Dans cette thèse, nous présentons une étude par spectroscopie infrarouge du couplage magnéto-électrique de matériaux multiferroïques. Nous avons commencé par mesurer la dépendance en température du spectre de phonons infrarouges du composé antiferromagnétique MnF2 et nous avons montré que les fréquences des phonons sont sensiblement modifiées par la transition magnétique. Nous avons ensuite mesuré le spectre de phonons de TbMnO3 et MnWO4, deux matériaux multiferroïques. Ces deux matériaux ont des comportements très différents lorsqu'on passe les transitions de phase. D'un côté le spectre de phonons de TbMnO3 subit une renormalisation similaire à celle observée dans MnF2 à la transition antiferromagnétique. On observe également un effet de la transition ferroélectrique sur un phonon polarisé dans la direction de la polarisation spontanée indiquant un lien entre les déplacements atomiques et la ferroélectricité. De son côté, le spectre de phonons de MnWO4, ne présente aucun changement aux transitions de phases magnétiques et ferroélectriques, ceci tend à appuyer l'hypothèse d'une ferroélectricité d'origine purement électronique dans ce matériau. Nous avons également étudié les excitations magnétiques de ces matériaux. Dans le cas de MnF2, des mesures de transmission infrarouge montrent une absorption due à l'activité dipolaire électrique d'un double magnon de bord de zone. Cette excitation est semblables aux électromagnons des matériaux multiferroïques car c'est une excitation magnétique sensible à un champ électrique oscillant. Dans TbMnO3, nous avons mesuré l'absorption due à l'électromagnon le plus intense situé autour de 60 cm−1 et nous avons montré que la totalité de son poids spectral vient de deux phonons polarisés dans la direction a. L'absence d'électromagnon dans le spectre d'absorption de MnWO4 semble indiquer que l'existence d'électromagnons dans les matériaux multiferroïques nécessite un fort couplage entre les phonons et l'ordre magnétique.

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Theoretical calculations have predicted the importance of two-photon absorptions in the third order optical nonlinearities of polydiacetylenes. We have used electro-absorption techniques to make these two-photon transitions slightly one-photon allowed and therefore observable in the one-photon absorption spectrum. Our findings, although unable to ascertain the existence of below-gap two-photon states, nevertheless provide an unexpected view of the conduction band in conjugated chain polymers. The observation of a field induced oscillatory change in the absorption around the vicinity of the band edge and comparison of this signal with theoretical calculations leads to the conclusion that the conduction band is composed of both one-photon and two-photon states. This oscillatory signal cannot be solely explained by one-photon or two-photon states but must include both types for this high energy signal to be reproduced in the theoretical calculations.

<p> Advancements in science and technology increasingly involve systems operating at the nanoscale. Interfaces are often present in these systems. Nanoscopic interfaces are ubiquitous in biological systems, nanofluidic devices, and integrated circuits. Properties at the interface may be quite different from the bulk, and in fact a true bulk may not be present in these systems. At the nanoscale the ratio of interface to volume is large, and the interface may have the dominant role in determining system behavior. Interfacial characteristics and their connection to interfacial properties are the focus of my thesis. Using molecular simulations of model interfaces we characterize how properties like chemistry, composition, and topography affect such phenomena such as hydrophobicity, heat transfer, and momentum transport at the nanoscale. An interface is defined simply as where two materials meet and a change in some structure or order parameter is observed. In aqueous systems, the type studied here, these changes are relatively sharp and occur within a distance of nanometers. Water molecules near the interface are expected to display sensitivity to the underlying surface. Indeed, water near a hydrophobic surface is more deformable and has greater fluctuations. The hydrophobicity of chemically heterogeneous surfaces and proteins are characterized using these nanoscopic measures. We find the effect of mixing hydrophobic and hydrophobic head group chemistries is asymmetric, i.e., it is easier to make a hydrophobic surface hydrophilic than the reverse. The role of hydrogen bonding in hydrophobic and ion hydration is also characterized using a short range water model. Hydrophobic and ion hydration are reasonably captured with the short range water model. These studies show the importance of chemical composition and local hydrogen bonding in determining surface hydrophobicity. Interfaces also lead to anomalous behavior in heat and momentum transport. Interfaces disrupt local structure and create boundary resistances that manifest in temperature discontinuities and interfacial slip. We explore the effects of chemical heterogeneity, nanoscale surface roughness, and directionality on thermal conductance across model solid-water interfaces. Interfacial conductance is directly influenced by the coupling strength or wettability of the surface. For chemically mixed surfaces, interfacial conductance does not precisely match with wettability. Surface roughness in general enhances conductance, but the improvement cannot be completely attributed to increased solvent accessible surfaced area. Momentum transport displays similar discontinuities at aqueous interfaces. These effects can be reduced through the use of osmolytes. Collectively this work highlights the influence of interfaces on heat and momentum transport. Insights are provided for modifying interfacial behavior and altering the property of interest.</p>

<p> This work contributes to the study of magnetic interactions in the low-dimensional antiferromagnets <i>M</i>(Sb,Ta)₂O₆, where <i> M</i> is a transition metal. By virtue of the trirutile structure, <i> M-O-O-M</i> chains propagate along [110] at <i>z</i> = 0 and [1<span style="text-decoration:overline">1</span>0] at <i>z</i> = 1/2 of the unit cell. These chains are separated along [001] by sheets of weakly-interacting diamagnetic ions. The spin-exchange coupling perpendicular to the chains is weak, permitting the low-dimensional classification. Single crystals have been grown using chemical vapor deposition and the floating zone method. Magnetization, in-field heat capacity, and high-resolution thermal expansion measurements have been performed along various axes, revealing significant anisotropy due to the peculiar magnetic structures and low dimensionality.</p><p> The Neel temperature, <i>T_N,</i> at which long-range order occurs is found to be unstable against the application of magnetic field above 2 T. Large fields tend to lower <i>T_N</i> of the set of moments with projections along the applied field. Moments which are aligned perpendicular to the field are significantly less affected. This can lead to the formation of a secondary peak in heat capacity when magnetic field is along either [110] or [1<span style="text-decoration:overline">1</span>0]. The change in heat capacity at the location of the newly formed peak means there is a change in entropy, which depends upon the direction of applied field with respect to the magnetic moments. Consequently, an anisotropic magnetocaloric effect arises due to the unique magnetic structure. The anisotropic nature of this effect has potential applications in magnetic refrigeration.</p>

<p> For a broad class of two-dimensional (2D) materials, the transition from isotropic fluid to crystalline solid is described by the theory of melting due to Kosterlitz, Thouless, Halperin, Nelson and Young (KTHNY). According to this theory, long-range order is achieved via elimination of the topological defects which proliferate in the fluid phase. However, many natural and man-made 2D systems posses spatial curvature and/or non-trivial topology, which require the presence of topological defects, even at T=0. In principle, the presence of these defects could profoundly affect the phase behavior of such a system. In this thesis, we develop and characterize an experimental system of charged colloidal particles that bind electrostatically to the interface between an oil and an aqueous phase. Depending on how we prepare the sample, this fluid interface may be flat, spherical, or have a more complicated geometry. Focusing on the cases where the interface is flat or spherical, we measure the interactions between the particles, and probe various aspects of their phase behavior. On flat interfaces, this phase behavior is well-described by KTHNY theory. In spherical geometries, however, we observe spatial structures and inhomogeneous dynamics that cannot be captured by the measures traditionally used to describe flat-space phase behavior. We show that, in the spherical system, ordering is achieved by a novel mechanism: sequestration of topological defects into freely-terminating grain boundaries (&ldquo;scars&rdquo;), and simultaneous spatial organization of the scars themselves on the vertices of an icosahedron. The emergence of icosahedral order coincides with the localization of mobility into isolated &ldquo;lakes&rdquo; of fluid or glassy particles, situated at the icosahedron vertices. These lakes are embedded in a rigid, connected &ldquo;continent&rdquo; of locally crystalline particles.</p><p>

The structural and magnetic properties in selected rare-earth transition metal compounds with ThMn$\sb{12}$- or Th$\sb2$Zn$\sb{17}$-type structures, which are of importance for permanent magnet applications, have been studied using X-ray diffraction and magnetometry techniques, in a temperature range from 2 K to their Curie temperatures. Included in this dissertation work are the $\rm R\sb2(Fe\sb{1-x}Co\sb{x})\sb{17}$ and $\rm R\sb2(Fe\sb{1-x}Co\sb{x})\sb{17}N\sb{3-\delta}$ (R = Ce, Pr and Nd, $\rm 0\le x\le 1$ and $\rm 0\le\delta\le 1)$ series; the $\rm Sm\sb2Fe\sb{17}N\sb{y}\ (y\sim 2.5)$ compound; the $\rm RCo\sb{10}Mo\sb2$ series (R = Y, Ce, Pr, Nd, Sm, Dy and Er); the $\rm RFe\sb{10-x}Co\sb{x}Mo\sb2$ series $\rm (0\le x\le 10;$ R = Y, Pr, Nd, Gd, Dy, Ho and Er); and the $\rm RFe\sb{10}Mo\sb2$ series (R = Y, Nd, Gd and Dy). All of the compounds in the $\rm RFe\sb{10-x}Co\sb{x}Mo\sb2$ series crystallize in the tetragonal ThMn$\sb{12}$-type structure, and their magnetic properties are strongly modified by the Co substitution. In the $\rm RCo\sb{10}Mo\sb2$ series, the 3d sublattice favors a uniaxial anisotropy, and in contrast to the R(Fe,TM)$\sb{12}$ type materials, the rare earths with $\alpha > 0$ (R = Sm, Er) favor a planar anisotropy, whereas the rare earths with $\alpha < 0$ (R = Nd, Pr, Dy) favor a uniaxial anisotropy. The molecular field theory on the basis of a two sublattice model is capable of rather accurately describing the temperature dependence of the magnetization for the $\rm RFe\sb{10}Mo\sb2,\ Sm\sb2Fe\sb{17},$ and $\rm Sm\sb2Fe\sb{17}N\sb{y}$ compounds. / Source: Dissertation Abstracts International, Volume: 55-04, Section: B, page: 1618. / Major Professor: Shahid A. Shaheen. / Thesis (Ph.D.)--The Florida State University, 1994.

High resolution helium atom scattering experiments have been employed to study epitaxial growth of alkali halides crystals: NaCl/NaCl(001), KBr/NaCl(001), NaCl/KBr(001) and KBr/RbCl(001). The growth behavior was characterized by measuring the deposition curves and the oscillations in the scattered intensities of the specular (and Bragg for NaCl/NaCl(001) and KBr/NaCl(001)) due to the layer-by-layer growth. The structures of the substrates and overgrowths were studied via angular distributions of the scattered intensity. Additionally, time-of-flight measurements were used to energy analyze the dynamics of thin epitaxial films for KBr/NaCl(001) and KBr/RbCl(001). For NaCl/NaCl(001), a terrace height of 2.8A or half of the bulk face-centered-cubic lattice spacing was found and scattering cross section for adsorbed NaCl were determined under a variety of conditions. For KBr/NaCl(001) and NaCl/KBr(001), a superstructure corresponding to the 6 KBr's or 7 NaCl's lattice spacing was observed during the growth, which disappeared after 6 monolayers were grown. The dynamics of two, three, four and seven monolayer KBr/NaCl(001) were studied by measuring the dispersion curves and comparing them with theoretical calculations. For KBr/RbCl(001), the data indicates that the step height of the first layer is different from that of subsequent layers, 3.71A compared with 3.32A expected from the bulk lattice spacing. Analysis of the time-of-flight spectra for the one monolayer film yields surface phonon dispersion curves which were compared with those of cleaved RbCl and KBr (001) surfaces. / Source: Dissertation Abstracts International, Volume: 53-11, Section: B, page: 5917. / Major Professor: James G. Skofronick. / Thesis (Ph.D.)--The Florida State University, 1992.

Consideration is given to the premise that certain electron paramagnetic resonance signals in natural quartz are the result of age-indicative accumulations of alpha recoil-induced lattice damage. The point defects potentially responsible for the observed EPR signals are reviewed, as a basis for consideration of point defect production by ionizing radiation of and alpha recoil fragment transport in quartz. The production of oxygen vacancies in the quartz lattice by individual alpha recoil fragments in both the $\sp{238}$U and $\sp{232}$Th decay chains is calculated using two different methods. The Monte Carlo transport method provides a detailed collision history for each of the recoil fragments. The EPR experiments focus on the size of the E' and oxygen hole-related center (OHRC) signals in five different types of quartz: pegmatitic, unmetamorphosed plutonic, metamorphosed plutonic, hydrothermal vein and volcanic. The effects of gamma and neutron irradiation on the EPR spectra are also considered. The concentrations of $\sp{238}$U, $\sp{232}$Th and $\sp{147}$Sm are determined using neutron activation analysis in many of the quartzes that were extracted from rocks (or were intimately associated with minerals) of known age. Significant correlations between the age of the quartz and the size of both the E' and OHRC signals are initially established, and improved correlation is observed after the signal sizes are normalized to the respective total alpha activity of each sample (based on its $\sp{238}$U, $\sp{232}$Th and $\sp{147}$Sm concentration). These correlations establish the basis for a potential quartz geochronometer. / Theoretical considerations involve factors related to (1) recoil damage efficiency, (2) paramagnetic fractions of precursor defect populations, (3) overlap of alpha recoil track volumes and (4) fluid inclusion-hosted alpha activity. It is hypothesized that spherical, alpha recoil-induced lattice damage halos may develop around alpha-active fluid inclusions. A method is proposed for quartz age determinations, and suggestions are made toward overcoming the present limitations. It is concluded that approximate age determinations ($\pm$15-20%) may eventually be possible, and that maximum age determinations might also eventually be possible. / Source: Dissertation Abstracts International, Volume: 51-07, Section: B, page: 3289. / Major Professors: Arthur Leroy Odom; George W. Devore. / Thesis (Ph.D.)--The Florida State University, 1990.