Analysis of magnetic field dependent mobility in ferromagnetic Ga1−xMnxAs layers

The magneto-transport properties of ferromagnetic Ga_1−xMn_xAs epilayers with Mn mole fractions in the range of x≈2.2-4.4% were investigated through Hall effect measurements. The magnetic field-dependent Hall mobility for a metallic sample with x≈2.2% in the temperature range of T=0-300 K was analyzed by magnetic field-dependent mobility model including an activation energy of Mn acceptor level. This model provides outstanding fits to the measured data up to T=300 K. It was found that the acceptor levels with activation energies of 112 meV at B=0 Oe decreased to 99 meV at B=5 kOe in the ferromagnetic region. The decrease in acceptor activation energy was due to the spin splitting of the Mn acceptor level in the ferromagnetic region, and was responsible for increase in carrier concentration.     

Investigation of carrier dynamics in Zn1−xMgxO by time-resolved photoluminescence

The influence of the Mg concentration and lattice temperature on the carrier recombination dynamics in Zn_1−xMg_xO alloys has been studied by time-resolved photoluminescence for different emission and excitation energies. Carrier localization effects are found to play a significant role, becoming increasingly important for lower temperatures and higher Mg concentrations. Emission energy dependent dynamics were analyzed by the application of the theoretical model, yielding a characteristic localization energy of 60±15 meV for the sample with the highest Mg concentration of x=0.21.     

Fundamental optical properties of Cd1-xMnxSe single crystals

The absorption near the fundamental edge of Cd_1-xMn_xSe was measured in the composition range 0<x<0.3 at room and liquid nitrogen temperature with the electric field of the radiation parallel and perpendicular to the hexagonal axis. An exponential dependence of the absorption coefficient versus photon energy was found, and a linear dependence of energy gap E₀ on composition was obtained. The room temperature reflectivity measurements in the energy range 2.5–5.2eV, for two polarization of light were performed, and a linear dependence of the interband transitions energies vs. alloy composition was found.     

Influence of indium content on the optical, electrical and crystallization kinetics of Se100−xInxthin films deposited by flash evaporation technique

Thin films of Se _100−xIn_x (x=10, 20 and 30 at%) have been prepared by the flash evaporation technique. The effect of the indium content on optical band gap of the Se_100−x In_x films has been investigated by the optical characterization. The optical band gap values of the Se_100−x In_x thin films were determined and are found to decrease with increasing indium content. This indium content changes the width of localized states in the optical band gaps of the thin films. It was found that the optical band gap, E_g, of the Se_100−x In_x films changes from 1.78 to 1.37 eV with increasing indium content from 10 to 30 at%, while the width of localized states in optical band gap changes from 375 to 342 meV. The temperature dependence of the dark electrical conductivity were studied in the temperature range 303-433 K and revealed two activation energies providing two electrical conduction mechanisms. The activation energy of the Se_100−x In_x films in the high temperature region changes from 0.49 to 0.32 eV with increasing indium content from 10 to 30 at%, while the hopping activation energy in the lower temperature region changes from 0.17 to 0.22 meV. The change in the electrical conductivity with time during the amorphous-to-crystalline transformation is recorded for amorphous Se_100−xIn_x films at two points of isothermal temperatures 370 and 400 K. The formal crystallization theory of Avrami has been used to calculate the kinetic parameters of crystallization.     

Modification of Cd1−xMnxTe crystal surface layers by nanopulsed laser irradiation

The surface modification of Cd_1−xMn_xTe (x = 0-0.3) crystal wafers under pulsed laser irradiation has been studied. The samples were irradiated by a Q-switched ruby laser with pulse duration of 80 ns. Optical diagnostics of laser-induced thermal processes were carried out by means of time-resolved reflectivity measurements at wavelengths 0.53 and 1.06 μm. Laser irradiation energy density, E varied in the range of 0.1-0.6 J/cm². Morphology of irradiated surface was studied using scanning electron microscopy. The energy density whereby the sample surface starts to melt, depends on Mn content and is equal to 0.12-0.14 J/cm² for x ≤ 0.2, in the case of x = 0.3 this value is about 0.35 J/cm². The higher Mn content leads to higher melt duration. The morphology of laser irradiated surface changes from a weakly modified surface to a single crystal strained one, with an increase in E. Under irradiation with E in the range of 0.21-0.25 J/cm², the oriented filamentary crystallization is observed. The Te inclusions on the surface are revealed after the irradiation of samples with small content of Mn.     

Ionic conductivity in the single crystals of non-stoichiometric fluorite phases M1−xRxF2+x (M=Ca,Sr, Ba; R=Y,La-Lu)

The ionic conductivity has been measured of single crystals of rare earth fluoride solid solutions in Ca, Sr and Ba fluorides described by the formula M_1?xR_xF_2+x (M=Ca, Sr, Ba; R=Y, La-Lu). The measurements have been done using dc and ac within the temperature range 300–850 K. With the increase of RF₃ concentration up to x=0.05–0.15 (in different systems) conductivity steeply increases. With the further concentration growth the conductivity increases only slightly, reaching saturation value for the solid solution Ba_1?xR_xF_2+x. The dependence of solid solution conductivity on the chemical composition has been studied for isoconcentrates M_0.9R_0.1F_2.1. It has been established that in CaF_2- based series the crystals with R=Gd, Tb possess maximum conductivity and minimum activation energy. For SrF₂ and BaF₂-based series the highest conductivity was observed for crystals containing LaF₃. It has been established that for all the crystals, independent of the chemical composition and defect concentration, the conductivity logarithm at definite temperature linearly depends on the activation energy. Within the investigated class of substances the optimum composition for solid electrolytes is Sr_0.69La_0.31F_2.31.     

Non-Fermi-liquid behavior in UCu4+xAl8−x compounds

We report on experimental studies of the Kondo physics and the development of non-Fermi-liquid scaling in UCu_4+xAl_8−x family. We studied 7 different compounds with compositions between x=0 and 2. We measured electrical transport (down to 65 mK) and thermoelectric power (down to 1.8 K) as a function of temperature, hydrostatic pressure, and/or magnetic field.Compounds with Cu content below x=1.25 exhibit long-range antiferromagnetic order at low temperatures. Magnetic order is suppressed with increasing Cu content and our data indicate a possible quantum critical point at x_cr≈1.15. For compounds with higher Cu content, non-Fermi-liquid behavior is observed. Non-Fermi-liquid scaling is inferred from electrical resistivity results for the x=1.25 and 1.5 compounds. For compounds with even higher Cu content, a sharp kink occurs in the resistivity data at low temperatures, and this may be indicative of another quantum critical point that occurs at higher Cu compositions.For the magnetically ordered compounds, hydrostatic pressure is found to increase the Néel temperature, which can be understood in terms of the Kondo physics. For the non-magnetic compounds, application of a magnetic field promotes a tendency toward Fermi-liquid behavior. Thermoelectric power was analyzed using a two-band Lorentzian model, and the results indicate one fairly narrow band (10 meV and below) and a second broad band (around hundred meV). The results imply that there are two relevant energy scales that need to be considered for the physics in this family of compounds.     

Raman scattering from GaSe1−xTex

The long-wavelength optical phonons of the layer GaSe_1?xTe_x have been investigated at room temperature by means of Raman scattering spectroscopy. The spectra of the Bridgman grown crystals were excited with the 1,06 μm line of the continuously operated YAG:Nd³⁺ laser. Detailed study of the Raman spectra of GaSe_1?xTe_x solid solutions showed that there is an abrupt change in the frequency-composition dependences for all observed modes. It is shown, that a phase transition from hexagonal ?-GaSe to monoclinic GaTe in GaSe_1?xTe_x solid solutions takes place in the composition range 0.27 ? × ? 0.72. Only one mode behaviour of the optical phonons was observed in GaSe_1?xTe_x system.     

Thermal dependence of impurity induced fluorescence in the system RbMgxMn1-xF3

The absorption spectrum of RbMnF₃ and the excitation spectra of the system RbMg_xMn_1-xF₃ at 10 K as well as the fluorescence spectra and lifetimes of Mn²⁺ in the systems RbMg_xMn_1-xF₃ and KMg_xMn_1-xF₃ in the region 10–300 K were measured. The lifetime and fluorescence temperature dependence suggest that the origin of the fluorescence occurs at Mn²⁺ sites slightly perturbed by impurity ions and that a non-radiative energy transfer mechanism is responsible for the observed thermal quenching. By using different Mn²⁺ concentrations in the above systems the dependence of the energy transfer on the Mn²⁺ concentration is shown. Finally, a preliminary observation on laser stimulated Mn²⁺ luminescence in the system RbMg_xMn_1-xF₃ is reported.     

Reflectivity of Sn(Se1-xSx)2 mixed crystals

Reflectivity measurements in fundamental absorption range (3.6–13 eV) have been made on Sn(Se_1-xS_x)₂ mixed compounds for several compositions (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0). The composition dependence of the energy location of the reflectivity structures is found to be non-linear and different according to the electronic transitions involved. This behaviour is discussed in terms of electronic structure evolution related to the anion substitution.     

Characterization of PbSe1−xTex thin films

The lead salts and their alloys are extremely interesting semiconductors due to their technological importance. The fabrication of devices with alloys of these compounds possessing detecting and lasing capabilities has been an important recent technological development. The high quality polycrystalline thin films of PbSe_1−xTe_x with variable composition (0≤x≤1) have been deposited onto ultra clean glass substrates by vacuum evaporation technique. As deposited films were annealed in vacuum at 350 K. The optical, electrical and structural properties of PbSe_1−xTe_x thin films have been examined. The optical constants (absorption coefficient and bandgap) of the films were determined by absorbance measurements in the wavelength range 2500-5000 nm using Fourier transform infrared spectrophotometer. The dc conductivity and activation energy of the films were measured in the temperature range 300-380 K. The X-ray diffraction patterns were used to determine the sample quality, crystal structure and lattice parameter of the films.     

Inelastic X-ray scattering investigations of lattice dynamics in SmFeAsO1−xFysuperconductors

We report measurements of the phonon density of states as probed with inelastic X-ray scattering in SmFeAsO_1−xF_y powders. An unexpected strong renormalization of phonon branches around 23 meV is observed as fluorine is substituted for oxygen. Phonon dispersion measurements on SmFeAsO_1−xF_y single crystals allow us to identify the 21 meV A_1g in-phase (Sm,As) and the 26 meV B_1g (Fe,O) modes to be responsible for this renormalization, and may reveal unusual electron-phonon coupling through the spin channel in iron-based superconductors.     

Small-signal ac response of Ba1−xLaxF2+x crystals

The small-signal ac response of single crystalline Ba_1?xLa_xF_2+x solid solutions has been studied in the temperature region 300–820 K, and in the frequency range 4×10^?2–5×10⁴ Hz. At low temperatures the frequency dispersion is dominated by processes in the bulk. Apart from the bulk conductance these processes include a dielectric response determined by broad distributions of dipole-like relaxations. The increase of the static dielectric constant with increasing solute content is explained. At higher temperatures the frequency dispersion reflects interfacial phenomena. In addition to an almost ideal interface capacitance, phenomena are observed which indicate the growth of a surface layer due to a reaction between the electrolyte, and residual oxygen and/or water vapour from the ambient.     

Specific heat and crystalline electric field parameters in TmxY1−xAl2

Specific heat measurements on Tm_xY_1?xAl₂ have been performed to determine the crystalline electric field parameters in Tm_xY_1?xAl₂. We find for the ground state the Γ⁽¹⁾₅ triplet and for the Lea, Leask and Wolf parameters X = (0.47 ± 0.03) and W = (0.034 ± 0.013) meV.     

Magnetic properties of (Ti1-xVx2O3 powders

The magnetic susceptibility of (Ti_1-xV_x2O₃ powders with 0 ? x ? 0.12 has been measured over the temperature range 4.2–300 K. The data show a Curie-Weiss behaviour between 20 and 150 K. As x is increased, the effective magnetic moment per V atom first decreases from 3.8 to 1.8 μ_B shows a steep increase for x = 0.02 and reaches a limit of 1.8 μ_B. These results are discussed in connection with a theoretical model proposed by Van Zandt, involving a narrow V impurity band.     

Experimental determination of intervalley energy gaps as a temperature function for Ga1−xAlxAs

The forward ln i-v characteristic of metal-compound semiconductor contacts shows abrupt variations of the n ideality factor, which can be explained by assuming that the satellite valleys give a bias dependent contribution to the total current. The knee voltage values at which such variations occur allow the determination of intervalley gaps by means of a simple relationship. We have measured such energy gaps for the Ga_1?xAl_xAs over the temperature range 100–300 K.     

Ionic conductivity in tysonite-type solid solutions La1−xBaxF3−x

The ionic conductivity of single crystals of tysonite-type solid solutions La_1?xBa_xF_3?x(0?x?0.095) has been studied parallel and perpendicular to the crystallographic c axis in the temperature range 293–1300 K. Three regions can be discerned in the compositional dependence of the ionic conductivity: (i) the “pure” crystal, in which at room temperature no exchange occurs between different types of anion sites in the tysonite structure; (ii) an intermediate region(0 < x < 7 × 10^-2) which reveals changes in both the conductivity activation enthalpy and the magnitude of the conductivity; (iii) a concentrated solid-solution region (x > 7 × 10^-2), where fluoride ions interchange easily among the different anion sublattices. Diffusion coefficients calculated from ionic conductivity results, are in good agreement with those calculated from ¹⁹F NMR measurements. Using the present data, along with ¹⁹F NMR data, dielectric relaxation data and structural considerations, mechanisms governing the ionic conductivity are proposed.     

CW and pulsed Pb1–xSnxSe diode laser spectroscopy of 14NH3 and 15NH3 molecules

The absorption spectra of ¹⁴NH₃ and ¹⁵NH₃ molecules from 930 cm^-1 to 1220 cm^-1 have been obtained with CW and pulsed Pb_1–xSn_xSe diode lasers. The laser emission frequency has been tuned by varying crystal composition, diode temperature, hydrostatic pressure, or injection current. The registration of the absorption spectra with CW PbSe laser continuously tuned by varying hydrostatic pressure has been accomplished. The possibility of gas isotope abundancies measurements by diode lasers is considered.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

Epr study of CoxZn1−xRh2O4 spinel solid solution

The EPR powder spectra of Co_xZn_1?xRh₂O₄ spinel solid solutions have been studied in the temperature range, 6–300 K for x ? 0.10. The principal features of the spectrum have been examined as a function of the temperature and of the composition. Evidence is presented that the solid solutions are antiferromagnetically ordered for x ? 0.40.