This work presents the development and application of analytical, numerical, and experimental methods for the study of thermal and electrical transport in nanoscale systems, with special emphasis on those materials and phenomena which can be important in thermoelectric and semiconductor device applications. Analytical solutions to the Boltzmann transport equation (BTE) using the relaxation time approximation (RTA) are presented and used to study the thermal and electrical transport properties of indium antimonide (InSb), indium arsenide (InAs), bismuth telluride (Bi₂Te₃), and chromium disilicide (CrSi₂) nanowires. Experimental results for the thermal conductivity of single layer graphene supported by SiO₂ were analyzed using an RTA-based model and compared to a full quantum mechanical numerical BTE solution which does not rely on the RTA. The ability of these models to explain the measurement results as well as differences between the two approaches are discussed. Alternatively, numerical solutions to the BTE may be obtained statistically through Monte Carlo simulation for complex geometries which may prove intractable for analytical methods. Following this approach, phonon transport in silicon (Si) sawtooth nanowires was studied, revealing that thermal conductivity suppression below the diffuse surface limit is possible. The experimental investigation of energy transport in nanostructures typically involved the use of microfabricated devices or non-contact optical methods. In this work, two such approaches were analyzed to ascertain their thermal behavior and overall accuracy as well as areas for possible improvement. A Raman spectroscopy-based measurement design for investigating the thermal properties of suspended and supported graphene was examined analytically. The resulting analysis provided a means of determining from measurement results the thermal interface conductance, thermal contact resistance, and thermal conductivity of the suspended and supported graphene regions. Previously, microfabricated devices of several different designs have been used to experimentally measure the thermal transport characteristics of nanostructures such as carbon nanotubes, nanowires, and thin films. To ascertain the accuracy and limitations of various microdevice designs and their associated conduction analyses, finite element models were constructed using ANSYS and measurements of samples of known thermal conductance were simulated. It was found that designs with the sample suspended were generally more accurate than those for which the sample is supported on a bridge whose conductance is measured separately. The effects of radiation loss to the environment of certain device designs were also studied, demonstrating the need for radiation shielding to be at temperatures close to that of the device substrate in order to accurately calibrate the resistance thermometers. Using a suspended microdevice like those analyzed using finite element analysis, the thermal conductivities of individual bismuth (Bi) nanowires were measured. The results were correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. Compared to bulk Bi in the same crystal direction, the thermal conductivity of a single-crystal Bi nanowires of 232 nm diameter was found to be 3 - 6 times smaller than bulk between 100 K and 300 K. For polycrystalline Bi nanowires of 74 nm to 255 nm diameter the thermal conductivity was reduced by a factor of 18 - 78 over the same temperature range. Comparable thermal conductivity values were measured for polycrystalline nanowires of varying diameters, suggesting a grain boundary scattering mean free path for all heat carriers in the range of 15 - 40 nm which is smaller than the nanowire diameters. An RTA-based transport model for both charge carriers and phonons was developed which explains the thermal conductivity suppression in the single-crystal nanowire by considering diffuse phonon-surface scattering, partially diffuse surface scattering of electrons and holes, and scattering of phonons and charge carriers by ionized impurities such as oxygen and carbon of a concentration on the order of 10¹⁹ cm⁻³. Using a similar experimental setup, the thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) of higher manganese silicide (HMS) nanostructures were investigated. Bulk HMS is a passable high temperature thermoelectric material which possesses a complex crystal structure that could lead to very interesting and useful nanoscale transport properties. The thermal conductivities of HMS nanowires and nanoribbons were found to be reduced by 50 - 60 % compared to bulk values in the same crystal direction for both nanoribbons and nanowires. The measured Seebeck coefficient data was comparable or below that of bulk, suggesting unintentional doping of the samples either during growth or sample preparation. Difficulty in determining the amorphous oxide layer thickness for nanoribbons samples necessitated using the total, oxide-included cross section in the thermal and electrical conductivity calculation. This in turn led to the determined electrical conductivity values representing the lower bound on the actual electrical conductivity of the HMS core. From this approach, the measured electrical conductivity values were comparable or slightly below the lower end of bulk electrical conductivity values. This oxide thickness issue affects the determination of the HMS nanostructure thermoelectric figure of merit ZT as well, though the lower bound values obtained here were found to still be comparable to or slightly smaller than the expected bulk values in the same crystal direction. Analytical modeling also indicates higher doping than in bulk. Overall, HMS nanostructures appear to have the potential to demonstrate measurable size-induced ZT enhancement, especially if optimal doping and control over the crystallographic growth direction can be achieved. However, experimental methods to achieve reliable electrical contact to quality four-probe samples needs to be improved in order to fully investigate the thermoelectric potential of HMS nanostructures. / text

We have studied the charge density wave (CDW) repolarization dynamics in blue bronze (K0.3MoO3) by applying symmetric bipolar square-wave voltages of different frequencies to the sample and measuring the changes in infrared transmittance, proportional to CDW strain. The frequency dependence of the electro-transmittance was fit to a modified harmonic oscillator response and the evolution of the parameters as functions of voltage, position, and temperature are discussed. We found that resonance frequencies decrease with distance from the current contacts, indicating that the resulting delays are intrinsic to the CDW with the strain effectively flowing from the contact. For a fixed position, the average relaxation time for most samples has a voltage dependence given by τ0 ∼ V −p, with 1 < p < 2. The temperature dependence of the fitting parameters shows that the dynamics are governed by both the force on the CDW and the CDW current: for a given force and position, both the relaxation and delay times are inversely proportional to the CDW current as temperature is varied. The long delay times (∼ 100 μs) for large CDW currents suggest that the strain response involves the motion of macroscopic objects, presumably CDW phase dislocation lines. We have done frequency domain simulations to study charge-density-wave (CDW) polarization dynamics when symmetric bipolar square current pulses of different frequencies and amplitudes are applied to the sample, using parameters appropriate for NbSe3 at T = 90 K. The frequency dependence of the strain at one fixed position was fit to the same modified harmonic oscillator response and the behavior of the parameters as functions of current and position are discussed. Delay times increase nonlinearly with distance from the current contacts again, indicating that these are intrinsic to the CDWwith the strain effectively flowing from the contact. For a fixed position and high currents the relaxation time increases with decreasing current, but for low currents its behavior is strongly dependent on the distance between the current contact and the sample ends. This fact clearly shows the effect of the phase-slip process needed in the current conversion process at the contacts. The relaxation and delay times computed (∼ 1 μs) are much shorter than observed in blue bronze (> 100 μs), as expected because NbSe3 is metallic whereas K0.3MoO3 is semiconducting. While our simulated results bear a qualitative resemblance with those obtained in blue bronze, we can not make a quantitative comparison with the K0.3MoO3 results since the CDW in our simulations is current driven, whereas the electro-optic experiment was voltage driven. Different theoretical models predict that for voltages near the threshold Von, quantities such as the dynamic phase velocity correlation length and CDW velocity vary as ξ ∼ |V/Von − 1| −ν and v ∼ |V/Von − 1|ξ with ν ∼ 1/2 and ζ = 5/6. Additionally, a weakly divergent behavior for the diffusion constant D ∼ |V/Von − 1|−2ν+ζ is expected. Motivated by these premises and the fact that no convincing experimental evidence is known, we carried out measurements of the parameters that govern the CDW repolarization dynamic for voltages near threshold. We found that for most temperatures considered the relaxation time still increases for voltages as small as 1.06Von indicating that the CDW is still in the plastic and presumably in the noncritical limit. However, at one temperature we found that the relaxation time saturates with no indication of critical behavior, giving a new upper limit to the critical regime, of |V/Von − 1| < 0.06.

L'alternance de deux groupes de polarités très différentes, CH2 et CF2, permet au poly fluorure de vinylidène (PVDF) d’être un polymère industriellement très intéressant. Cependant, cette spécificité mène aussi à d’importantes inversions du monomère lors de la polymérisation vinylique. Pendant la polymérisation, en complément de la propagation tête-queue, CH2CF2CH2CF2, les monomères inversés conduisent à l’addition en queue-queue, CF2CH2CH2CF2, et tête-tête, CH2CF2CF2CH2. Le taux de transformation de polymère se trouve expérimentalement entre 3 et 7%. Ce pourcentage élevé entraine sans aucun doute la modification de propriétés macroscopiques. En utilisant la dynamique moléculaire, cette thèse a pour but de montrer l'effet de ces défauts sur la température de transition vitreuse (Tg), la dynamique locale et sur la température de fusion (Tm) du PVDF. En phase amorphe, le PVDF avec différents pourcentages de régio-défauts a été étudié : 3.6, 4.1, 9.3 et 23%. Cette étude permet de prédire le comportement de polymères qui ne sont pas synthétisés. Étant donné que les Tg simulées et expérimentales concordent avec précision, les motifs moléculaires qui donnent lieu à l'effet plastifiant de l'inversion de monomères peuvent être envisagés. En plus d'accentuer leur effet de plastifiant, la conclusion significative est que la relaxation de la chaîne peut être révélée en abordant explicitement des mouvements locaux. Car cette procédure ne peut pas être déduite de la connaissance du Tg, nous avons basé notre analyse sur le fait cela : 1) Nous avons démontré que des relations linéaires directes entre Tg et l'énergie d'activation conformationnelle de transition (Ea) extraite à partir d'un graphe d'Arrhenius, existent. Ce diagramme correspond au logarithme naturel des taux de transition entre les états rotameriques contre l'inverse de la température. La pente de cette courbe rapporte directement à cet Ea efficace. Un tel lien a été seulement spéculé dans la littérature. 2) Nous avons calculé des relations d'Arrhenius pour différents genres de torsions le long de la chaîne d'épine dorsale. En conséquence, une barrière d'énergie potentielle, ea, est associée à la rotation d'un lien dans un environnement spécifique. L'addition de ces énergies pesées par le pourcentage de chaque lien le long de l'épine dorsale, donne un ea moyen qui est équivalent à l'ea efficace. À l'aide de cette procédure, nous avons maintenant accès au mouvement local de la chaîne entière. 3) Nous avons vérifié cette procédure pour calculer une valeur pour le Tg du copolymère alternatif du l'éthylène-tétrafluoroéthylène (E-TFE), qui possède les segments qui sont présents le PVDF changé. L'ambiguïté concernant la valeur de la Tg du copolymère E_TFE peut être résolue grâce à cette approche, puisque le PVDF avec 50% de défauts régio-isomériques conduit à l'E_TFE. D'ailleurs, nous avons étudié les temps de relaxation pour la fonction d'autocorrélation de torsion au-dessus d'un large éventail de température. La dynamique locale est alors spécifiquement étudiée. L'équation Vogel-Fulcher-Tammann (VFT) est utilisée pour décrire le processus de relaxation associée aux mouvements coopératifs des segments le long de la chaîne. Nous avons également étudié le possibilité d'utiliser le Kohlrausch-Williams-Watts (KWW), fonction exponentielle étirée, afin de décrire la dépendance temporelle du processus de relaxation, ce travail a été effectué à différentes températures. Les résultats concordent bien avec les données expérimentales. L'objectif principal de cette section est d'étudier conjointement la fréquence des transitions conformationnelles et le temps de relaxation obtenu par la fonction d’autocorrelation de torsion, sur une plage importante de température, afin d’établir un entre les fréquences des transitions conformationnelles et le comportement de type VFT. Nous montrons pour la première fois qu’une relation linéaire peut être établie entre la barrière de transition conformationnelle et l’énergie d’activation effective. Nous montrons pour la première fois qu'une relation linéaire peut être établie entre la barrière de transition conformationnelle, Ea et l'énergie d'activation effective, B, responsables de la dynamique locale. Parmi les cinq phases cristallines que présente le PVDF, les cristaux α et ß présentent des propriétés particulières intéressantes et ont fait l'objet d'une attention significative. Ces deux structures cristallines sont celles que l’on rencontre le plus souvent, la phase α est la plus thermodynamiquement stable le cristal β possède des propriétés ferroélectriques. Toutefois, le comportement lors de la fusion de ces deux phases cristallines n’est pas encore totalement compris. Certains chercheurs pensent que la température de fusion de la phase β est supérieure à la phase alpha . D'autres affirment que le pic endothermique vu sur le thermogramme obtenue par calorimétrie différentielle à balayage (DSC) a été attribué par erreur à la phase β, cela à cause d’une confusion dans les références . À cet égard, le comportement de la Tm des cristaux α et β par rapport à leur épaisseur est obtenu par la dynamique moléculaire. Différents types de nanocristaux composés de chaînes de PVDF, sans ou avec 10% de régio-défauts, ayant des longueurs différentes ont ainsi été simulées dans les phases α et β. On applique l'équation de Gibbs-Thomson (G-T) afin de déterminer l'énergie de surface et l’enthalpie de fusion des nanocristaux. Les valeurs déterminées sont en accord avec les données expérimentales. Nous avons montré que le PVDF en phase β pur a une température de fusion inférieure à celle du PVDF en phase α pur. Cependant, en insérant des défauts à l'intérieur du cristal, la phase α modifiée présente une température de fusion inférieure à celle de la phase β modifiée. / Abstract : Alternating two groups, CH2 and CF2, of very different polarities along the backbone chain of polyvinylidene fluoride (PVDF) leads to very interesting properties, such as ferroelectricity. However, these properties are affected by the presence of regioisomerism defects (monomer inversion) that appear during the synthesis. During the polymerization, in addition to the Head-to-Tail (HT) sequences, CH2CF2CH2CF2, the reversed monomer units lead to formation of Tail-to-Tail (TT), CF2CH2CH2CF2, and Head-to-Head (HH), CH2CF2CF2CH2, links. The rate of this chain alteration experimentally lies between 3 and 7 %. This percentage undoubtedly brings changes in macroscopic properties. The aim of this thesis is to reveal the impact of these defects on the glass transition temperature (Tg), local dynamics and melting temperature (Tm) of PVDF by using Molecular Dynamics (MD) simulation. In amorphous phase, PVDF chains with different percentages of regiodefects were investigated: 0, 3.6, 4.1, 9.3, and 23 %. This study makes it possible to predict the experimental behavior of polymers which have not yet been synthesized. Once Tg is acquired, the relaxation of the chain can be investigated through the calculation of the activation energy (Ea) of the conformational transition. The significant conclusion is that the relaxation of the chain can be revealed by addressing the local motions. More specifically: a) We demonstrate a direct linear relationship between Tg and Ea extracted from an Arrhenius plot. This diagram corresponds to the natural logarithm of transition rates between rotameric states versus the inverse of the temperature. The slope of this curve yields directly Ea. Such a link was only speculated in the literature. b) A significant finding of this work is that the mobility of the chain can be associated with different types of bonds in PVDF with regiodefects. c) Based on the analysis of Ea for the different bond contributions, we proposed a value for the Tg of ethylene-tetrafluoroethylene (E-TFE), an isomeric polymer of PVDF with 50% regiodefects. Experimentally, the available data for the Tg of E-TFE are limited and highly variable. For example, it has been reported as varying from -108 °C to 145 °C. The ambiguity of Tg for this copolymer can be resolved with this approach. Furthermore, we studied the relaxation time associated with the torsional autocorrelation function (TACF) over a wide temperature range. The Vogel-Fulcher-Tammann (VFT) equation was used to describe the temperature dependence of the relaxation time. The Kohlrausch Williams Watts (KWW) stretched exponential function is then applied to fit the time dependence of the relaxation process at various temperatures. The results obtained from this work were found to be in good agreement with the experimental data. A particular interest in this study is the question of how the non-Arrhenius VFT of relaxation process is related to the Arrhenius behavior of conformational jump rates near the glass transition. In both cases, the energies (the conformational transition energy (Ea) and the effective activation energy (B) in VFT equation), were very close to the value of a single torsional barrier. However, in contrast to the relaxation time associated with TACF, the rates of conformational jumps show the activation energy higher than the single barrier value. We have shown that a linear relationship can be established between the conformational transition energy and the effective activation energy. In crystalline PVDF, among the five typical phases, the α and β crystals are of particular interest. The α phase is the most thermodynamically stable form and the β crystal possesses ferroelectric properties. The melting behaviour of these two crystal phases is not so clear. Some researchers believe that the melting temperature of the β phase is higher than that of the α phase. Others have claimed that the higher melting temperature of the peak in Differential Scanning Calorimetry (DSC) has been mistakenly attributed to β phase melting, due to confusion in the referencing of literature sources. In this regard, the melting temperatures of α and β crystals (with and without regiodefects) with respect to their thickness are captured by MD simulation. We then applied the Gibbs-Thomson (G-T) equation to determine the melting temperature, as well as the surface energy and enthalpy of fusion, for α and β nanocrystals. We have shown that pure β phase PVDF has a lower melting temperature than pure α phase PVDF. However, by inserting regiodefects randomly inside the crystal, the α phase with regiodefects shows a lower melting temperature than that of the β phase with regiodefects. We attributed this behaviour to the different structures of the two phases.

<p>Many of the earth’s ecosystems are experiencing large species losses due to human impacts such as habitat destruction and fragmentation, climate change, species invasions, pollution, and overfishing. Due to the complex interactions between species in food webs the extinction of one species could lead to a cascade of further extinctions and hence cause dramatic changes in species composition and ecosystem processes. The complexity of ecological systems makes it difficult to study them empirically. The systems often consist of large species numbers with lots of interactions between species. Investigating ecological communities within a theoretical approach, using mathematical models and computer simulations, is an alternative or a complement to experimental studies. This thesis is a collection of theoretical studies. We use model food webs in order to explore how biodiversity (species number) affects the response of communities to species loss (Paper I-III) and to environmental variability (Paper IV).</p><p>In paper I and II we investigate the risk of secondary extinctions following deletion of one species. It is shown that resistance against additional species extinctions increases with redundancy (number of species per functional group) (Paper I) in the absence of competition between basal species but decreases with redundancy in the presence of competition between basal species (Paper II). It is further shown that food webs with low redundancy run the risk of losing a greater proportion of species following a species deletion in a deterministic environment but when demographic stochasticity is included the benefits of redundancy are largely lost (Paper II). This finding implies that in the construction of nature reserves the advantages of redundancy for conservation of communities may be lost if the reserves are small in size. Additionally, food webs show higher risks of further extinctions after the loss of basal species and herbivores than after the loss of top predators (Paper I and II).</p><p>Secondary extinctions caused by a primary extinction and mediated through direct and indirect effects, are likely to occur with a time delay since the manifestation of indirect effects can take long time to appear. In paper III we show that the loss of a top predator leads to a significantly earlier onset of secondary extinctions in model communities than does the loss of a species from other trophic levels. If local secondary extinctions occur early they are less likely to be balanced by immigration of species from local communities nearby implying that secondary extinctions caused by the loss of top predators are less likely to be balanced by dispersal than secondary extinctions caused by the loss of other species. As top predators are vulnerable to human-induced disturbances on ecosystems in the first place, our results suggest that conservation of top predators should be a priority. Moreover, in most cases time to secondary extinction is shown to increase with species richness indicating the decay of ecological communities to be slower in species-rich than in species-poor communities.</p><p>Apart from the human-induced disturbances that often force species towards extinction the environment is also, to a smaller or larger extent, varying over time in a natural way. Such environmental stochasticity influences the dynamics of populations. In paper IV we compare the responses of food webs of different sizes to environmental stochasticity. Species-rich webs are found to be more sensitive to environmental stochasticity. Particularly, species-rich webs lose a greater proportion of species than species-poor webs and they also begin losing species faster than species-poor webs. However, once one species is lost time to final extinction is longer in species-rich webs than in species-poor webs. We also find that the results differ depending on whether species respond similarly to environmental fluctuations or whether they are totally uncorrelated in their response. For a given species richness, communities with uncorrelated species responses run a considerable higher risk of losing a fixed proportion of species compared with communities with correlated species responses.</p>

Many of the earth’s ecosystems are experiencing large species losses due to human impacts such as habitat destruction and fragmentation, climate change, species invasions, pollution, and overfishing. Due to the complex interactions between species in food webs the extinction of one species could lead to a cascade of further extinctions and hence cause dramatic changes in species composition and ecosystem processes. The complexity of ecological systems makes it difficult to study them empirically. The systems often consist of large species numbers with lots of interactions between species. Investigating ecological communities within a theoretical approach, using mathematical models and computer simulations, is an alternative or a complement to experimental studies. This thesis is a collection of theoretical studies. We use model food webs in order to explore how biodiversity (species number) affects the response of communities to species loss (Paper I-III) and to environmental variability (Paper IV). In paper I and II we investigate the risk of secondary extinctions following deletion of one species. It is shown that resistance against additional species extinctions increases with redundancy (number of species per functional group) (Paper I) in the absence of competition between basal species but decreases with redundancy in the presence of competition between basal species (Paper II). It is further shown that food webs with low redundancy run the risk of losing a greater proportion of species following a species deletion in a deterministic environment but when demographic stochasticity is included the benefits of redundancy are largely lost (Paper II). This finding implies that in the construction of nature reserves the advantages of redundancy for conservation of communities may be lost if the reserves are small in size. Additionally, food webs show higher risks of further extinctions after the loss of basal species and herbivores than after the loss of top predators (Paper I and II). Secondary extinctions caused by a primary extinction and mediated through direct and indirect effects, are likely to occur with a time delay since the manifestation of indirect effects can take long time to appear. In paper III we show that the loss of a top predator leads to a significantly earlier onset of secondary extinctions in model communities than does the loss of a species from other trophic levels. If local secondary extinctions occur early they are less likely to be balanced by immigration of species from local communities nearby implying that secondary extinctions caused by the loss of top predators are less likely to be balanced by dispersal than secondary extinctions caused by the loss of other species. As top predators are vulnerable to human-induced disturbances on ecosystems in the first place, our results suggest that conservation of top predators should be a priority. Moreover, in most cases time to secondary extinction is shown to increase with species richness indicating the decay of ecological communities to be slower in species-rich than in species-poor communities. Apart from the human-induced disturbances that often force species towards extinction the environment is also, to a smaller or larger extent, varying over time in a natural way. Such environmental stochasticity influences the dynamics of populations. In paper IV we compare the responses of food webs of different sizes to environmental stochasticity. Species-rich webs are found to be more sensitive to environmental stochasticity. Particularly, species-rich webs lose a greater proportion of species than species-poor webs and they also begin losing species faster than species-poor webs. However, once one species is lost time to final extinction is longer in species-rich webs than in species-poor webs. We also find that the results differ depending on whether species respond similarly to environmental fluctuations or whether they are totally uncorrelated in their response. For a given species richness, communities with uncorrelated species responses run a considerable higher risk of losing a fixed proportion of species compared with communities with correlated species responses.

Nuclear magnetic resonance (NMR) is a common tool utilized in core analysis. NMR can reveal important information about pore structure, fluid configuration and wettability. However, standard NMR core analysis techniques look at the sample as a whole or only at thin slices. Two NMR pulse sequences are introduced that allow for the determination of relaxation time or diffusion-relaxation time distributions as a function of sample height. One-dimensional T 2 and D-T 2 profiles can be determined with a low-field Maran Ultra spectrometer by implementing nuclear magnetic resonance imaging techniques. Frequency encoding gradients impart spatial resolution to the measurements and allow for the creation of T, and D-T 2 profiles without having to perform multiple slice selective measurements. The first technique, denoted as RARE, relies solely on resolving transverse relaxation, T 2 , as a function of height. The second method, D-T 2 profiling, allows for the determination of both the diffusion coefficient, D, and T 2 as a function of height. The ability to resolve D in addition to T 2 allows fluids with overlapping relaxation times to be distinguished, and therefore it is not necessary to use D 2 O in order to differentiate the water signal from the oil signal. Implementation of these two methods allows for the determination of porosity and saturation profiles. Experiments were performed with a sandpack in order to demonstrate the applicability of these two techniques, and saturation profiles of a sandpack were determined at various stages of the flooding process.

In this thesis, we have performed systematical study of the complex impedance spectra(CIS) with the manganeses oxide thin films by the equivalent circuit model(ECM) composed of resistance and capacitance. The ECM has been utilized in analog of the electrical and dielectric properties of the granular films. The purpose of this research is to understand how the electrical- and magneto-transport properties in La0.67Ca0.33MnO3(LCMO),La0.8Ba0.2MnO3(LBMO),La0.67Sr0.33MnO3(LSMO(113)) and La0.67Sr1.33MnO4 (LSMO(214)) thin films, at various magnetic fields and temperatures. First of all, we demonstrate that the LSMO(214) and LSMO(113) can be sensitively affected by magnetic states on the manganite films. Our result provides further understanding of the dielectrics variation during the phase transition from an AFM insulating phase and/or a ferromagnetic metallic phase to a paramagnetic PM metallic phase. It is known that the strong correlation between the itinerant carriers and the local magnetic moments is the mechanism for FM/PM phase transition for LSMO(113), while the direct magnetic exchange coupling governed the AFM/PM phase transition and an indirect coupling to the status of intrinsic carriers for LSMO(214) films. These transitions can not be concludes directly by using a dc resistance measurement but can be clearly distinguished by the CIS measurement. On the other hand, the dc resistance (Rdc) and the relaxation time(£n) have the same tendency that this indicates the changes of £n matches to the electric transport properties for LCMO_90min and LSMO(214) thin films. We focus on the the dielectric properties of both samples are insensitive to temperature, revealing that the dielectric behavior is independent of magnetic phase transition but strongly associated with the transport properties. Therefore, the magnetic transitions can be most thoroughly investigated by combining CIS measurements and RC ECM, as well as by making dc resistance measurements. Moreover, the relative change of M£q(ac) is nearly larger than the dc resistive variation. This phenomenon, called giant magneto-impedance effect (GMI), implies that thehigh-frequency magnetotransport effect may enhance the performance of these manganese oxides for sensing the magnetic field. The CMI, have been analyzed by ECM, including two sets of parallel R and capacitance (C) components in series. The analyzing results the specific feature of grain boundaries(GBs) can be attributed to the interplay of magnetic moment spin disorder to ordering. The grain boundary (GB) effect can enhance low field magnetoresitance (LFMR) for artificial GBs, but shows very limited enhancement for those GBs in epitaxial films. This study finds that artificial GBs, which exhibit large LFMR, can be modeled as a non-conductive layer which disconnects the lattice periodicity of adjacent grains and contains no magnetic ions. The GBs in the present fully strained epitaxial film, which shows a relatively smaller LFMR, are more similar to a semi-continuous grain with continuous distribution of magnetic ions that align loosely parallel to the grain magnetic moment. In addition, we report in this study the high frequency magneto-transport properties, based on the classical model, of La0.8Ba0.2MnO3 and La0.67Ca0.33MnO3 thin films around their ferromagnetic transitions and under an external magnetic field. It is found that the specific features of magneto-impedance can be correlated with the complex magnetization response and the dielectric relaxation in corresponding phase states. The fast dielectric relaxation time, £nE, and the slow magnetic response, £nH, reflect the interplay of itinerant carriers and the magnetic coupling to the ac electromagnetic wave, indicating that the double exchange, or hopping, of carriers between O 2P and Mn 3d-eg states occur prior to the indirect magnetic coupling of adjacent Mn ions via strong Hunt¡¦s rules. Applied magnetic field enhances both electric and magnetic dipoles are now responding faster to the electromagnetic wave. The results of our work may provide a fundamental understanding of high frequency magnetic and electrical properties of the manganite films, and imply tips for device application of the films.

Organic polymer materials have been broadly applied in optical storage, optical communication and optical signal process. It has been revealed that these organic materials have some superior characteristics such as larger electro-optical (EO) coefficients, broader bandwidth and shorter response time, which make it good for EO modulator application. In our study, the goal is to study the EO coefficient of novel polymer material to be used in low driving voltage EO modulator. During the experiment, the dependence of the second harmonic generation (SHG) intensity on doping concentration of DR1/PMMA was observed and a reasonable explanation of the nonlinear dependency was given. We measured the EO coefficient of a new material, ASF/PMMA, using a Mach-Zehnder interferometer. We also observed the relaxation process of SHG intensity of this new material, which was compared with that of DR1/PMMA. Under the same measurement condition, we found that the EO coefficient of ASF/PMMA (13.1 pm/V) is significantly larger than that of DR1/PMMA (3 pm/V). It is also found that relaxation time of ASF/PMMA and DR1/PMMA are 22 and 8.5 seconds, respectively. Because of the superior characteristics of this material, it is suitable to be used in EO modulator.

No description available.

No description available.