Ferromagnetism of semiconducting structures with the indirect interaction of magnetic impurities through a quasi-two-dimensional conduction channel

Ferromagnetism in a layer of magnetic impurity atoms indirectly interacting through a quasi-two-dimensional conduction channel in the nearest quantum well is considered in the mean-field model. The interaction appears through the RKKY mechanism due to the extension of the wavefunction of charge carriers localized in a model triangular well to the region where the impurities are localized. The Curie temperature and its dependence on the carrier density in the well and well depth are determined. The results are compared with the experimental data concerning the structures based on the diluted magnetic semiconductors Ca_1?x Mn _x As.     

Binding energies of excitons in symmetric and asymmetric quantum wells in a magnetic field

The binding energy of the exciton in the symmetric and asymmetric GaAs/Ga_1 − xAl_xAs quantum wells is calculated with the use of a variational approach. Results have been obtained as a function of the potential symmetry, and the size of the quantum well in the presence of an arbitrary magnetic field. The applied magnetic field is taken to be parallel to the axis of growth of the quantum well structure. The role of the asymmetric barriers, magnetic field, and well width in the excitonic binding is discussed as the tunability parameters of the GaAs/Ga_1 − xAl_xAs system.     

The energy spectra and wavefunctions of MSQW

On the basis of the quasistationary state approximation, the asymptotic transfer method (ATM) was extended to calculate the energy spectra, envelope functions of carriers and dispersion relations of Ga_1−xAl_xAs multiple sawtooth quantum wells (MSQW) in the presence of an electric field applied along the growth direction and a magnetic field parallel to the MSQW interfaces. It turned out that the energy spectra revealed the Landau-like behavior of carriers under a magnetic field. The envelope functions of carriers displayed obvious fluctuation and the dispersion relations showed a non-parabolic shape.     

Ferromagnetism in heterostructures based on a dilute magnetic semiconductor

Ferromagnetism of magnetic impurity atoms located in the barrier regions of various heterostructures (solitary heterojunction, single quantum well, double quantum well, or superlattice) is considered theoretically. The indirect magnetic interaction of impurities occurs via charge carriers localized in quasi-two-dimensional conducting channels of these structures due to “penetration” of the wavefunction of charge carriers into the barrier regions. The wavefunctions defined analytically in the triangular potential model are virtually the same as in “exact” numerical calculations (joint solution of the Poisson and Schrödinger equations). The corresponding Curie temperatures are determined, which may attain approximately 500 K in Ga_{1 ? x} Mn _x As-based structures according to calculations.     

Intense laser effects on nonlinear optical absorption and optical rectification in single quantum wells under applied electric and magnetic field

In this work the effects of intense laser on the electron-related nonlinear optical absorption and nonlinear optical rectification in GaAs-Ga_1−xAl_xAs quantum wells are studied under, applied electric and magnetic field. The electric field is applied along the growth direction of the quantum well whereas the magnetic field has been considered to be in-plane. The calculations were performed within the density matrix formalism with the use of the effective mass and parabolic band approximations. The intense laser effects are included through the Floquet method, by modifying the confining potential associated to the heterostructure. Results are presented for the nonlinear optical absorption, the nonlinear optical rectification and the resonant peak of these two optical processes. Several configurations of the dimensions of the quantum well, the applied electric and magnetic fields, and the incident intense laser radiation have been considered. The outcome of the calculation suggests that the nonlinear optical absorption and optical rectification are non-monotonic functions of the dimensions of the heterostructure and of the external perturbations considered in this work.     

Shallow impurity binding energy in lateral parabolic confinement under an external magnetic field

We have described the calculation of hydrogenic impurity binding energies in cylindrical GaAs–Ga_1−xAl_xAs quantum well wires (QWWs) with lateral parabolic confinement in the presence of an axial magnetic field. The numerical calculations of this system have been performed with the use of a variational procedure in the effective mass approximation. We observed sharp changes in binding energy for critical spatial confinement radius and B$B$ magnetic field values.     

Influence of electric current and magnetic field on terahertz photoconductivity in Pb1 − x Sn x Te(In)

Photoconductivity of Pb_{1 ? x} Sn _x Te(In) solid solutions in the terahertz spectral range is defined by a new type of local electron states linked to the quasi-Fermi level. The paper deals with investigation of the influence of electric current and magnetic field on the amplitude of the terahertz photoconductivity in Pb_{1 ? x} Sn _x Te(In) alloys of different composition. It is shown that the density of local electron states responsible for the positive persistent photoconductivity decreases with increasing electric current via a sample, as well as with transition to the hole conductivity in samples with a high content of tin telluride (x > 0.26). It is found that the magnetic field dependence of the positive photoconductivity is non-monotonous and has a maximum. The maximum position in magnetic field is proportional to the terahertz radiation quantum energy. Mechanisms responsible for the effects observed are discussed.     

Carrier collection in a semiconductor quantum well

Data are presented showing that a GaAs quantum well, sandwiched between two epitaxial Al_xGa_1-xAs(x ～ 0.4) confining layers, loses its effectiveness as a collector of excess carriers and as a source of recombination radiation for well dimensions L_z < 100 Å. It is shown that this behavior is expected because of the difficulty in scattering carriers to the bottom of the quantum well as L_z → l_p, the path length for scattering (LO phonon).     

Spin polarization and spin-dependent transmittance in II–VI diluted magnetic semiconductorheterostructure

In this work, we carried out detailed investigation of a Cd_1?xMn_xTe/CdTe/Cd_1?xMn_xTe diluted magnetic semiconductor based quantum well. Our theoretical results are based on an accurate self-consistent resolution of the one-dimension Schrödinger and Poisson’s equations in the framework of the mean-field approximation for spin-up and spin-down orientations of carriers coupled via the sp–d exchange interaction. From the calculation of spin-dependent carrier densities for ferromagnetic and anti-ferromagnetic coupling, we evidence the spin-up and spin-down space separation for holes in quantum well for different values of band offsets. From deduced spin polarizations, we show that the CdTe region acts as a layer of spin rearrangement and spin reversal, respectively, in the ferromagnetic and anti-ferromagnetic coupling. The transmittance coefficients T₊ and T_? of injected spin-up and spin-down carriers are evaluated as a preliminary work to assess the spin-dependent currents in devices consisting of alternatively layers of non-magnetic and diluted magnetic semiconductors.     

On the photogalvanic effect in asymmetric quantum wells of different shapes

In this work we studied the charge carriers' behaviour in quantum structures where the symmetry with respect to space coordinates and time-reversal symmetry are broken simultaneously. As the models of such structures we considered finite triangular as well as finite semi-parabolic quantum wells placed in external magnetic field. We have shown by numerical analysis that the energy spectra of charge carriers in such structures are anisotropic with respect to in-plane (transverse) motion ?n(+kx)≠?n(−kx). This leads to the anisotropy of charge carrier's in-plane momentum transfer which can be very naturally explained by introducing the concept of charge carriers ‘renormalized’ effective masses. The anisotropy of momentum transfer leads to interesting photo-galvanic effect, the anisotropy of photo-conductivity σ(+kx)≠σ(−kx) and as it follows from our calculations, the effect though not very great, could be measurable for the magnetic field of about few T.     

Electric-field-induced energy spectra and dispersions of ZnO/MgxZn1−xO MQWs

The energy spectra and dispersion relations of carriers in the presence of an electric field applied along the growth direction in ZnO/Mg_xZn_1−xO multiple quantum wells (MQW) are calculated using the asymptotic transfer method (ATM) on the basis of the quasistationary state approximation. The energy spectra of the carriers induce some quasi-bound levels under electric fields. The dispersion relations for the energy of the ground state and lower excitation states still have parabolic shapes for both the electrons and the heavy holes in the presence of a moderate electric field. Our results also reveal that the number of energy levels increases with increasing number of ZnO quantum wells and that the energies increase with both increasing Mg composition x and electric field strength.     

Linear and nonlinear optical properties in a semiconductor quantum well under intense laser radiation: Effects of applied electromagnetic fields

In this work we are studying the intense laser effects on the electron-related linear and nonlinear optical properties in GaAs–Ga_1?xAl_xAs quantum wells under applied electric and magnetic fields. The calculated quantities include linear optical absorption coefficient and relative change of the refractive index, as well as their corresponding third-order nonlinear corrections. The nonlinear optical rectification and the second and third harmonic generation coefficients are also reported. The DC applied electric field is oriented along the hererostructure growth direction whereas the magnetic field is taken in-plane. The calculations make use of the density matrix formalism to express the different orders of the dielectric susceptibility. Additionally, the model includes the effective mass and parabolic band approximations. The intense laser effects upon the system enter through the Floquet method that modifies the confinement potential associated to the heterostructure. The results correspond to several configurations of the dimensions of the quantum well, the applied electric and magnetic fields, and the incident intense laser radiation. They suggest that the nonlinear optical absorption and optical rectification are nonmonotone functions of the dimensions of the heterostructure and of the external perturbations considered in this work.     

On field effect studies and superconductor-insulator transition in high-Tc cuprates

We summarize previous field effect studies in high-T _c cuprates and then discuss our method to smoothly tune the carrier concentration of a cuprate film over a wide range using an applied electric field. We synthesized epitaxial one-unit-cell thick films of La_2?x Sr _x CuO₄ and from them fabricated electric double layer transistor devices utilizing various gate electrolytes. We were able to vary the carrier density by about 0.08 carriers per Cu atom, with the resulting change in T _c of 30 K. The superconductor-insulator transition occurred at the critical resistance very close to the quantum resistance for pairs, R _Q = h/(2e)² = 6.5 kΩ. This is suggestive of a quantum phase transition, possibly driven by quantum phase fluctuations, between a “Bose insulator” and a high-T _c superconductor state.     

Quantum wells under an in-plane magnetic field: Effect of the composition parameters on excited electron energy splitting

The dependence of the electronic spin-splitting energy on the composition parameters (x,y) in In_xGa_1?xAs–In_yAl_1?yAs-based quantum wells, has been calculated. InGaAs narrow gap structures, subjected to in-plane magnetic fields, have been selected because these structures have a big Landè factor. The dependence of the Landé factor both on the applied fields and composition parameters has been included for fixed well width and external electric field. Contributions from the interfaces and strain, which also depend on the composition, are included. Spin-splitting energy and density of states show a strong dependence on the above parameters.     

Influence of quantum confinement on the photoemission from superlattices of optoelectronic materials

We study the photoemission from quantum wire and quantum dot superlattices with graded interfaces of optoelectronic materials on the basis of newly formulated electron dispersion relations in the presence of external photo-excitation. Besides, the influence of a magnetic field on the photoemission from the aforementioned superlattices together with quantum well superlattices in the presence of a quantizing magnetic field has also been studied in this context. It has been observed taking into account HgTe/Hg_1−xCd_xTe and In_xGa_1−xAs/InP that the photoemission from these nanostructures increases with increasing photon energy in quantized steps and exhibits oscillatory dependences with the increase in carrier concentration. Besides, the photoemission decreases with increasing light intensity and wavelength, together with the fact that said emission decreases with increasing thickness exhibiting oscillatory spikes. The strong dependences of the photoemission on the light intensity reflects the direct signature of light waves on the carrier energy spectra. The content of this paper finds six applications in the fields of low dimensional systems in general.     

Effects of applied electric and magnetic fields on the nonlinear optical rectification and second-harmonic generation in a graded quantum well under intense laser field

In this present study, the effects of electric and magnetic fields on the nonlinear optical rectification and second-harmonic generation in a graded quantum well under intense laser field have been investigated theoretically. The energy eigenvalues and their corresponding eigenfunctions are obtained by solving Schrödinger equation within the framework of effective mass approximation. The analytic expressions for the optical properties are calculated by the compact-density-matrix approach and iterative method. The numerical results are presented for a typical GaAs/Ga_1?x Al _x As quantum well. The results show that the nonlinear optical rectification and second-harmonic generation coefficients are considerably affected by the electromagnetic fields and intense laser field.     

Combined effects of magnetic and electric fields on the interband optical transitions in InAs/InP quantum wire

Combined effects of magnetic and electric fields on the confined exciton in an InAs_1−xP_x/InP (x=0.2) quantum well wire are investigated taking into account the geometrical confinement effect. Variational formulism, within the frame work of effective mass approximation, is applied to obtain the exciton binding energy. The second order harmonic generation and the optical gain are carried out using compact density method. The strain effects are included with the confinement potential in the Hamiltonian. The energy difference of the ground and the first excited state is found in the presence of magnetic and electric fields taking into the consideration of spatial confinement effect. The result shows that the optical properties are more influenced taking into account the effects of geometrical confinement, magnetic field and electric field. It is shown that the telecommunication wavelength can be achieved with the suitable doping barrier material with the wire material and the external perturbations.     

Low temperature electronic transports in the presence of a density gradient

In this paper, we review low temperature electronic transport results in high quality two-dimensional electron systems. We discuss the quantization of the diagonal resistance, R_xx, at the edges of several quantum Hall states. Each quantized R_xx value turns out to be close to the difference between the two adjacent Hall plateaus in the off-diagonal resistance, R_xy. Moreover, peaks in R_xx occur at different positions in positive and negative magnetic fields. All three R_xx features can be explained quantitatively by a ∼1% cm electron density gradient. Furthermore, based on this observation, the well known but still enigmatic resistivity rule, relating R_xx to , finds a simple interpretation in terms of this gradient. In another sample, at 1.2 K, R_xx we observe a strongly linear magnetic field dependence. Surprisingly, this linear magnetoresistance also originates from the density gradient. Our findings throw an unexpected light on the relationship between the experimentally measured R_xx and the diagonal resistivity ρ_xx.     

Simple theoretical analysis of the Fowler-Nordheim field emission from microstructures and quantum wires of optoelectronic materials

We present a simplified theoretical formulation of the Fowler-Nordheim field emission (FNFE) under magnetic quantization and also in quantum wires of optoelectronic materials on the basis of a newly formulated electron dispersion law in the presence of strong electric field within the framework of k.p formalism taking InAs, InSb, GaAs, Hg_1−xCd_xTe and In_1−xGa_x As_yP_1−y lattice matched to InP as examples. The FNFE exhibits oscillations with inverse quantizing magnetic field and electron concentration due to SdH effect and increases with increasing electric field. For quantum wires the FNFE increases with increasing film thickness due to the existence van-Hove singularity and the magnitude of the quantum jumps are not of same height indicating the signature of the band structure of the material concerned. The appearance of the humps of the respective curves is due to the redistribution of the electrons among the quantized energy levels when the quantum numbers corresponding to the highest occupied level changes from one fixed value to the others. Although the field current varies in various manners with all the variables in all the limiting cases as evident from all the curves, the rates of variations are totally band-structure dependent. Under certain limiting conditions, all the results as derived in this paper get transformed in to well known Fowler-Nordheim formula.