Positive and negative linear magnetoresistance of graphite

The magnetoresistance (MR) of bulk graphite with different particle sizes is investigated. The MR of the graphite decreases with the particle size decreases. The graphite with micro-sized particles has a positive MR and exhibits positive linear field dependence of MR at about 50 K, whereas the graphite with particle size of 30.2 nm has a negative MR and exhibits negative linear field dependence of MR at about 25 K. The possible mechanism for the MR of graphite can be partially understood using ordinary MR theory, weak localization theory and diffuse scattering theory.     

Magnetic polarons in Ca1−xYxMnO3

Experimental evidence shows that in the magnetoresistive manganite Ca_1−xY_xMnO₃, ferromagnetic (FM) polarons arises in an antiferromagnetic (AF) background, as a result of the doping with yttrium. This hypothesis is supported in this work by classical Monte Carlo calculations performed on a model where FM double exchange and AF superexchange compete.     

Synthesis and dielectric properties of Bi1−xNdxFeO3 perovskites

Single-phase polycrystalline samples of Bi_1−xNd_xFeO₃ were prepared by standard solid state reaction method. X-ray diffraction (XRD) patterns of the powder samples were recorded and analyzed for the confirmation of crystal structure lattice parameters. Further, these samples were characterized by IR technique to identify and understand the aspect of bonding in the present samples. The dielectric measurements were carried out on the samples as a function of frequency in the range 100 Hz to 1 MHz at room temperature and also as a function of temperature in the range 300–750 K at certain fixed frequencies. The DC and AC electrical resistivity studies were carried out in order to understand conduction mechanism in the samples.     

The order of magnetic phase transition in La(Fe1−xCox)11.4Si1.6 compounds

Magnetic transitions in La(Fe_1−xCo_x)_11.4Si_1.6 compounds with x=0–0.08, have been studied by DC magnetic measurements and Mössbauer spectroscopy. The temperature dependence of the Landau coefficients has been derived by fitting the magnetization, M(μ₀H), using the Landau expansion of the magnetic free energy. For x0.02 there is a strongly first-order magnetic phase transition between ferromagnetic and paramagnetic (F–P) states in zero external field and a metamagnetic transition from paramagnetic to ferromagnetic (P–F) above T_c. Increasing the cobalt content drives the F–P transition towards second order and eliminates the metamagnetic transition.     

Laterally patterned high mobility two-dimensional electron gases obtained by overgrowth of focused ion beam implanted Al1−xGaxAs

The use of focused ion beam implantation doping of an inverted GaAs/Al_1−xGa_xAs heterostructure during a growth interruption allows for the lateral modulation of the heterostructure doping. Hence, laterally patterned two dimensional electron gases (2DEGs) are obtained with no further processing steps required. We have performed the direct writing of a 2DEG with a Hall-bar pattern, such that only the application of ohmic contacts was necessary and the sample surface remained unharmed otherwise. The 2DEG has an electron density of 3.6×10¹¹ cm⁻² and an electron mobility of 4.8×10⁵ cm²/V s, as determined by magnetotransport measurements. A conventional mesa-etched Hall-bar with almost identical electronic properties has also been studied. Different behaviour of the longitudinal as well as the transversal magnetoresistance for the two Hall-bars is observed and can be concluded to be due to a different confinement potential.     

Structural and vibrational properties of dilute GaNxAs1−x(P1−x)

We report a comprehensive analyzes of the Fourier transform infrared (FTIR) absorption and Raman scattering data on the structural and vibrational properties of dilute ternary GaAs_1−xN_x,[GaP_1−xN_x] (x<0.03) alloys grown on GaAs [GaP] by metal organic chemical vapor deposition (MOCVD) and solid source molecular beam epitaxy (MBE). By using realistic total energy and lattice dynamical calculations, the origin of experimentally observed N-induced vibrational features are characterized. Useful information is obtained about the structural stability, vibrational frequencies, lattice relaxations and compositional disorder in GaNAs (GaNP) alloys. At lower composition (x<0.015) most of the N atoms occupy the As [P] sublattice {N_As[N_P]}—they prefer moving out of their substitutional sites to more energetically favorable locations at higher x. Our results for the N-isotopic shifts of local mode frequencies compare favorably well with the existing FTIR data.     

Magnetic properties of the pyrite type FexCo1−x−yNiyS2 system

The magnetic phase diagram of the Fe_xCo_1−x−yNi_yS₂ system was determined. Ferromagnetic (F)-, antiferromagnetic (AF )- and paramagnetic (P) phases were found. In the triangular diagram, the F-phase appears along the CoS₂-FeS₂ line, the AF-phase around NiS₂ and the P-phase between the F- and AF-phases. Discussions are given on the F-phase.     

Influence of chemical disorder on the magnetic behaviour and phase diagram of the FexNi1−x system

In this paper, we calculate the equilibrium phase diagram and the magnetic moment curve for the Fe_xNi_1−x system and simulate their Mössbauer spectra assuming a binomial distribution to reproduce the chemical disorder in these alloys. We also assume that the high-spin/low-spin transition for a central iron atom is governed by the number of nearest neighbours and next nearest neighbours of the iron atoms. The calculated equilibrium phase diagram and the magnetic moment curve are very close to that presented in the literature and the simulated Mössbauer spectra are in excellent agreement with that of their corresponding phases measured in our lab.     

Magnetic and structural properties of magnetoresistive FexAu100−x alloys produced by mechanical alloying

Fe_xAu_100−x alloys have been produced for the first time by mechanical alloying. X-ray diffractograms show FCC peaks. From the X-ray diffracion peak-widths we estimate the final grain size, which vary with x from 112 nm (for x=15) to 32 nm (for x=30). Lattice parameter decreases with concentration (minimum 0.401 nm at x=30), but above Vegard's law values. Susceptibility measurements show cluster-glass behaviour. Critical temperatures are consistently lower than similar alloys produced by arc melting followed by fast quenching. A magnetic phase diagram is presented. Giant magnetoresistance is present in all samples, with a maximum at x=25. This effect is caused by the dispersion of small iron clusters produced by the mechanical work.     

Nax/2Bi1−x/2MoxV1−xO4 and Bi1−x/3MoxV1−xO4 New Scheelite-related phases

New Scheelite-related solid solutions of the compositions Na_x/2Bi_1−x/2Mo_xV_1−xO₄ (0≤x≤1) and Bi_1−x/3 Mo_xV_1−xO₄(0≤x≤0.2) have been synthesised by the substitution of Na and Mo at the A and B sites respectively of the ABO₄ type ferroelastic BiVO₄. The phases were characterised using chemical analysis, powder X-ray diffraction, scanning electron microscopy, EDAX, and Raman spectroscopy. While almost a continuous solid solution is obtained for the series Na_x/2Bi_1−x/2Mo_xV_1−xO₄, the absence of Na at the A-site results only in a narrow stability region for the other series, Bi_1−x/3 Mo_xV_1−xO₄ where 0≤x≤0.2. Raman spectra of selected samples at room temperature also suggest that vanadium and molybdenum atoms are disordered at the tetrahedral sites.     

Luminescence from Si-Si1−xGex/Si1−yCy-Si structures

Near band edge photoluminescence has been obtained from Si_1−yC_y quantum well (QW) and neighboring Si_1−xGe_x/Si_1−yC_y double QW (DQW) structures. Enhanced no-phonon recombination is observed from the DQW structures and it is attributed to a breaking of the k-selection rule in the presence of the heterointerface. The luminescence persists for measurement temperatures up to 30–50 K and the intensity exhibits a quenching behavior with an activation energy equal to 8–20 meV. In electroluminescence only recombination in the Si_1−xGe_x layer has been observed from neighboring Si_1−xGe_x and Si_1−yC_y DQW structures.     

Phonon Raman scattering in Nd1+xBa2−xCu3O7−δ and Pr1+xBa2−xCu3O7−δ single crystals

The polarized Raman spectra of Nd_1+xBa_2−xCu₃O_7−δ (−0.023≤x≤0.107) and Pr_1+xBa_2−xCu₃O_7−δ (0.01≤x≤0.15) single crystals have been investigated. It was found that the Cu(2) A_g mode softens by 6 cm⁻¹ in Nd 1:2:3 and 4 cm⁻¹ in Pr 1:2:3 as x increases. These frequency shifts cannot be explained by the change in the relevant bond lengths due to Nd(Pr)-substitution for Ba. The variations with x of the two low frequency modes may be affected by change of their hybridization and/or change of their force constants. The linewidths of Ba mode in Pr 1:2:3 are broader than those in Y 1:2:3. This result suggests that the Pr substitution on Ba sites occurred even in a very small value of x. In x(yy) geometry the relative intensity of the Ba and O(4) modes in Nd 1:2:3 is greater than those in Pr 1:2:3. The difference between Nd 1:2:3 and Pr 1:2:3 in the relative intensity of the Ba and O(4) modes may be produced by the chains.     

Quantum critical point, scaling and universality in high Tc(CaxLa1−x)(Ba2−c−xLac+x)Cu3Oy

Using charge transport in sintered ceramic samples it is observed that at all doping, including non superconducting overdoped samples, there exists a temperature in which below it dR/dT < 0. This suggests that either the quantum critical point is not necessarily inside the superconducting dome or that the CuO₂ plane is never overdoped. Data relating experimental Cooper pair density, conductivity and T_c suggest that Homes’ relation might need a more specific definition of the conductivity σ.     

Negative luminescence from In1−xAlxSb and CdxHg1−xTe diodes

Indium aluminium antimonide (In_1−xAl_xSb) and cadmium mercury telluride (Cd_xHg_1−xTe) heterostructure diodes, which comprise a near intrinsic active region bounded by more highly doped contact regions, exhibit positive or negative luminescence at medium to long infrared wavelengths when forward or reverse biased respectively at room temperature. In reverse bias, the carrier densities in the near intrinsic region are reduced below their equilibrium values by the effects of exclusion and extraction. In consequence, the radiative recombination is reduced and the devices emit less infrared radiation than the thermal equilibrium value. The observed intensity of the negative luminescence is in general agreement with expected values.     

Magnetic properties of pseudobinary compounds CrSb1−xAsx

Magnetic susceptibility and crystal structure were studied on CrSb_1−xAs_0.6, structural transformation occurs between MnP-type and NiAs-type at 270 K, the Néel temperature is 300 K. Paramagnetic moments of the system are 2.45−2.7μ_B/Cr, considerably larger than antiferromagnetic moments.     

Blue cathodoluminescence from thulium implanted AlxGa1−xN and InxAl1−xN

Room temperature cathodoluminescence (RTCL) was obtained from Tm implanted Al_xGa_1−xN with different AlN contents (in the range 0≤x≤0.2) and from implanted In_xAl_1−xN with different InN contents (x=0.13 and 0.19) close to the lattice match with GaN. The Tm³⁺ emission spectrum depends critically on the host material. The blue emission from Al_xGa_1−xN:Tm peaks in intensity for an AlN content of x0.11. The emission is enhanced by up to a factor of 50 times with an increase of annealing temperature from 1000 to 1300 C. The blue emission from In_0.13Al_0.87N:Tm, annealed at 1200 C, is more than ten times stronger than that from Al_xGa_1−xN:Tm, x≤0.2. However, the intensity decreases significantly as the InN fraction increases from 0.13 to 0.19.     

Hydrogen adsorption on Mo1−xRex(110) (x=0–0.25) surfaces

The effects of adsorbed H on the Mo_1−xRe_x(110), x=0, 0.05, 0.15, and 0.25, surfaces have been investigated using low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS). For the x=0.15 alloy only, a c(2×2) LEED pattern is observed at a coverage Θ0.25 ML. A (2×2) pattern is observed for H coverages around Θ0.5 ML from surfaces with x=0, 0.05, and 0.15. Both c(2×2) and (2×2) patterns are attributed to reconstruction of the substrate. At higher coverages, a (1×1) pattern is observed. For the alloy surface with x=0.25, only a (1×1) pattern is obtained for all H coverages. Two H vibrations are observed in HREELS spectra for all Re concentrations, which shift to higher energies at intermediate coverages. Both peaks exhibit an isotopic shift, confirming their assignment to hydrogen. For Re concentrations of x=0.15 and higher, a third HREELS peak appears at 50 meV as H (D) coverage approaches saturation. This peak does not shift in energy with isotopic substitution, yet cannot be explained by contamination. The intrinsic width of the loss peaks depends on the Re concentration in the surface region and becomes broader with increasing x. This broadening can be attributed to surface inhomogeneity, but may also reflect increased delocalization of the adsorbed hydrogen atom.     

The structural features of Cu1−xPbx liquid alloys

Structure of Cu_1−xPb_x (x=0, 0.025, 0.05, 0.075, 0.1, 0.125) molten alloys has been studied by means of X-ray diffraction method. Structural parameters obtained from structure factors (SF) and radial distribution functions were analyzed. Partial structure factors were calculated by using Reverse Monte-Carlo method. It is shown that tendency to preferred interaction of atoms of the same kind grows with increasing of lead content.     

Photoluminescence study of proton-implanted InP1−xAsx:Yb

Photoluminescence (PL) studies of low-energy (60 keV, H⁺₂), proton-implanted InP_1-xAs_x (0 x 17) crystals doped with Yb are reported. In the implanted samples we observed remarkable intensity reduction of all the PL lines. After annealing, the PL spectra did not recover to their preirradiation values indicating high thermal stability of the generated damage. We have not seen any influence of hydrogen or lattice defects on the shape of the Yb intra-4f-shell luminescence. We show that hydogen-implantation-induced defects stabilize surfaces of InP and InPAs compounds, preventing their decomposition during capless thermal annealing up to 650°C. It is suggested that this enhanced surface stabilization is due to the presence of defects saturated with hydrogen atoms which are bound tightly to the phosphorus atoms.     

Structural characteristics and morphology of SmxCe1−xO2−x/2 thin films

Effect of the deposition temperature (200 and 500 °C) and composition of Sm_xCe_1−xO_2−x/2 (x = 0, 10.9–15.9 mol%) thin films prepared by electron beam physical vapor deposition (EB-PVD) and Ar⁺ ion beam assisted deposition (IBAD) combined with EB-PVD on structural characteristics and morphology/microstructure was investigated. The X-ray photoelectron spectroscopy (XPS) of the surface and electron probe microanalysis (EPMA) of the bulk of the film revealed the dominant occurrence of Ce⁴⁺ oxidation state, suggesting the presence of CeO₂ phase, which was confirmed by X-ray diffraction (XRD). The Ce³⁺ oxidation states corresponding to Ce₂O₃ phase were in minority. The XRD and scanning electron microscopy (SEM) showed the polycrystalline columnar structure and a rooftop morphology of the surface. Effects of the preparation conditions (temperature, composition, IBAD) on the lattice parameter, grain size, perfection of the columnar growth and its impact on the surface morphology are analyzed and discussed.