Combined translational-rotational jumps in two orientationally ordered solids, (alpha)-CO and CO(,2), have been measured using C('13) NMR. In the Pa3 structure of these solids, a molecule which jumps to a neighboring (presumably vacant) site will reorient, due to the orientationally ordered structure.;The rates of translation and rotation have been measured independently by using different NMR techniques. The rotations were detected at high field (14.7 MHz) through the modulation of the chemical shift anisotropy; spin echoes and stimulated echoes were used. The translational jumps modulate the dipolar interactions and were studied at low fields (1.256 MHz) with line narrowing and Slichter-Ailion slow motion (T(,1D)) experiments. The rates of translations and rotations agree, indicating that they are two aspects of one combined motion.;The shift anisotropies of (alpha)-phase C('13)O and C('13)O(,2) were found to be 350 (+OR-) 15 ppm and 325 (+OR-) 15ppm, respectively; both values are in good agreement with previous NMR measurements. The jump rates of the combined motion in both materials obey the thermally activated expression: (omega)(,j) = (tau)(,j)('-1) = (omega)(,0) exp(-E(,a)/kT). The activation parameters are E(,a)/k = 2100 K and (omega)(,0) = 2 x 10('18) s('-1) for (alpha)-CO and E(,a)/k = 6600 K and (omega)(,0) = 2 x 10('17) s('-1) for CO(,2). The activation energies of (alpha)-CO and CO(,2) from this study agree by corresponding states analysis with that found previously for the same motion in N(,2)O. All three molecular solids belong to the family of solids composed of small, linear molecules with Pa3 crystal structure. Unusually high frequency prefactors ((omega)(,0)) are seen in all three solids and are not understood. The high prefactors are also shown to appear in other molecular solids such as benzene and ammonia.

Monovacancy diffusion is a thermally activated process characterized by an activation energy E(,d). The diffusion of atoms requires the formation and migration of vacancies. The concentration of vacancies n/N is given by n/N (DBLTURN) exp(-E(,f)/kT). It can be shown that the activation energy E(,d) is the sum of the energy to form a vacancy E(,f) and the energy required for an atomic jump E(,m): E(,d) = E(,f) + E(,m). Furthermore, the atomic jump rate (omega)(,j) can be shown to be thermally activated and given by (omega)(,j) (DBLTURN) (nu)(,o)(n/N)exp(-E(,m)/kT) ((nu)(,o) is the attempt frequency).;NMR offers many techniques to measure the activation energy, E(,d). However, there are no NMR techniques available for the determination of the migration energy. This thesis presents an NMR experiment for the measurement of the migration energy. It is a simple NMR experiment performed on a sample prepared with a nonequilibrium concentration of vacancies. By preparing the sample such that the vacancy concentration does not change with temperature, the jump rate has only one thermally activated term. Thus, the jump rate now has an activation energy equal to the migration energy E(,m).;This experiment was performed on pure aluminum metal. The migration energy was found to be .69 ev (+OR-) 20% (the accepted value is .67 ev); however, we do not consider this an accurate determination. We do consider the experiment successful and promising. The activation energy of self diffusion was also determined; its value is 1.33 ev.

Pulsed deuterium NMR experiments have been performed on deuterated samples of poly(vinylidene fluoride), PVF(,2), repeat unit CH(,2)CF(,2) and the copolymer poly(vinylidene fluoride)-tetrafluoroethylene (80-20), (PVF(,2)-F(,4)E). A deuterium line shape study has been employed to characterize the orientational distribution of the dipole moments in both poled and unpoled samples. In addition, the orientational distributions of chain axis alignment has been measured in stretched samples.;PVF(,2) and PVF(,2)-F(,4)E exhibit unique piezoelectric and pyroelectric properties which are due to their crystalline components (approximately 50% in typical material). Four crystalline phases exist in the homopolymer PVF(,2), the (alpha), (beta), (gamma), and the (delta). The relatively electrically inactive, melt solidified (alpha) phase can be transformed into the ferroelectric (beta) phase by mechanical deformation (stretching) or by application of large electric fields (poling- 200 Mv/meter). The copolymer melt solidifies directly into the (beta) phase. The (beta) phase is characterized by a net dipole moment per unit cell due to the parallel alignment of the CF(,2) dipole moments in the unit cell. The orthorhombic unit cell dimensions of the (beta) phase render a nearly pseudohexagonal packing structure of the molecular chains and is thought to allow for dipole reorientations via 60(DEGREES) steps during the poling process.;The orientationally dependent quadrupole interaction has been used to characterize the orientational distributions of the molecular dipole moments about the poling direction in both poled and unpoled samples of PVF(,2) and PVF(,2)-F(,4)E. No orientational dependence was found in either the poled or unpoled samples. This is in contrast to recent x-ray results which report substantial orientational anisotropy in rolled, poled protonated films. We have also measured the degree of chain axis alignment in stretched samples of both PVF(,2) and PVF(,2)-F(,4)E. Alignment in both samples is characterized by a gaussian distribution function about the stretch direction with a half width 1/e maximum of 22(DEGREES) and 18(DEGREES) respectively.;The characterization of the experimental results was done by comparing computer simulated line shapes with the experimental line shapes.

The local density approximation (LDA) has been proved to be a powerful starting point for calculating electronic and structural properties for many real materials. We have studied the effects of particular forms of exchange-correlation potentials (the X{dollar}\alpha{dollar} and Hedin-Lundqvist form) upon the structural properties for the 3d Ti and 4d Zr using a highly accurate linearized augmented plane wave (LAPW) method. The calculated equilibrium volumes differ by 6-8% for these two forms (with X{dollar}\alpha{dollar} results in better agreement with experiment) with proportional differences in other structural properties, which we take to be an indication of the intrinsic reliability of the LDA. Considerable sensitivity in the calculated structural properties to the particular exchange-correlation potential (the X{dollar}\alpha{dollar}, Wigner, and Hedin-Lundqvist) was also found for the fcc and the high temperature bcc La. The calculation on SmS reveals that the LDA is inadequate for this very localized 4f electron system, while the LDA works fairly well for the chemically similar material LaS. For HgTe and HgSe, the fully occupied 5d electrons in Hg has been found to be important in determining the structural properties through the Hg d - chalcogen p interactions, however this p-d hybridization appears to be relatively unchanged through the various pressure induced phase transitions. We calculated the total energy of the seven layer slabs of Pd(111) surface as a function of the top layer spacing, the relaxation is found to be very small ({dollar}<{dollar}1%).

Single crystals of K(CN)$\sb{\rm x}$/(Br)$\sb{\rm 1-x}$ enriched in $\sp{15}$N have been studied by $\sp{15}$N NMR. Because of the anisotropic chemical shift, the NMR frequency spectrum of the orientational glass state reflects the distribution of cyanide orientations. By studying the spectrum at many orientations of each crystal, the orientational probability distribution function P($\Omega$) has been determined for x = 0.50 and for x = 0.20. The probability function is largest along the (100) directions, intermediate in the (110) directions, and nearly 0 along the (111) directions. The NMR determined P($\Omega$) is presented graphically and in terms of the Kubic Harmonic functions. The NMR results are in sharp contrast to molecular dynamics calculations but agree with neutron scattering data for x = 0.53.

Deuteron NMR selective inversion (SI) is developed to study slow molecular motions in solids. Theoretical and practical aspects of selective inversion of spin-1 nuclei in solids are presented. Differences between powdered solids and liquids are considered. Double sideband modulated (DSBM) shaped pulses are shown to improve the performance of SI pulses. DSBM and DANTE selective pulses are compared for off-resonance SI applications. Common spin-{dollar}{lcub}1{rcub}\over{lcub}2{rcub}{dollar} shaped pulses are tested for suitability to solid state deuteron NMR. Simple, short pulses are shown to be most effective due to fast spin-spin relaxation and large underlying homogeneous linewidths in solids.;The power and utility of selective inversion is demonstrated by investigating the molecular dynamics of polycrystalline dimethylsulfone-{dollar}\rm d\sb6{dollar} (DMS). Quantitative information on the slow two-site jump motion in DMS is obtained. The complementary nature of selective inversion and quadrupolar echo lineshape (QELS) experiments is explored. Combination of QELS and SI extends the range of rates observed, increasing the accuracy of the information obtained. For DMS, motional rates were measured over a 80 K range (motional rates of {dollar}\rm 3\times 10\sp1{lcub}-{rcub}5\times 10\sp{lcub}4{rcub}\ s\sp{lcub}-1{rcub}),{dollar} permiting the barrier to slow rotation to be accurately characterized. SI was also used to observe the limiting homogeneous linewidth as a function of resonance offset and temperature. The temperature dependence of the homogeneous linewidth is shown to be sensitive to motion in the kilohertz range.;SI was used to quantify the molecular dynamics in some complex systems. SI and QELS experiments were performed on host urea-{dollar}d\sb4{dollar} in different urea inclusion compounds (UIC). Rotation rates about both the CO and CN bonds of urea-{dollar}d\sb4{dollar} in the UICs were determined as a function of temperature. Activation energies were obtained, and the results are discussed with respect to guest-host interactions in the UICs. Preliminary results are presented on quadrupolar echo lineshape simulations which include empirical, anisotropic homogeneous linewidth corrections. The modified simulations give significantly different best-fit motional rates than conventional QELS analysis. This allows some of the systematic errors of QELS analysis to be evaluated.;SI experiments were conducted, over a 125 K range, to study backbone motion in bisphenol-A polycarbonate. These experiments failed to detect the presence of any large angle slow motion of methyl groups in the isopropylidene moiety. This negative result suggests that cis-trans isomerization of the carbonate group is not the origin of the backbone motion.

The purpose of this study is to characterize radiation-induced luminescence of terbium-doped silicate glasses. Experiments performed investigated the optical properties, isothermal time-evolution, and temperature dependence of the radiation-induced luminescence of two commercially available terbium-doped glasses. A problem common to this type of glass is the persistent luminescence, or afterglow, that occurs following the end of excitation from an external source of radiation. While the processes that govern characteristic luminescence of rare earth ions, including terbium, are well understood, the processes that give rise to afterglow in doped glasses are not. Identifying the source of long-term luminescence is essential for controlling problems that may arise from practical applications of luminescent glasses.;It was determined that the stimulation of terbium fluorescence is the result of direct excitation from the external radiation source, and indirect excitation from the delayed recombination of charge carriers releasing from traps in the host glass. The range of trap depths is found to be well represented by quasi-continuous distribution functions. The characteristic decay time during the initial response of both glasses studied is approximately 3.5 milliseconds. Decay of the afterglow was observed to persist for several hours, depending on the acquired dose of radiation. Comparison of the response to x-rays and ultraviolet radiation yielded the same results, indicating that the same processes are involved in producing afterglow for both cases. This result suggests a more efficient means of characterizing scintillating glasses by using ultraviolet lasers instead of x-rays.

The discovery of tunneling magneto-resistance has led to a great deal of interest in the study of ferromagnet-insulator-ferromagnet (FIF) systems due to potential sensor and magnetic storage applications. An analysis of the band structure of the 3d ferromagnets shows that the conduction electrons become spin polarized by the molecular field. The transmission coefficient of these electrons across a tunneling gap therefore depends upon the relative alignment of the molecular field between the two ferromagnets.;In this work the manufacture of such tunneling gaps through compression molding of powdered ferromagnetic iron with a high performance polyimide has been studied for the first time. The percent change in the resistance with applied magnetic field depends critically on the volume percentage of ferromagnetic material in the composite. A peak in the tunneling magnetoresistance (TMR) occurs at a volume concentration just beneath the percolation threshold of the ferromagnetic material. The change in resistance relative to the resistance at zero field, DeltaR/R0, obtains a room temperature peak value of -4.5% at 20% iron volume concentration.;Granular conducting systems near the percolation threshold are also subject to variable range hopping (VRH) conduction. The charging energy of small metallic grains results in an energy barrier for the acceptance of an additional electron. Electronic conduction requires thermal activation over this barrier along with tunneling through the insulating regime. The result of these two combined processes is a temperature dependent tunneling distance and a conductivity of the form ln sigma ∝ T-x, with 1/4 ≤ x ≤ 1/2.;The theoretical development and experimental measurements of TMR and VRH in iron polyimide nanocomposites are thoroughly developed and analyzed in this work. Ferromagnet particle size and band structure effects on TMR are also explored in an effort to optimize the material for sensor applications.

In this dissertation we study the electronic properties of certain two-dimensional chiral electron systems. We study the static and dynamic screening of gapped bilayer graphene and find important qualitative differences between the dielectric screening function obtained using a simplified 2-band model and that obtained using a more sophisticated 4-band model. We also formulate a continuum model to study the low-energy electronic properties of heterostructures formed by graphene on a strong three-dimensional topological insulator (TI) both for the case of commensurate and incommensurate stacking. We find that the proximity of the TI induces a strong enhancement of the spin-orbit coupling in graphene that can be tuned via the twist angle. Additionally, we examine the effect of a spin-active interface on the symmetry of proximity-induced superconducting pairing amplitudes in topological insulators. We compare our results to those for normal metals and ferromagnetic materials finding that the nontrivial spin chirality of the TI leads to qualitatively different behavior of the pairing amplitude. Lastly, we study the many-body instabilities of the Dirac states predicted to arise on the surfaces of topological Kondo insulators identifying regions of parameter space in which the system exhibits spin density wave, and charge density wave order.

No description available.