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The prospect of implementing quantum device architectures with technologically mature III–V semiconductors requires precisely controlled topologically protected edge states and bulk insulation. However, experimentally reaching this regime with III–V heterostructure epitaxy has been difficult due to charged bulk defects. Charged defects prevent bulk insulation and disturb the stability of edge states. Hence, we study carrier properties in a hybridized III–V InAs0.9Sb0.1/GaSb superlattice (SL) structure. We realize that an electron density (mobility) is limited to approximately 1012 cm−2 (104 cm2/V s). In order to understand these limits, the authors investigate the in-plane mobility of hybridized SLs as a function of current-carrying layer thickness, L, to determine scattering mechanisms that restrict carrier mobility. Although theory predicts the in-plane mobility is proportional to L6 dominated by interface roughness scattering (IRS) at low temperatures, we report that mobility follows the fourth power dependence, which is a weaker than expected from IRS theory. We attribute the discrepancy between experiment and the model to interface intermixing and wave function penetration into barrier regions. We use this understanding to develop a strategy for realizing high-performance topological materials. [ABSTRACT FROM AUTHOR]

The flexibility of amorphous anodized alumina (AAO) in developing radiation dosimeter for hadron therapy is reported by controlled carbon ion implantation, followed by thermoluminescence (TL) measurements. The efficacy of amorphous AAO in controlling TL sensitivity is found to be governed by an increase in F+ defect centers as a function of carbon concentration, as revealed from the close resemblance of the trend in photoluminescence intensity. Moreover, its nanoporous structure is demonstrated to be advantageous for defect engineering due to the increase in the surface-to-volume ratio. Detailed X-ray photoelectron spectroscopy analysis suggests the formation of F+ centers by substituting Al3+ ions with C2+ in the vicinity of oxygen vacancies, where depth-dependent study showed the evolution of conducting channels owing to sp2 hybridized C–C bonding, leading to a differential charging effect. This work provides a direction to tune nanoporous AAO in its amorphous form for future ion beam dosimetry. [ABSTRACT FROM AUTHOR]

The sol-gel route was used to prepare nickel-silica nanocomposites, to grow nickel oxide films.

Sol-gel technique, followed by reduction treatment, is used in the preparation of glass-metal nanocomposite powders in Fe/SiO2 and Ni/SiO2 systems. Hot pressing of these powders yields bulk nanocomposites. The metal particles have a diameter in the range between 8.9 and 14.8 nanometers. Enhanced coercivities are revealed by the materials. Ferromagnetic component superimposed on a superparamagnetic doublet constitutes the Mossbauer spectrums of Fe/SiO2 samples.

Ni0.5Zn0.5Fe2O4 nano powders were synthesized by an auto combustion method and then heat treated at different temperatures in air for a fixed time. As a consequence, a distribution in particle size and strain was incorporated within the specimens, as estimated from the Rietveld refinement analysis of the powder x-ray diffraction data. The changes in the microstructure and crystal structure parameters were carefully extracted through the refinement analysis. Thermal annealing causes increment in the dispersion and mean of the size distribution. Reallocation of cations in the lattice sites occur as a consequence of the heat treatment which was manifested in their altered unit cell length (a), r.m.s. strain ({ε2}½), oxygen positional parameter (u), metal-oxygen bond lengths (ROA and ROB), and the band positions (v1and v2) in the vibrational spectroscopy. We also investigate the hyperfine and magnetic properties of the samples using different instrumental techniques (with different operating time scales) like Mössbauer spectroscopy, electron paramagnetic resonance spectroscopy, and superconducting quantum interference device magnetometry. Results show that the effect of particle size distribution was manifested in their hyperfine field distribution profile, paramagnetic resonance spectra, and magnetic anisotropy energy distribution profile. Co-existence of superparamagnetic and ferrimagnetic phase was recorded at room temperature in the samples when annealed at lower temperature. However, with increase in annealing temperature, the nature of the size distribution changes and ferrimagnetic ordering predominates for the larger size nanoparticles. Thus, the effect of particle size distribution on the structural, hyperfine, and magnetic properties of various Ni0.5Zn0.5Fe2O4 nanoparticles was investigated herein which hitherto has not been discussed in the literature. [ABSTRACT FROM AUTHOR]

The Magnetic and electrical transport properties of La0.67Ca0.33MnO3 (LCMO): xSiCN (x=0-40 vol percent) composites are discussed. The saturation moment M(sub S) of 68 emu/gm at 5 K and 5 kOe field of pure LCMO shifts to 79 emu/gm in the sample with x=0.05 and declines monotonically as x increases further.

The large magnetoresistance in (La(sub 1-x)Ca(sub x)MnO3)(sub 1-y) (LCMO) ZrO2 (x approximately equal to 0.3 and 0.0 less than or equal to y less than or equal to 0.40 mole%) composites, which are synthesized by using (non Pechini type) sol-gel technique, is described. The phase evolution in the LCMO:ZrO2 composites were evaluated by x-ray powder diffraction and correlated to the magnetic and electrical properties and the composites show higher magnitude of magnetoresistance (MR) compared to pure LCMO.

This study presents the effects of substitution of Zn2+ for Co2+ at low concentrations and the effects of temperature variations on the structural, magnetic, and magnetostrictive properties of cobalt ferrite. Although the Zn-substituted cobalt ferrite samples, Co1-xZnxFe2O4 (x = 0.02, 0.04, 0.06, 0.09, and 0.17) did not show observable changes in crystal structure, the magnetic and magnetostrictive properties were strongly affected. The variation in magnetic susceptibility with composition can be related to the variations in magnetization, coercive field and magnetocrystalline anisotropy. The changes in coercive field were found to be primarily due to the variations in the magnetocrystalline anisotropy. The effect of magnetocrystalline anisotropy on magnetization was stronger at lower cation concentration than at higher concentrations. The decrease in magnetization around 150 K is attributed to the high magnetocrystalline anisotropy at low temperatures which prevented the maximum applied field of 4 MA/m from causing the saturation of magnetization in the samples. Because the magnetocrystalline anisotropy was determined with the magnetization data using the Law of Approach to saturation magnetization, the reliability of the result was found to decrease with decrease in temperature. Peak-to-peak magnetostriction amplitude and the strain sensitivity decreased with increase in Zn substitution. [ABSTRACT FROM AUTHOR]

Effect of ambient gases present in air on photoluminescence (PL) and electrical conductivity of films prepared using ZnO nanoparticles (NPs) have been investigated. It is observed that NPs of size below 20 nm kept inside a chamber exhibit complete reduction in their visible PL when oxygen partial pressure of the surrounding gases is decreased by evacuation. However, the visible PL from ZnO NPs is insensitive to other major gases present in the ambient air. The rate of change of PL intensity with pressure is inversely proportional to the ambient air pressure and increases when particle size decreases due to the enhanced surface to volume ratio. On the other hand, an assembly of ZnO NPs behaves as a complete insulator in the presence of dry air and its major components like N2, O2, and CO2. Electrical conduction having resistivity ∼102-103 Ωm is observed in the presence of humid air. The depletion layer formed at the NP surface after acquiring donor electrons of ZnO by the adsorbed oxygen has been found to control the visible PL and increases the contact potential barrier between the NPs which in turn enhances the resistance of the film. [ABSTRACT FROM AUTHOR]

The present article describes the size induced changes in the structural arrangement of intrinsic defects present in chemically synthesized ZnO nanoparticles of various sizes. Routine x-ray diffraction and transmission electron microscopy have been performed to determine the shapes and sizes of the nanocrystalline ZnO samples. Detailed studies using positron annihilation spectroscopy reveals the presence of zinc vacancy. Whereas analysis of photoluminescence results predict the signature of charged oxygen vacancies. The size induced changes in positron parameters as well as the photoluminescence properties, has shown contrasting or nonmonotonous trends as size varies from 4 to 85 nm. Small spherical particles below a critical size (∼23 nm) receive more positive surface charge due to the higher occupancy of the doubly charge oxygen vacancy as compared to the bigger nanostructures where singly charged oxygen vacancy predominates. This electronic alteration has been seen to trigger yet another interesting phenomenon, described as positron confinement inside nanoparticles. Finally, based on all the results, a model of the structural arrangement of the intrinsic defects in the present samples has been reconciled. [ABSTRACT FROM AUTHOR]

Defects present in ZnO nanocrystals prepared by a wet chemical method have been characterized by photoluminescence (PL) and positron annihilation spectroscopy (PAS) techniques. The as-prepared sample was heat treated at different temperatures to obtain nanocrystals in the size range of 19–39 nm. X ray diffractograms confirmed the single-phase wurtzite structure formation. Photoluminescence measurements showed a strong violet band at 434 nm, which has been identified as due to electronic transitions from the zinc interstitial defect level to the top of the valence band. A marked decrease in the intensity of the violet emission with increasing heat-treatment temperature has been observed, which is attributed to recombination of zinc interstitials with zinc vacancies. Positron annihilation spectroscopy has been employed to understand the dynamics of the vacancy-type defects and their annealing behavior. The observed variation of the defect related lifetime components with heat-treatment temperature has been successfully explained by using a three-state trapping model. The results of PL and PAS studies in the present case are found to be complementary to each other. [ABSTRACT FROM AUTHOR]

Composites of copper core-copper oxide shell nanoparticles in a gel-derived silica glass were prepared by a chemical method. The core-shell structure formed a percolative system within the glass. The copper core had diameters in the range 3.5-4.8 nm with the shell thickness varying from 1.0 to 1.6 nm. The dielectric modulus for these nanocomposites was measured over the frequency range 100 Hz-6 MHz at temperatures varying from 330 to 535 K. The modulus spectra were analyzed by a Kohlrausch-Williams-Watts stretched exponential relaxation function. The exponentβ for the nanocomposites was found to have values∼ 0.47 whereas the value obtained in the case of the precursor glass was 0.67. The activation energies for the dielectric relaxation were found to be in reasonable agreement with those obtained earlier for dc conductivity. [ABSTRACT FROM AUTHOR]

Iron nanoparticles with a shell of Fe[sub 3]O[sub 4] phase with a total diameter of 5.3 nm have been grown within a silica gel matrix in the percolative configuration by suitable reduction followed by oxidation treatments, dc electrical resistivity measurements were carried out in the temperature range 80-300 K. The resistivity of the nanocomposites was found to be about 7 orders of magnitude lower than that of the reference gel. The electrical conduction has been explained on the basis of a small polaron hopping mechanism. The activation energy in the case of the composites was calculated from experimental data to be about one-fifth that for the reference sample. An interfacial amorphous phase is believed to cause such reduction in resistivity. The effective dielectric constant of this phase was estimated to be about four times that of the reference glass. Magnetization measurements on these specimens were carried out in the temperature range 5-300 K both in zero field cooled and field cooled states. A peak in the magnetization at ∼120 K was ascribed to an order-disorder (Verwey) transition. Another peak at ∼55 K was explained as arising due to a spin glass like disorder at the interface between the ferromagnetic iron ores and the ferrimagnetic Fe[sup 3]O[sub 4] shell. A loop shift was observed as a result of the spin freezing below this temperature. [ABSTRACT FROM AUTHOR]

A sol-gel route was used to prepare iron nanoparticles in copper matrix. It was shown that the iron nanoparticles have coercivities ranging from 200 to 500 Oe which are characteristic of single-domain iron grains. Alpha-Fe particles were accounted for in the Mossbauer spectrum. In addition, a finite value was obtained which was consistent with an electron transfer between iron atoms and the copper matrix.

Iron nanoparticles with a shell of Fe3O4 phase of a total diameter of 5.3 nm is grown within a silica gel matrix in the percolative configuration by suitable reduction followed by oxidation treatments. The dc electrical resistivity measurements carried out in the temperature range of 80-300 K is found to be about 7 orders of magnitude lower than that of the reference gel.

Nanocrystalline particulates of Er doped cobalt-ferrites CoFe(2-x)ErxO4 (0⩽ x ⩽ 0.04), were synthesized, using sol-gel assisted autocombustion method. Co-, Fe-, and Er- nitrates were the oxidizers, and malic acid served as a fuel and chelating agent. Calcination (400-600 °C for 4h) of the precursor powders was followed by sintering (1000 °C for 4 h) and structural and magnetic characterization. X-ray diffraction confirmed the formation of single phase of spinel for the compositions x = 0, 0.01, and 0.02; and for higher compositions an additional orthoferrite phase formed along with the spinel phase. Lattice parameter of the doped cobalt-ferrites was higher than that of pure cobalt-ferrite. The observed red shift in the doped cobalt-ferrites indicates the presence of induced strain in the cobalt-ferrite matrix due to large size of the Er+3 compared to Fe+3. Greater than two-fold increase in coercivity (~66 kA/m for x=0.02) was observed in doped cobalt-ferrites compared to CoFe2O4 (~29 kA/m). [ABSTRACT FROM AUTHOR]

The magnetic and electrical transport properties of La0.67Ca0.33MnO3 (LCMO): xSiCN (x=0–40 vol %) composites, synthesized by mechanical mixing of citrate gel derived LCMO and polymer derived ceramics SiCN, have been investigated as a function of composition. The ability of SiCN to behave as a sintering additive leads to interesting magnetic and electrical properties. Si+4, being strongly favored for tetrahedral coordination, cannot enter the perovskite lattice. Saturation moment, MS of 68 emu/gm at 5 K and at 5 kOe field of pure LCMO shifts to 79 emu/gm in the sample with x=0.05 and then decreases monotonically as x increases further. The lower MS, TC and high electrical resistivity of the sample with x=0, compared to those which have been reported in the literature, is due to small particle size in nm range. SiCN as a sintering aid increases the particle size of LCMO to 1–2 μm for x=0.10. Three orders of magnitude change in electrical resistivity (204 Ω cm for x=0;0.5 Ω cm for x=0.05) is explained on the basis of the difference in the mobility of the charge carriers, by fitting the experimental transport data with a correlated polaron hopping model. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Colossal magnetoresistance (CMR) composite materials have been synthesized to explore the possibility of improving magneto-transport and structural properties in CMR systems. In this work we describe (La[SUB1-x]Ca[SUBx]MnO[SUB3])[SUB1-y] (LCMO) (ZrO[SUB2])[SUBy] (x≈0.3 and 0.0⩽y⩽0.40 mole %) composites that have been synthesized using a modified (non Pechini type) sol-gel technique. Magnetoresistivity of the composites was evaluated at 5 T field and in the temperature range 5-300 K. The composites show higher magnitude of MR compared to pure LCMO. The MR rises from a base value 76%, for the case y50, to a maximum value of 93.8%, obtained at y50.05. dc susceptibility measurements show a distinct ferromagnetic to paramagnetic transition in all composites. The ferromagnetic transition temperature (T[SUBC]) drops from 225 K in pure LCMO (y=0) to 121 K in y=0.05 and then slowly rises to 157 K as y increases. The plots of zero field cooled susceptibility ξ[SUBZFC](T) and field cooled susceptibility ξ[SUBFC] (T) diverge clearly below T[SUBC], indicating magnetic irreversibility. The composite exhibits a clear metal -- insulator transition (T[SUBMI])at or just above the magnetic transition. The peak resistivity (r)MI (at the metal -- insulator transition)also exhibits interesting changes. For pure LCMO polycrystals, ρ[SUBMI]=102Ω cm, but it increases to 228 Ω cm for y=0.05 and then gradually decreases to 1.94 Ω cm for y⩾0.10. The phase evolution in the LCMO:ZrO[SUB2] composites was studied by x-ray powder diffraction and correlated to the magnetic and electrical properties. [ABSTRACT FROM AUTHOR]

Provides information on the production of aligned samarium-cobalt deposits using plasma spraying and c-axis alignment. Cause of rapid deterioration in coercivity and second-quadrant behavior of the magnets; Maximum-energy product values of the sprayed magnets; Use of x-ray diffraction; Limitations on the achieved values of the maximum-energy product.