/GaSb quantum dots, using x and y values in the range of 0.11–0.15 and 0.10–0.14, respectively, are studied here. These ranges were chosen according to experimental results obtained from the samples under study. Spherical potential wells of finite depth determined by the discontinuity in the conduction band ΔEC for electrons and the discontinuity in the valence band ΔEV for holes were used. In the calculations, the variational procedure within the effective-mass approximation was used and electrons and holes in a type-I band alignment formed by two semiconductors with similar parabolic conduction bands were considered. From these results, it was concluded that higher electronic confinements such as quantum dots were possibly formed during the growth process in some areas of the heterostructure. [ABSTRACT FROM AUTHOR]

The effects of the Dresselhaus spin splitting on the Landé g factor associated with conduction electrons in GaAs–(Ga,Al)As quantum wells are studied by using the nonparabolic Ogg–McCombe effective Hamiltonian. The g factor and cyclotron effective mass are calculated as functions of applied magnetic fields (along both the growth and in-plane directions) and GaAs well widths of the heterostructure. Present calculations indicate that in GaAs–(Ga,Al)As heterostructures, the inclusion of the Dresselhaus term leads to very small corrections in the effective Landé factor. Taking into account the effects of nonparabolic and anisotropic terms in the Hamiltonian is fundamental in obtaining quantitative agreement with experimental measurements. Moreover, the present results suggest that previous theoretical work on the Dresselhaus spin-splitting effects on the effective Landé factor should be viewed with caution if nonparabolic and anisotropic effects are not taken into account. [ABSTRACT FROM AUTHOR]

We have performed a theoretical study of the cyclotron effective mass and electron effective Landé g∥ factor in semiconductor GaAs–Ga1-xAlxAs quantum wells under an applied magnetic field parallel to the growth direction of the quantum well. The theoretical approach is within the nonparabolic and effective-mass approximation and via an Ogg-McCombe effective Hamiltonian [Proc. Phys. Soc. London 89, 431 (1969); Phys. Rev. 181, 1206 (1969)] for the electron in the conduction band of the GaAs–Ga1-xAlxAs heterostructure, which allows a unified treatment of both the cyclotron mass and g∥ factor. Calculations are performed for different widths of the GaAs–Ga1-xAlxAs quantum wells and as functions of the applied magnetic field, with results in very good agreement with reported experimental measurements of the electron cyclotron effective mass and g∥ factor. [ABSTRACT FROM AUTHOR]

A theoretical study is presented of the effects of in-plane magnetic fields on the I–V characteristic curves associated with resonant electron tunneling through shallow impurity states in GaAs/(Al,Ga)As double-barrier heterostructures. A simple one-band tight-binding Hamiltonian is used and the current is obtained by adopting Keldysh diagrammatic techniques for nonequilibrium processes. As the field strength increases the peak associated with the donor-assisted tunneling shifts to lower voltages whereas the resonance associated with tunneling through the first subband shifts to higher ones. Also, the intensities are both reduced, in qualitatively good agreement with the experimental reports of Sakai et al. [Phys. Rev. B. 48, 5664 (1993)]. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have studied the effects of an uniaxial stress on the binding energy of a shallow donor impurity in a parallelepiped-shaped GaAs–(Ga,Al)As quantum dot. In the calculations we have used a variational technique within the effective-mass approximation. The stress was applied in the z direction and the donor impurity was located at various positions along the z axis. Our results show that the donor binding energy increases with increasing stress and for decreasing sizes of the quantum dot. Also, we have found that the binding energy for various values of the donor position along the z axis for constant quantum well box size increases with the proximity of the impurity to the center of the structure. Moreover, we obtain the shallow-donor binding energies as functions of uniaxial stress in the limit in which the quantum dot turns into either a quantum well or a quantum-well wire. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Hydrogenic donor binding energies in infinite and finite GaAs-(Ga,Al)As spherical quantum dots are obtained as a function of the donor position for various radii inside the effective mass approximation. An increase in the donor binding energy of the quantum dots is observed when the obtained results are compared to the quantum well wires and quantum wells. This indicates the presence of higher geometrical electronic confinement in the GaAs-(Ga,Al)As systems.

The hydrostatic pressure effects on the conduction-electron Lande g factor in GaAs-[Ga.sub.1-x][Al.sub.x]As quantum heterostructures (QHs) under the impact of applied magnetic fields are studied. The competition between the geometrical and magnetic confinement versus hydrostatic pressure effects has shown that the geometrical confinement has commanded the behavior of the g factor.