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.     

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.     

Optical and magneto-optical studies of a multiferroic GaFeO<Subscript>3</Subscript> with a high Curie temperature

Single crystals of a noncentrosymmetric orthorhombic pyroelectric ferrimagnet Ga_2?xFe_xO₃ with a Curie temperature within 260–345 K have been grown by the flux method. It has been found that the electrical properties of the single crystals varied over a broad range from 10⁵ to 10¹³ Ω cm depending on the presence of transitionmetal oxide impurities. The dispersion relations for all three principal dielectric functions of orthorhombic GaFeO₃ have been determined in the range 0.7–5.4 eV by spectroscopic ellipsometry. The spectra of the dielectric functions of the orthorhombic Ga_2?xFe_xO₃ crystals are compared with the spectra of the trigonal crystals. The Faraday effect and second-harmonic generation are studied, and the law of the transition to the paramagnetic state has been determined. The crystallographic and magnetic contributions to the second-harmonic generation are analyzed.     

The phonon dispersion relations in a disordered Ni1-xPtx system

The phonon dispersion relations of disordered alloys Ni_1-xPt_x were measured for five specimens covering the full range of atomic concentration by means of neutron inelastic scattering. At the resonant frequencies well separated double peak structures were observed in the phonon spectra. For the specimens with $\text{x ≧ 0.30}$, the energy splitting of the phonon dispersion curve for the L-branch takes place at a higher frequency than that for the T-branch. The maximum phonon energy decreases with increasing Pt concentration but deviates from the theoretical prediction based on the mass defect CPA model. For x = 0.95 the localized mode cannot be observed as a well defined peak and does not seem to be split off from the main phonon band.     

Defects related room temperature ferromagnetism in p-type (Mn, Li) co-doped ZnO films deposited by reactive magnetron sputtering

We report on the defects related room temperature ferromagnetic characteristics of Zn_0.95-xMn_xLi_0.05O (x = 0.01, 0.03, 0.05 and 0.08) thin films grown on glass substrates using reactive magnetron sputtering. By increasing the Mn content, the films exhibited increases in the c-axis lattice constant, fundamental band gap energy, coercive field and remanent magnetization. Comparison of the structural and magnetic properties of the as-deposited and annealed films indicates that the hole carriers, together with defects concentrations, play an important role in the ferromagnetic origin of Mn and Li co-doped ZnO thin films. The ferromagnetism in films can be described by bound magnetic polaron models with respect to defect-bound carriers.     

Tilted magnetic field effect on the subbands of a GaAlAs diode with a GaAs quantum well

We have calculated the subbands in the GaAs quantum well at the n-side of the junction in a Ga_1−xAl_xAs diode. We show that the density of carriers confined in the quantum well increases by the increasing magnetic field strength but also decreases depending on the magnetic length. We have observed oscillatory behavior of the density of carriers due to the in-plane magnetic field.     

Terahertz spectroscopy of quantum-well narrow-bandgap HgTe/CdTe-based heterostructures

The energy spectra of quantum-well narrow-bandgap Hg_{1 ? y} Cd _y Te/Cd _x Hg_{1 ? x} Te heterostructures have been studied. The dependences of the effective cyclotron mass on the density (in classical magnetic fields) and the transition energy (in quantizing fields) have been obtained from the cyclotron resonance measurements. These dependences confirm the near-linear dispersion law for the electrons with small mass at the band bottom (the minimum cyclotron mass measured is 0.003 m ₀). The interband photoconductivity of the CdHgTe-based structures with the long-wavelength photoresponse edge lower than 6 meV has been demonstrated.     

Absorption spectra near the absorption edge of the system HgxZn1−xCr2Se4

Optical absorption spectra near the absorption edge of the system Hg_xZn_1?xCr₂Se₄ have been measured for various compositions in the temperature range between room and liquid helium temperatures. A correlation between the optical properties and magnetic structures becomes obvious from the measurements of the energy at the absorption edge and the change in the magnetic property with composition variation, and by measuring the optical absorption spectra under the applied magnetic field.     

Kinetic effects in manganites La1 − x Ag y MnO3 (y ≤ x)

We have measured the resistivity, magnetoresistance, and thermopower of ceramic manganite samples La_{1 ? x} Ag _y MnO₃ (y ≤ x) doped with silver as functions of temperature (4.2–350 K) and magnetic field (up to 26 kOe). A metal-insulator phase transition is observed in all investigated samples at temperatures close to room temperature. The behavior of the resistivity and thermopower in the high-temperature paramagnetic region is interpreted using the concept of small radius polaron; the activation energy decreases with increasing doping level. The resistivity in the low-temperature ferromagnetic region is approximated by the expression ρ_FM(T) = ρ₀ + AT ² + BT ^4.5 presuming the existence of electron-electron and electron-magnon interactions. A resistivity minimum and a strong magnetoresistive effect are observed at low temperatures. The latter effect is associated with scattering of charge carriers at grain boundaries, which are antiferromagnetically ordered relative to one another. The temperature dependence of thermopower in the magnetically ordered phase is described in the framework of a model taking into account the drag of charge carriers by magnons.     

Influence of silicon and atomic order on the magnetic properties of (Fe80Al20)100– x Si x nanostructured system

Mechanically alloyed (Fe₈₀Al₂₀)_100???x Si _x alloys (with x?=?0, 10, 15 and 20) were prepared by using a high energy planetary ball mill, with milling times of 12, 24 and 36 h. The structural and magnetic study was conducted by X-rays diffraction and Mössbauer spectrometry. The system is nanostructured and presents only the BCC disordered phase, whose lattice parameter remains constant with milling time, and decreases when the Si content increases. We found that lattice contraction is influenced 39% by the iron substitution and 61% by the aluminum substitution, by silicon atoms. The Mössbauer spectra and their respective hyperfine magnetic field distributions show that for every milling time used here, the ferromagnetism decreases when x increases. For samples with x?≥?15 a paramagnetic component appears. From the shape of the magnetic field distributions we stated that the larger ferromagnetic phase observed in the samples alloyed during 24 and 36 h is a consequence of the structural disorder induced by mechanical alloying.     

Magnetic cluster developement in In1−x Mn x Sb semiconductor alloys

X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM) and magnetic measurements as a function of applied magnetic field and temperature for In_1?x Mn _x Sb (0.05≤x≤0.2) system are reported. Magnetic measurements performed at high and small magnetic field in ZFC and FC indicate the coexistence of ferromagnetic In_1?x Mn _x Sb solid solution and two types of magnetic cluster: ferromagnetic MnSb and ferrimagnetic Mn₂Sb. XPS valence band and Mn 2p core level spectra have confirmed the presence of MnSb and Mn₂Sb phases. TEM images show some manganese antimonide phase microinclusions with dimension between (30–40) nm.     

Magnetic polarons,clusters, and their effect on the electric properties of weakly doped lanthanum manganites

The resistivity, magnetoresistance, thermopower, and magnetic susceptibility of La_1?xA_xMnO₃(A≡Ca,Sr;x=0.07–0.1) single crystals are investigated in the temperature range from 77 to 400 K. Sharp changes in the properties (the resistivity activation energy ΔEρ, its temperature coefficient γ, the thermopower activation energy ΔE _S , the magnetoresistance, and the appearance of spontaneous magnetization) of these crystals occur near a temperature of 275±25 K, which is approximately twice as high as their Curie point T_C and approximately half of the structural transition temperature. The results are explained by the phase separation: the formation of ferromagnetic clusters. The phase separation occurs through the coalescence of small-radius unsaturated magnetic polarons, in which only two or three magnetic moments of Mn are polarized, into a large-radius ferromagnetic polaron (a cluster about 10–12 Å in size) with several charge carriers. As a result, the short-range order occurs in the cluster at a temperature of about 275 K, which is close to T _C of conducting doped manganites. The results of the experimental studies of the resistivity and the magnetoresistance as functions of temperature and magnetic field and the estimates agree well with the cluster model.     

Optical and magnetic properties of (Ga1−xMnx)N/GaN digital ferromagnetic heterostructures

(Ga_1−xMn_x)N/GaN digital ferromagnetic heterostructures (DFHs) and (Ga_1−xMn_x)N/GaN grown on GaN buffer layers by using molecular beam epitaxy have been investigated. The photoluminescence (PL) spectra showed band-edge exciton transitions. They also showed peaks corresponding to the neutral donor-bound exciton and the exciton transitions between the conduction band and the Mn acceptor, indicative of the Mn atoms acting as substitution. The magnetization curves as functions of the magnetic field at 5 K indicated that the saturation magnetic moment in the (Ga_1−xMn_x)N/GaN DFHs decreased with increasing Mn mole fraction and that the saturation magnetic moment and the coercive field in the (Ga_1−xMn_x)N/GaN DFHs were much larger than those in (Ga_1−xMn_x)N thin films. These results indicate that the (Ga_1−xMn_x)N/GaN DFHs hold promise for potential applications in spintronic devices.     

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.     

Conductivity in a disordered medium and carrier localization in weakly doped lanthanum manganites

The dc and 9.2-GHz electrical resistivities and magnetoresistance observed in La_1?xA_xMnO₃ crystals (A=Sr, Ce, x≤0.1) in the temperature interval 77–300 K are accounted for by the contributions due to carriers, both nonlocalized and localized in the valence-band tail and near the Fermi level. The localized-state tail extends to a depth of 0.15–0.25 eV inside the band gap, and the hopping activation energy varies from 0.06 to 0.15 eV, depending on the sample composition. Within the temperature region where magnetic ordering sets in, the variations of the electrical resistivity and magnetoresistance with temperature and magnetic field are caused by variations in the carrier mobility and concentration.     

Photoluminescence of ZnMnTe quantum-well structures in a magnetic field

Photoluminescence spectra of strained structures Zn_{1 ? x} Mn _x Te/Zn_{1 ? y} Mg _y Te with magnetic quantum wells and nonmagnetic barriers are studied. The Zeeman splitting of the heavy exciton is found to follow an unusual behavior: both spin components shift down in energy. The heavy-exciton photoluminescence Zeeman components are observed to be inversely distributed in intensity, with the higher energy component being stronger than the lower energy component. The Zeeman splitting of the exciton in a magnetic field is calculated. The data obtained permit refinement of some parameters of the energy spectrum and magnetic properties of these structures.     

Interband optical absorption in a strained CdxZn1−xO/ZnO quantum dot

Numerical calculations of the excitonic absorption spectra in a strained Cd_xZn_1−xO/ZnO quantum dot are investigated for various Cd contents. We calculate the quantized energies of the exciton as a function of dot radius for various confinement potentials and thereby the interband emission energy is computed considering the internal electric field induced by the spontaneous and piezoelectric polarizations. The optical absorption as a function of photon energy for different dot radii is discussed. Decrease of exciton binding energy and the corresponding optical band gap with the Cd concentration imply that the confinement of carriers decreases with composition x. The main results show that the confined energies and the transition energies between the excited levels are significant for smaller dots. Non-linearity band gap with the increase in Cd content is observed for smaller dots in the strong confinement region and the magnitude of the absorption spectra increases for the transitions between the higher excited levels.     

Exciton binding energy and the optical absorption coefficient in a strained CdxZn1−xO/ZnO quantum dot

Numerical calculations of the excitonic absorption spectra in a strained Cd_xZn_1?xO/ZnO quantum dot are investigated for various Cd contents. We calculate the quantized energies of the exciton as a function of dot radius for various confinement potentials and thereby the interband emission energy is computed considering the internal electric field induced by the spontaneous and piezoelectric polarizations. The optical absorption as a function of photon energy for different dot radii is discussed. Decrease of exciton binding energy and the corresponding optical band gap with the Cd concentration imply that the confinement of carriers decreases with composition x. The main results show that the confined energies and the transition energies between the excited levels are significant for smaller dots. Non-linearity band gap with the increase in Cd content is observed for smaller dots in the strong confinement region and the magnitude of the absorption spectra increases for the transitions between the higher excited levels.     

Superconducting properties of PbxSn1−xTe

Superconducting transition temperatures and critical magnetic field curves on low carrier concentration Pb_xSn_1?xTe samples where 0.25 ?x?0.55 are reported. These data are interpreted in terms of a changing band structure from that of SnTe to one which is similar to that of GeTe. The importance of carriers (holes) in secondary maxima of the valence band is emphasized.     

Magnetic polaron contribution to donor bound exciton in Cd1−xMnxSe

Photoluminescence spectra associated with the donor bound exciton has been studied in Cd_1-xMn_xSe as a function of composition (x = .10, .05, and 0.0) and temperature in zero magnetic field. In comparison with the previously studied neutral donor, the bound exciton shows a large enhancement in its binding energy and broadening, attributed to the exchange interaction with the Mn⁺⁺-ion 3d electrons. In particular, a transition from a bound polaron to a fluctuation dominated regime is in evidence for x = .10 in this multiparticle system.