The Thermopower and Resistivity of Nearly Magnetic Dilute Alloys

<p>In some nearly magnetic dilute alloys, in which the host and impurity are transition metals of similar electronic structure, the thermopower is observed to form a "giant" peak at about the spin fluctuation temperature Tsf deduced from resistivity measurements. Two explanations for these peaks have been postulated: the first is that the peaks are a diffusion thermopower component involving scattering off localized spin fluctuations (LSF) at the impurity sites; the second is that they are an LSF drag effect. We examine the thermopower and resistively of two nearly magnetic alloy systems: Rh(Fe) and Pt(Ni). In the first part of this thesis we describe measurements of the low temperature thermopower and resistivity of several Rh(Fe) alloys to clarify discrepancies in previous measurements and we show, by using a modified Nordheim-Gorter analysis, that the observed thermopower peaks are a diffusion and not a drag effect. In the second part of the thesis we describe measurements of the low temperature thermopower and resistivity of Pt (Ni), for which no previous data had been available. The Pt(Ni) samples are manufactured as thin, evaporated films on glass substrates. However, due to the difficulty encountered in controlling the very high residual resistivity of these samples, we are not able to draw definite conclusions regarding either the thermopower or the resistivity.</p>

The Thermopower and Resistivity of Nearly Magnetic Dilute Alloys

<p>In some nearly magnetic dilute alloys, in which the host and impurity are transition metals of similar electronic structure, the thermopower is observed to form a "giant" peak at about the spin fluctuation temperature Tsf deduced from resistivity measurements. Two explanations for these peaks have been postulated: the first is that the peaks are a diffusion thermopower component involving scattering off localized spin fluctuations (LSF) at the impurity sites; the second is that they are an LSF drag effect. We examine the thermopower and resistively of two nearly magnetic alloy systems: Rh(Fe) and Pt(Ni). In the first part of this thesis we describe measurements of the low temperature thermopower and resistivity of several Rh(Fe) alloys to clarify discrepancies in previous measurements and we show, by using a modified Nordheim-Gorter analysis, that the observed thermopower peaks are a diffusion and not a drag effect. In the second part of the thesis we describe measurements of the low temperature thermopower and resistivity of Pt (Ni), for which no previous data had been available. The Pt(Ni) samples are manufactured as thin, evaporated films on glass substrates. However, due to the difficulty encountered in controlling the very high residual resistivity of these samples, we are not able to draw definite conclusions regarding either the thermopower or the resistivity.</p>

Infinite-Volume Limit of Stochastic s-d System with Single-Component Impurity and s-Electron Spins

A Hamiltonian [Formula: see text], with locally smeared Ising-type s-d exchange between s-electrons and magnetic impurities, in a dilute magnetic alloy, is investigated. The Feynman-Kac theorem, Laplace expansion and Bogolyubov inequality are applied to obtain a lower and upper bound (lb and ub) on the system’s free energy per conducting electron [Formula: see text]. The two bounds differ, in the infinite-volume limit by a term [Formula: see text], linear in impurity concentration: lb[Formula: see text], ub[Formula: see text], [Formula: see text] denoting the Hamiltonian of the approximating mean-field s-d system. [Formula: see text] represents randomly positioned impurities interacting with a mean field implemented by the gas of conduction s-electrons, the latter interacting with the field of barriers and wells (according to the s-electron’s spin orientation) localized at the impurity sites. The inequality [Formula: see text] demonstrates increasing accuracy of the mean-field [Formula: see text]-theory, with decreasing impurity concentration.

Photocatalytic Reactivity of Carbon–Nitrogen–Sulfur-Doped TiO2 Upconversion Phosphor Composites

Sol–gel synthesized N-doped and carbon–nitrogen–sulfur (CNS)-doped TiO2 solutions were deposited on upconversion phosphor using a dip coating method. Scanning electron microscopy (SEM) imaging showed that there was a change in the morphology of TiO2 coated on NaYF4:Yb,Er from spherical to nanorods caused by additional urea and thiourea doping reagents. Fourier transform infrared (FTIR) spectroscopy further verified the existence of nitrate–hyponitrite, carboxylate, and SO42− because of the doping effect. NaYF4:Yb,Er composites coated with N- and CNS-doped TiO2 exhibited a slight shift of UV-Vis spectra towards the visible light region. Photodecomposition of methylene blue (MB) was evaluated under 254 nm germicidal lamps and a 300 W Xe lamp with UV/Vis cut off filters. The photodegradation of toluene was evaluated on TiO2/NaYF4:Yb,Er and CNS-doped TiO2/NaYF4:Yb,Er samples under UV light illumination. The photocatalytic reactivity with CNS-doped TiO2/NaYF4:Yb,Er surpassed that of the undoped TiO2/NaYF4:Yb,Er for the MB solution and toluene. Photocatalytic activity is increased by CNS doping of TiO2, which improves light sensitization as a result of band gap narrowing due to impurity sites.

METHODS OF THERMAL-STTMULATED CURRENTS FOR RESEARCH OF IMPURITY SITES AT LTGHT-EMTTTTNG DTODES

Impurity sites in LEDs based on of Gai-xAlxAs for fiber-optic communication lines using thermal-stimulated current method were researched. The causes of the degradation of such diodes are clarified. The installation for research using the method of thermal-stimulated currents is described. Given in p-n junctions based on GaAlAs. The dependence curves of thermal-stimulated currents and temperature are shown at various heating rates. Was made a research of light-emitting diodes degradation by their power supply of current pulses up to 10 A, with a duration of 100 ns and a frequency of 300 Hz, as well as at 50 mA, 20 mA and a temperature of 800C. A connection was found in the process of degradation of LEDs with an increasing of the concentration of impurity sites. The curves of thermal-stimulated currents determined the concentration of impurity sites before and after the degradation of LEDs. It is shown that the main reason for the change in the electrical characteristics of the p-n junctions of the studied samples upon passing a direct current is the accumulation of impurity sites.

Electronic modes in carbon nanotubes with single and double impurity sites

A theoretical study of isolated and doubly-clustered impurities is presented for the electronic excitations in a carbon nanotube lattice. Using a matrix operator formalism and a tight-binding model where the interactions between atoms take place via nearest-neighbor hopping, the properties of the excitations are deduced. A geometry consisting of long, single-walled carbon nanotubes is assumed with the defects introduced in the form of substitutional impurity atoms, giving rise to the localized electronic modes of the nanotube as well as the propagating modes of the pure (host) material. The impurities are assumed to be in a low concentration, having the form of either a single, isolated defect or a small cluster of two defects close together. A tridiagonal matrix technique is employed within a Green’s function formalism to obtain the properties of the discrete modes of the system, including their frequencies and localization. The numerical examples show a dependence on the nanotube diameters and on the relative spatial configurations of the impurities. The results contrast with the previous studies of line impurities since there is no translational symmetry along the longitudinal axis of the nanotubes in the present case.