Browsing by Subject "Magnetic fields"
Now showing 1 - 9 of 9
- Results Per Page
- Sort Options
Item Berry phase modification to electron density of states and its applications(2007-05) Xiao, Di, 1979-; Niu, QianWe study the Berry phase correction to the electron density of states and present a number of its applications. It is now well recognized that the Berry phase of the electronic wave function plays an important role in the dynamics of Bloch electrons. For instance, the electron will acquire an anomalous velocity term transverse to the applied electric field, giving rise to an intrinsic contribution to the anomalous Hall effect. On the other hand, we find that the Berry phase also has a fundamental effect on the electron phase space, and leads to a modification of the phase-space density of states. This surprising result has a number of applications, which we shall discuss in detail. We first derive an explicit expression of the orbital magnetization (zero and finite temperature), where it is shown that contributions to the orbital magnetization can be classified into a local rotation of the electron and global centerof-mass motion. Based on this formula, we develop a theory of the Berry-phase effect in anomalous transport in ferromagnets driven by statistical forces such as the gradient of temperature or chemical potential. We also study the Berry phase effect on magnetotransport, showing that a linear (in field) magnetoresistance is possible in ferromagnets. Finally, we propose that in graphene with broken inversion symmetry, a valley Hall effect exists and the finite valley polarization can be detected by measuring the magnetization.Item Contributions to the electrochemistry of the cell(1933) Rosene, H. F. (Hilda Florence), 1897-1978; Lund, E. J. (Elmer Julius), 1884-1969Item Global instabilities in rotating magnetized plasmas(2009-05) Pino, Jesse Ethan, 1981-; Mahajan, Swadesh M.; Hazeltine, R. D. (Richard D.)The Magnetorotational Instability (MRI) is believed to be the primary mechanism for angular momentum transfer in astrophysical accretion disks. This instability, which exists in ionized disks in the presence of weak magnetic fields, can either transfer angular momentum directly, or give rise to anomalous viscosity via non-linear turbulence. While many previous analytical treatments are concerned with the local theory of the MRI, when the length scale of rotation shear is comparable to the length scale of the most unstable modes, a global analysis is necessary. In this dissertation we investigate the global theory of the linear MRI. In particular, we show how rotation shear can localize global modes and how the global growth rates can differ signicantly from the local approximation in certain cases. Changes in the equilibrium density are considered. In addition, the effects of Hall Magnetohydrodynamics on the MRI are studied in both the local and global cases.Item An investigation of the physical parameters of young stellar objects(2011-12) Deen, Casey Patrick; Jaffe, D. T.; Lacy, John; Sneden, Chris; Scalo, John; Johns-Krull, Christopher; Evans, Neal J.Studies of the temporal evolution of young stars and their associated properties rely upon the ability of astronomers to determine ages and masses of objects in different evolutionary states. The best method for determining the age and mass of a young stellar object is to place the object on the Hertzsprung-Russell (HR) diagram and to compare to theoretical evolutionary tracks. Accurate ages allow the investigation of the temporal evolution of properties associated with stellar youth (accretion rates, X-ray activity, circumstellar excess, etc...). One property intimately linked with stellar youth is the presence (or absence) of an optically thick primordial circumstellar disk. Objects in "young" star forming regions are more likely to show evidence for a disk than objects in "older" clusters. Within a single cluster, the picture is not as clear. There exist objects in very young clusters (~1 Myr) which show no evidence for circumstellar disks, and there exist objects in very old clusters (~10 Myr), which show evidence for robust disks, suggesting a variable other than stellar age is driving the evolution of the disks. To investigate whether these outliers are due to age spreads, initial conditions, or simply appear anomalous due to erroneous age determinations, we must determine better placements in the HR diagram by carefully transforming observable quantities (spectral type and apparent magnitude) into the quantities necessary for comparison evolutionary models (effective temperature and luminosity). In the Ophiuchus star forming region, I investigate whether or not objects with disks are younger than disk-less objects. I find no difference in the ages of the two populations, but the systematic and random uncertainties are large enough to mask all but the largest age differences. In the hope of better determining the physical parameters of young stellar objects, I embark on a spectral synthesis campaign to produce comparison synthetic spectra which account for the effects of magnetic fields. This requires the modification of the MOOG spectral synthesis program to handle the full Stokes vector treatment for polarized radiation through a magnetized medium. I create a grid of synthetic spectra covering ranges in effective temperature, surface gravity, and average magnetic field strength relevant for studies of young stellar objects, and develop a Chi-squared minimization routine to determine the best fit synthetic spectrum for a given observed spectrum at an arbitrary resolving power. This grid of synthetic spectra will be an invaluable complement to future near infrared, large band-pass, high-resolving power spectrographs (i.e. IGRINS). In addition to these observational and theoretical attempts to reduce systematic errors, I also helped to develop a suite of silicon and KRS-5 grisms for use in the FORCAST instrument, a mid infrared camera on the SOFIA telescope. These grisms will afford the imaging instrument a mid infrared spectroscopic capability at wavelengths normally inaccessible from the ground. I also report on my work to help write FG Widget, the quick-look reduction software package developed to support grism observations.Item Magnetic field enhancement of Coulomb blockade conductance oscillations in metal-metal oxide double barrier tunnel devices fabricated using atomic force microscope nanolithography(2005) Wiemeri, Jeffrey Charles; Shih, Chih-KangMagnetic field enhancement of Coulomb blockade conductance oscillations in metal oxide double barrier tunnel devices fabricated using atomic force microscope nanolithography is reported for the first time. Anodic oxidation by this method was accomplished on lithographically patterned Ti and Ni device layers. This is the first time Ni has been reported to be oxidized via scanning probe lithography. Magnetoresistance measurements were taken on selected devices in a Hall effect cryogenic system where tunneling conductance behavior was observed at 1.8, 10, 25, and 50 K in the Ti devices and 150 K in the Ni devices. Coulomb blockade conductance features were observed at vii 1.8, 10 and 50 K in the Ti devices and 10 and 25 K in the Ni devices. Applying a 9T magnetic field enhanced the conductance oscillations and clarified the Coulomb staircase apparent in the I-V curves for both devices. From theoretical fits of the experimental conductance behavior for the Ti devices, this is attributed to a suppression of cotunneling currents in the device. Additionally, in multiple Ti devices, a zero-bias anomaly peak was observed at ~ 2 K and is attributed to contaminant particles in the metal oxide barrier creating a localized magnetic moment in the junction leading to spin-flip and s-d exchange scattering assisted tunneling according to the Anderson-Appelbaum model. This is the first time these zero-bias anomalies have been observed and reported in planar tunnel junctions.Item Magneto-optical studies of field-driven propagation dynamics of domain walls in permalloy nanowires and scaling of magnetic energy losses in permalloy films and microstructures(2006) Nistor, Corneliu; Erskine, James L.The magneto-optical Kerr effect is used to study the propagation dynamics of field-driven magnetic domain walls in Permalloy nanowires. The domain wall velocity- field characteristic is found to be linear below a threshold field and highly non-linear above the threshold. An analytical model is proposed for the propagation dynamics of domain walls in ferromagnetic nanowires at low fields. The magnetic energy loss in Permalloy thin films and microstructures driven by sinusoidal magnetic fields is studied as a function of the applied field frequency over a wide range of frequencies. The magnetic energy loss is shown to be constant at low frequencies and rapidly increasing at higher frequencies. The measurements are found to be in agreement with a model of magnetization reversal based on domain wall motion.Item Overcoming pathogenic biofilm recalcitrance with magnetic mitigation therapies(2019-05) Heersema, Lara Ashley; Smyth, Hugh D. C.; Dunn, Andrew; Ghosh, Debadyuti; Gordon, Vernita; Cui, ZhengrongPathogenic biofilms are associated with the chronicity of infectious diseases and are difficult to treat using current therapies. Biofilm recalcitrance enables antibiotic resistance up to 1,000 fold compared to non-biofilm related infections and requires longer treatment regimens. This increase in the use of antimicrobials in the fight against biofilms contributes to the alarming increase in antibiotic resistant microorganisms globally. Therefore, there is a need for new treatments that are effective against multiple types of pathogenic biofilms, can target biofilms in surface and deep tissues, exhibit low or no toxicity, and can synergize with current pharmacological therapies without inducing resistance. Towards achieving efficient biofilm disruption and removal, I report on the use of magnetic nanoparticle and magnetic field treatments optimized with current therapeutics against pathogenic biofilms. The first section of this dissertation focuses on the use and characterization of in vitro pathogenic biofilm models for susceptibility testing. Reported here is the development and characterization or a novel high-throughput multispecies oral caries associated biofilm model. This model was designed to facilitate screening of novel therapies against a relevant oral caries model with both commensal and pathogenic microorganisms. The second section of this dissertation focuses on the use of magnetic nanoparticles and magnetic fields to mechanochemically disrupt and remove biofilms. The synthesis reaction parameters of iron oxide nanoparticles was also assessed using statistical modeling to allow for large scale synthesis of magnetic nanoparticles. The third section of this dissertation focuses on the synergism of magnetic nanoparticles and magnetic fields with current antimicrobials through the use of FDA approved inactive ingredients. Potential formulations were evaluated using an accelerated stability and efficacy study, while prototype magnetic field devices for treating oral caries biofilms were also developed. In sum, this work represents key steps towards slowing the rise in antibiotic resistance while ensuring treatment of chronic biofilm infections by using magnetic-based mitigation of biofilmsItem The role of rotation and magnetic fields in a core collapse supernova(2006-08) Akiyama, Shizuka; Wheeler, J. CraigWhile the process that converts implosion into explosion in core collapse supernovae is poorly understood, their observed asphericity provides new constraints on the physics of these events. Since pulsars are rotating and magnetized neutron stars, there is no doubt that rotation and magnetic fields are inherent to the exploding engine. We have shown that magnetic field amplification is an inevitable by-product of the differential rotation that accompanies core-collapse. We performed 1D core-collapse simulations of rotating iron cores with various rotational profiles and velocities. We found that differential rotation was a generic feature of rotating iron core collapse. As a result, the magnetorotational instability (MRI) generates magnetic fields of order 10¹⁵⁻¹⁷ G in a few tens of milliseconds where the negative shear is the strongest. Although magnetic fields of order 10¹⁵⁻¹⁷ G are very strong, they are not strong enough to modify the equation of state of degenerate electron gas near the proto-neutron star. The corresponding MHD luminosity available is ~10⁵² erg s⁻¹, which can modify the explosion dynamics if the power is sustained for a fraction of a second. When rotational effects are included, we found that there is a critical iron core rotation rate that gives the most rapidly rotating proto-neutron star, faster than which the rotational velocity of the proto-neutron star decreases due to centrifugal support. This non-monotonic behavior of post-collapse core rotation suggests that the progenitor of the most rapidly rotating proto-neutron star is not the most rapidly rotating iron core, but that those iron cores with nearly the critical initial rotation rate may produce the maximum proto-neutron star rotation, the strongest magnetic fields, and the most robust supernova explosions. Even small rotation may induce non-axisymmetric instabilities, which drive magneto-acoustic flux in to the mantle, transporting enegy out of the proto-neutron star to the region near the stalled shock. Further implications for rotation and magnetic fields, pulsars and magnetars, and jet formation mechanisms are discussed.Item Transport of protostellar cosmic rays in turbulent dense cores(2022-07-08) Fitz Axen, Margot; Offner, StellaRecent studies have suggested that low-energy cosmic rays (CRs) may be accelerated inside molecular clouds by the shocks associated with star formation. We use a Monte Carlo transport code to model the propagation of CRs accelerated by protostellar accretion shocks through protostellar cores. We calculate the CR attenuation and energy losses and compute the resulting flux and ionization rate as a function of both radial distance from the protostar and angular position. We show that protostellar cores have non-uniform CR fluxes that produce a broad range of CR ionization rates, with the maximum value being up to two orders of magnitude higher then the radial average at a given distance. In particular, the CR flux is focused in the direction of the outflow cavity, creating a ’flashlight’ effect and allowing CRs to leak out of the core. The radially averaged ionization rates are less than the measured value for the Milky Way of [zeta] ≈ 10⁻¹⁶ s⁻¹; however, within r ≈ 0.03 pc from the protostar, the maximum ionization rates exceed this value. We show that variation in the protostellar parameters, particularly in the accretion rate, may produce ionization rates that are a couple of orders of magnitude higher or lower than our fiducial values. Finally, we use a statistical method to model unresolved sub-grid magnetic turbulence in the core. We show that turbulence modifies the CR spectrum and increases the uniformity of the CR distribution but does not significantly affect the resulting ionization rates.