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.

This document consists of two very different projects but the common thread is in the interest of magnetic fields. It describes the effect of magnetic fields in two Interstellar Medium regions in the Galaxy. Electromagnetic force is one of the four fundamental forces in physics. It is not known where magnetic field has initially risen in the Universe, but what is certain is that it has significant effect in the dynamics of star formation and galaxy formation. The studies aim to better understand the effects of field in an active star forming region and in the halo of the Galaxy. We observed the HI 21 cm spectral line via the Zeeman effect in attempt to detect line-of-sight magnetic field strengths in both of the projects. For the star forming region project in Chapter 2, towards the Eagle Nebula, an upper limit of the field strength was determined. From the observational results, physical conditions of the region were modeled. For the second project in Chapter 3, we attempted to detect magnetic fields via Zeeman effect towards non galactic disk objects. All of the observed positions have radial velocities that cannot be explained by the simple galactic rotation. Hence, they are considered to be non galactic disk sources and often grouped as High Velocity Clouds. With a unique observational technique and analysis, we derived the best fit line-of-sight magnetic fields. A particular interest to us is the Smith Cloud. From the detection of magnetic field, we attempted to estimate the density of the ambient medium in the halo, which will be useful for studying the galaxy formation.

One of the phenomenal results emerging from string theory is the AdS/CFT correspondence or gauge-gravity duality: In certain cases a theory of gravity is equivalent to a "dual" gauge theory, very similar to the one describing non-gravitational interactions of fundamental subatomic particles. A difficult problem on one side can be mapped to a simpler and solvable problem on the other side using this correspondence. Thus one of the theories can be understood better using the other. The mapping between theories of gravity and gauge theories has led to new approaches to building models of particle physics from string theory. One of the important features to model is the phenomenon of confinement present in strong interaction of particle physics. This feature is not present in the gauge theory arising in the simplest of the examples of the duality. However this N = 4 supersymmetric Yang-Mills gauge theory enjoys the property of being integrable, i.e. it can be exactly solved in terms of conserved charges. It is expected that if a more realistic theory turns out to be integrable, solvability of the theory would lead to simple analytical expressions for quantities like masses of the hadrons in the theory. In this thesis we show that the existing models of confinement are all nonintegrable--such simple analytic expressions cannot be obtained. We moreover show that these nonintegrable systems also exhibit features of chaotic dynamical systems, namely, sensitivity to initial conditions and a typical route of transition to chaos. We proceed to study the quantum mechanics of these systems and check whether their properties match those of chaotic quantum systems. Interestingly, the distribution of the spacing of meson excitations measured in the laboratory have been found to match with level-spacing distribution of typical quantum chaotic systems. We find agreement of this distribution with models of confining strong interactions, conforming these as viable models of particle physics arising from string theory.

<p> The process of star formation is an integral part of the new field of astrobiology, which studies the origins of life. Since the gas that collapses to form stars and their resulting protoplanetary disks is known to be weakly ionized and contain magnetic fields, star formation is governed by multifluid magnetohydrodynamics. In this thesis we consider two important problems involved in the process of star formation that may have strongly affected the origins of life, with the goal of determining the thermal effects of these flows and modeling the physical conditions of these environments.</p><p> We first considered the outstanding problem of how primitive bodies, specifically asteroids, were heated in protoplanetary disks early in their lifetime. Reexamining asteroid heating due to the classic unipolar induction heating mechanism described by Sonett et al. (1970), we find that this mechanism contains a subtle conceptual error. As original conceived, heating due to this mechanism is driven by a uniform, supersonic, fully-ionized, magnetized, T Tauri solar wind, which sweeps past an asteroid and causes the asteroid to experience a motional electric field in its rest frame. We point out that this mechanism ignores the interaction between the body surface and the flow, and thus only correctly describes the electric field far away from the asteroid where the plasma streams freely. In a realistic protoplanetary disk environment, we show that the interaction due to friction between the asteroid surface and the flow causes a shear layer to form close to the body, wherein the motional electric field predicted by Sonett et al. decreases and tends to zero at the asteroid surface. We correct this error by using the equations of multifluid magnetohydrodynamics to explicitly treat the shear layer. We calculate the velocity field in the plasma, and the magnetic and electric fields everywhere for two flows over an idealized infinite asteroid with varying magnetic field orientations. We show that the total electric field in the asteroid may either be of comparable strength to the electric field predicted by Sonett et al. or vanish depending on the magnetic field geometry. We include the effects of dust grains in the gas and calculate the heating rates in the plasma flow due to ion-neutral scattering and viscous dissipation. We term this newly discovered heating mechanism &ldquo;electrodynamic heating&rdquo;, use measurements of asteroid electrical conductivities to estimate the upper limits of the possible heating rates and amount of thermal energy that can be deposited in the solid body, and compare these to the heating produced by the decay of radioactive nuclei like Al²⁶.</p><p> For the second problem we modeled molecular line emission from time-dependent multifluid MHD shock waves in star-forming regions. By incorporating realistic radiative cooling by CO and H₂ into the numerical method developed by Ciolek &amp; Roberge (2013), we present the only current models of truly time-dependent multifluid MHD shock waves in weakly-ionized plasmas. Using the physical conditions determined by our models, we present predictions of molecular emission in the form of excitation diagrams, which can be compared to observations of protostellar outflows in order to trace the physical conditions of these environments. Current work focuses on creating models for varying initial conditions and shock ages, which are and will be the subject of several in progress studies of observed molecular outflows and will provide further insight into the physics and chemistry of these flows.</p>

In this thesis we address fundamental questions about what constitutes and limits an HI disc, probing the distribution of neutral gas in the outer parts of galaxies. We use a subsample of galaxies observed as part of the THINGS survey to investigate the HI extent of spiral galaxy discs. We revisit previous work on the extent of HI discs, showing the limitations set by insufficient linear resolution. We then exploit the high spatial and velocity resolution combined with good sensitivity of THINGS to investigate where the atomic gas discs end and what might shape their edges. We find that the atomic gas surface density across most of the disc is constant at 5 – 10 x 10^20 atoms/cm^2 and drops sharply at large radius. The general shape of the HI distribution is well described by a Sérsic-type function with a slope index, n = 0.14 - 0.22 and characteristic radius ri. We propose a new column density threshold of 5 x 10^19 atoms/cm^2 to define the extent of the gas disc. This limit is well within reach of modern instruments and is at the level where disc gas becomes susceptible to ionisation by an extragalactic radiation field. We argue that at this level the HI column density has decreased to one tenth of that across the inner disc and that by going to yet lower column density the disc is unlikely to grow by more than 10% in radius. The HI column density at which the radial profiles turn over is too high for it to be caused by ionisation by an extragalactic UV field and we postulate that the HI extent is set by how galaxy discs form. Ionisation by extragalactic radiation will only play a rôle at column densities below 5 x 10^19 atoms/cm^2, if any. To study the crucial relation between observed edges and how closely these reproduce the intrinsic distribution of gas through our interferometric measurements, we created an ensemble of models based on four radial density distributions. We conclude that the observed edges in spiral galaxies faithfully reflect their intrinsic shape. Only in very specific cases of highly inclined (>75º) and/or large vertical scaleheight discs do we see strong deviations from the intrinsic surface density of the observed shape of the edges in spiral galaxies. In the case of NGC 3198 we concluded that there is no significant difference in the radial profiles obtained with either constant or exponentially increasing vertical gas distributions, when scaleheights are not higher than 1 kpc at the outskirts of the disc. We infer an upper limit to the scaleheight of NGC 3198 of 2 kpc. To address the distribution of neutral gas at larger scales, we study an HI rich, giant LSB galaxy, NGC 765. We present HI spectral line and radio-continuum VLA data, complemented by optical and Chandra X-ray maps. NGC 765 has the largest HI-to-optical ratio known to date of any spiral galaxy and one of the largest known HI discs in absolute size with a diameter of ~ 240 kpc measured at a surface density of 2 x 10^19 atoms/cm^2. We derive a total HI mass of M_HI = 4.7 x 10^10 M_sun, a dynamical mass of M_dyn = 5.1 x 10^11 M_sun and an HI mass to luminosity ratio of M_HI/L_B = 1.6, making it the nearest and largest “crouching giant”. Optical images reveal evidence of a central bar with tightly wound low-surface brightness spiral arms extending from it. Radio-continuum (L_1.4 GHz = 1.3 x 10^21 W/Hz) and X-ray (L_x ~ 1.7 x 10^40 erg/s) emission is found to coincide with the optical core of the galaxy, compatible with nuclear activity powered by a low-luminosity AGN. We may be dealing with a galaxy that has retained in its current morphology traces of its formation history. In fact, it may still be undergoing some accretion, as evidenced by the presence of HI clumps the size (< 10 kpc) and mass (10^8 -10^9 M_sun) of small (dIrr) galaxies in the outskirts of its HI disc and by the presence of two similarly sized companions. In an exploration of future work, we engaged in a study of the edges in the HI discs of dwarf irregular galaxies, their parameterisation and simulation. A collection of simulations were created based on the dwarf galaxy NGC 2366, similar to what was done for the larger spiral galaxies, showing that line-of-sight column densities are affected for discs with inclinations higher than 60º. Five out of eleven of the dwarfs from THINGS which are less inclined than 60º were analysed and parameterised with our Sérsic-type function. Their discs have average central column densities spread evenly from log_10 NHI = 20.7 atoms/cm^2 to log_10 NHI =21.3 atoms/cm^2. Their radial decline is shallower (slope index peaks around n ~ 0.3) than for spirals. The up-coming Local Irregular That Trace Local Extremes (LITTLE) THINGS project, will likely enlarge the number of local dwarf irregular (dIm) galaxies to which this type of analysis can be applied and for which these preliminary results verified.

Carbon nanotube (CNT)/polymer composite sheets can be extremely high strength and lightweight, which makes them attractive for fabrication of mechanical structures. This thesis demonstrates a method whereby smooth, thin CNT/polymer composite sheets can be fabricated and patterned on the microscale using a process of photolithography and plasma etching. CNT/polymer composites were made from CNTs grown using chemical vapor deposition using supported catalyst growth and floating catalyst growth. The composite sheets had a roughness of approximately 30nm and were about 61¼m or 261¼m depending on whether they were made from supported catalyst grown or floating catalyst grown CNTs. The composites were patterned using an oxygen plasma as the etchant and a hard mask of silicon nitride.

Herbig Ae/Be (HAeBe) stars are intermediate mass (2-10 solar mass) pre-main sequence stars with circumstellar disks. Observing planets within these young disks would greatly aid in understanding planet formation processes and timescales particularly around massive stars. So far, only one planet, HD 100546 b, has been confirmed to orbit a HAeBe star. With over 250 HAeBe stars known, and several observed to have disks with structures thought to be related to planet formation, it seems likely that there are as yet undiscovered planetary companions within the circumstellar disks of some of these young stars. Direct detection of a low-luminosity companion near a star requires high contrast imaging, often with the use of a coronagraph, and the subtraction of the central star's point spread function (PSF). Several processing algorithms have been developed in recent years to improve PSF subtraction and enhance the signal-to-noise of sources close to the star. However, many HAeBe stars were observed via direct imaging before these algorithms came out. We used the PSF subtraction program PynPoint to reprocess archival images of HAeBe stars from the Advanced Camera for Surveys on the Hubble Space Telescope to increase the likelihood of detecting a planet in their disks. We believe we have recovered the known planet around HD 100546 and possibly its candidate second companion. We also detect new candidate sources in the vicinities of HD 141569 and HD 163296. Further observations are needed to confirm the reality of these detections and also establish their association with the host stars.

Energy-based acoustic quantities provide vital information about acoustic fields and the characterization of acoustic sources. Recently, the phase and amplitude gradient estimator (PAGE) method has been developed to reduce error and extend bandwidth of energy-based quantity estimates. To inform uses and applications of the method, analytical and experimental characterizations of the method are presented. Analytical PAGE method bias errors are compared with those of traditional estimation for two- and three-microphone one-dimensional probes. For a monopole field when phase unwrapping is possible, zero bias error is achieved for active intensity using three-microphone PAGE and for specific acoustic impedance using two-microphone PAGE. A method for higher-order estimation in reactive fields is developed, and it is shown that a higher-order traditional method outperforms higher-order PAGE for reactive intensity in a standing wave field. Extending the applications of PAGE, the unwrapped phase gradient is used to develop a method for directional sensing with improved bandwidth and arbitrary array response.

This thesis describes the development of the real-time convolution system (RTCS) for a little-studied talker/listener in virtual acoustic environments. We include descriptions of the high-resolution directivity measurements of human speech, the RTCS system components, the measurement and characterization of oral-binaural room impulse responses (OBRIRs) for a variety of acoustic environments, and the compensation filter necessary for its validity. In addition to incorporating the high-resolution directivity measurements, this RTCS improved on that developed by Cabrera et al. [1] through the derivation and inclusion of the compensation filter. Objective measures in the time- and frequency-domains, as well as subjective measures, were developed to asses the validity of the RTCS. The utility of the RTCS is demonstrated in the study on vocal effort, and the results of an initial investigation into the vocal effort data are presented.

The MiMeS (Magnetism in Massive Stars) project is a large-scale, high-resolution, sensitive spectropolarimetric investigation of the magnetic properties of O- and early B-type stars. Initiated in 2008 and completed in 2013, the project was supported by three Large Program allocations, as well as various programmes initiated by independent principal investigators, and archival resources. Ultimately, over 4800 circularly polarized spectra of 560 O and B stars were collected with the instruments ESPaDOnS (Echelle SpectroPolarimetric Device for the Observation of Stars) at the Canada–France–Hawaii Telescope, Narval at the Télescope Bernard Lyot and HARPSpol at the European Southern Observatory La Silla 3.6 m telescope, making MiMeS by far the largest systematic investigation of massive star magnetism ever undertaken. In this paper, the first in a series reporting the general results of the survey, we introduce the scientific motivation and goals, describe the sample of targets, review the instrumentation and observational techniques used, explain the exposure time calculation designed to provide sensitivity to surface dipole fields above approximately 100 G, discuss the polarimetric performance, stability and uncertainty of the instrumentation, and summarize the previous and forthcoming publications.