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Browsing by Subject "Physics"

Now showing 1 - 20 of 114
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    AC Magnetic Susceptibility of a Thin Film of Permalloy
    (2014-10-02) Gray, Isaiah;
    Measurements of noise in magnetic thin lms have variously reported 1=f noise, white noise, and random telegraph noise. The 1=f noise experiments claim magnetic noise by relating the magnetic noise to the susceptibility with use of the uctuation-dissipation theorem. However, neither the linearity of the susceptibility necessary for application of uctuation-dissipation nor the frequency dependence of susceptibility was explored. To investigate more fully the frequency dependence and linearity of the magnetic susceptibility of a magnetic lm, we measured the AC susceptibility of a 100 nm thick lm of permalloy as a function of the magnitude HAC and frequency f of an applied AC magnetic eld over the full hysteresis loop of the lms, i.e., the measurements were performed while slowly varying an applied DC magnetic eld, HDC. The AC frequency range was from 20 Hz to 5 kHz while the AC eld range was between 0.2 G and 1 G. At HDC = 0 the response of the system was measured as a function of HAC at 200 Hz. It was found to be nonlinear but became reasonably close to linear for HAC < 1G - the coercive eld was approximately 15 G. The in-phase and out-of-phase components approximately follow power laws with frequency, with exponents 0.69 and -0.2. The behavior of the out-of-phase component is roughly consistent with previously measured white magnetic noise and a simple harmonic oscillator model, but the in-phase component does not follow the prediction of this model.
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    Advanced analysis and background techniques for the cryogenic dark matter search.
    (2010-01) Qiu, Xinjie
    The Cryogenic Dark Matter Search (CDMS) used Ge and Si detectors, operated at 50mK, to look for Weakly Interacting Massive Particles (WIMPs) which may make up most of the dark matter in our universe. This dissertation describes the simulation, analysis, and results of the first WIMP-search data runs of the CDMS experiment between October 2006 and July 2007 with its 5 Towers of detectors at the Soudan Underground Laboratory. A blind analysis, incorporating improved techniques for event reconstruction and data quality monitoring, resulted in zero observed events. The results of this work place the most stringent limits yet set upon the WIMP-nucleon spin-independent cross section for WIMP masses above 44 GeV/c2, as well as setting competitive limits on spin-dependent WIMP-nucleon interactions.
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    Anisotropic hardware injection of gravitational waves into LIGO detectors.
    (2011-08) Pihlaja, Miranda
    The goal of LIGO (Laser Interferometer Gravitational-wave Observatory) is to measure very small changes in the length of its arms caused by gravitational waves stretching and compressing space-time. The purpose of this project was to perform a search pipeline test on the LIGO detector using a point source stochastic signal. Here we report an effort to test the LIGO stochastic data-analysis pipeline using a simulated hardware injection. A hardware injection is an end-to-end test that begins with the generation of a signal and ends with the recovery of the signal from the detector’s strain data. The signal is injected into the interferometers by actuating on a test mass (or mirror) in order to shake it to simulate a gravitational wave. We successfully recovered the hardware injection with measured parameters consistent with injected parameters, thus providing an end-to-end test of the stochastic data-analysis pipeline.
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    Automated Analysis of Shubnikov-de Haas Oscillations in 2D Material
    (2024-04-18) Li, Yaotian; Davydov, Konstantin; Wang, Ke
    In recent years, some remarkable discoveries, such as superconductivity and ferromagnetism, have been made in few-layer graphene heterostructures. Understanding the new physics relies on analyzing the samples’ magnetoresistance, such as studying the periodic dependence of resistance on the magnetic field and charge carrier density, known as Shubnikov-de Haas (SdH) oscillations. This research aims to automate the analysis of SdH oscillations in 2D materials, specifically focusing on graphene, to accelerate data processing while ensuring accuracy. The project employs machine learning techniques, particularly Density-Based Spatial Clustering of Applications with Noise (DBSCAN), to separate data into clusters, where each cluster corresponds to a local resistance minimum. All the local minima form a Landau fan. The position of the Landau fan is determined by a linear relation between the magnetic field and charge carrier density, which can be found by applying a linear fit to the clusters. From that, the filling factor of a particular cluster can be extracted. This helps determine essential parameters like twist angles and band structure degeneracy in the system. Overall, this project efficiently determines the filling factor of 70% of visible minima, providing sufficient information to comprehend the system's properties.
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    Axial-scan fluorescence fluctuation spectroscopy: initial development and experimental challenges.
    (2010-05) Chen, Yun
    Summary abstract not available.
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    Bolometric detectors for EBEX: a balloon-borne cosmic microwave background polarimeter.
    (2009-12) Hubmayr, Johannes
    We discuss the design and performance of arrays of millimeter-wave, bolometeric detectors for EBEX, the E and B Experiment. EBEX is a balloon-borne telescope designed to measure the polarization in the cosmic microwave background (CMB) radiation with 8 arc-minute resolution at 150, 250 and 410 GHz during a 14-day long duration balloon flight in Antarctica. On June 11, 2009 EBEX launched an engineering test flight from NASA's Columbia Scientific Ballooning Facility (CSBF) achieving 10 hours at float altitudes of 115,000 ft. EBEX is the first experiment to successfully operate transition edge sensor (TES) bolometers readout by superconducting quantum interference devices (SQUIDs) from a balloon platform. We present the EBEX instrument design, review TES bolometer and SQUID theory of operation and elaborate on frequency domain multiplexing. Following the analysis of the detector and readout system, we detail measurements that characterize the bolometers and conclude with a discussion of the EBEX receiver optical efficiency and in-flight bolometer loading during the engineering flight.
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    Calibration and design of the E and B EXperiment (EBEX) cryogenic receiver
    (2014-08) Zilic, Kyle Thomas
    I discuss the design, construction, and calibration of the E \& B EXperiment (EBEX), a balloon-borne telescope designed to measure the B-mode polarization anisotropy of the cosmic microwave background (CMB). EBEX observes the sky with 8 arcmin resolution in three frequency bands centered on 150, 250, and 410 GHz, with over 1,500 detectors. Polarimetry is performed through use of a continuously rotating achromatic half-wave plate with fixed wire-grid polarizer. The experiment was designed to detect the gravitational-lensed B-mode signal and detect or set an upper limit for the inflationary B-mode signal. In this thesis, I describe the design and structure of various subsystems of the EBEX receiver and predict their experimental performance. Several calibrating instrumental response experiments are described and the results reported and compared to predictions. A brief review of the 2012-2013 long duration balloon (LDB) flight from McMurdo Station, Antarctica, is provided and a summary of the receiver performance during flight characterized.
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    Cell free expression in emulsions and vesicles.
    (2010-08) Monina, Nadezda
    In this study we address the effects that membrane composition has on expression within two common forms of encapsulation: the single layered micelle and the bilayered vesicle. We then proceed to show the effects of oxygen on transcription machinery and the role the membrane plays in optimizing oxygen concentration for protein expression. In each case, the surfactant encapsulation is prepared in mineral oil and encloses an Escherichia coli cytoplasmic extract. Measurement of expression efficiency is conducted by both endpoint and kinetic measurement of eGFP (enhanced green fluorescent protein) concentrations via fluorescent microscopy. In the case of micelle encapsulation, results show that micelles of long block copolymer (LBCP) have the highest rate of expression as well as final concentration at 8 hours. Results also show a large discrepancy in eGFP expression between well oxygenated and less oxygenated environments. Similarly, a comparison between lipids suggests longer fatty acid tail length corresponds to more effective inhibition of oxygen diffusion leading to greater expression efficiency. Further expression tests in vesicles show membranes composed of phosphotidylcholine (PC) lipid mixed with LBCP have the most efficient eGFP as well as α hemolysin expression among vesicles of differing composition. With this information comes a newfound knowledge of the effects of synthetic membrane composition, oxygen concentration and the relationship between these two parameters on protein expression.
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    Characterization of Variant Phase Transitions in Smectic-C* Liquid Crystals by Computer Simulation
    (2009-04-08) Martin, Stephen
    Recent experiments have identified a smooth transition from smectic-C*[alpha1] to smectic-C*[alpha2] in mixtures of some liquid crystalline compounds. However, phenomenological theories implementing only short-range interactions have predicted the intervention of smectic-C*[FI2] in this transition. Using a parameterized free-energy model, this smooth transition in smectic-C*[alpha] is rationalized by variation of three interaction parameters independently in a computer simulation. A phase diagram characterizing the results of our simulation is presented.
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    Commissioning of the New Beam Halo Monitoring System for the CMS Experiment
    (2015) Stifter, Kelly;
    A new Beam Halo Monitor (BHM) detector system has been installed in the CMS cavern to measure the machine-induced background (MIB) from the LHC. This back- ground originates from interactions of the primary beam halo with the nal set of collimators before the CMS experiment and from beam gas interactions. The BHM detector uses the directional nature of Cherenkov radiation and event timing to select particles coming from the direction of the beam and to suppress those originating from the interaction point. It consists of 40 quartz cylinders, placed on each side of the CMS detector, coupled to UV-sensitive PMTs. For each bunch crossing, the PMT signal is digitized by a charge integrating ASIC and the arrival time of the signal is recorded. The data are processed in real time to yield a precise measurement of per-bunch-crossing MIB rate for each beam. This measurement is made available to CMS and the LHC, in order to provide real-time feedback on the beam quality and improve the e ciency of data taking. The BHM detector is now in the commissioning phase, and rst results have been obtained.
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    Comprehensive study of the chemical reactions resulting from the decomposition of chloroform in alkaline aqueous solution.
    (2009-11) Mews, Jorge Estevez
    Chloroform (CHCl3) is a volatile liquid, which has a rather slow rate of decomposition in ground water. It is a known carcinogen and one of the most common contaminants found at toxic waste sites. The dominant degradation process for chloroform in both the atmosphere and the groundwater is the reaction with the hydroxyl radical or hydroxide ion. This process triggers a sequence of reactions which ultimately yield carbon monoxide, hydrogen chloride, and formic acid. The rate of chloroform degradation is considerably larger in solution than that in the gas phase and it increases dramatically with increasing pH. However, only one of the viable reactions had been studied previously at a high level of theory in solution. It is of great interest to gain a deeper understanding of the decomposition reaction mechanism. Quantum mechanical methods are well suited for studying the mechanism of organic reactions. However, a full quantum mechanical treatment of the entire fluid system is not computationally feasible. In this work, combined quantum mechanical and molecular mechanical (QM/MM) methods are used for studying chemical reactions in condensed phases. In these calculations, the solute molecules are treated quantum mechanically (QM), whereas the solvent molecules are approximated by empirical (MM) potential energy functions. The use of quantum mechanics and statistical sampling simulation is necessary to determine the reaction free energy profile. In the present study, the ab initio Hartree-Fock theory along with the 3-21G basis set was used in the quantum mechanical calculations to elucidate the reaction pathways of chloroform decomposition, with a focus on basic reaction conditions. Statistical mechanical Monte Carlo approach was then applied in molecular mechanical simulations, employing the empirical TIP3P model for water. We employed state-of-the-art electronic structure methods to determine the gas-phase inter-nuclear potential energy profile for all the relevant reactions. Each gas-phase potential energy profile obtained at a high level of theory was used as a post-correction of the corresponding reaction free energy profile in aqueous solution. A detailed picture of the actual mechanism driving the decomposition pathway of chloroform has emerged from these simulations.
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    Correlations in polymer blends: simulations, perturbation theory, and coarse-grained theory.
    (2009-09) Chung, Jun Kyung
    A thermodynamic perturbation theory of symmetric polymer blends is developed that properly accounts for the correlation in the spatial arrangement of monomers. By expanding the free energy of mixing in powers of a small parameter α which controls the incompatibility of two monomer species, we show that the perturbation theory has the form of the original Flory-Huggins theory, to first order in α. However, the lattice coordination number in the original theory is replaced by an effective coordination number. A random walk model for the effective coordination number is found to describe Monte Carlo simulation data very well. We also propose a way to estimate Flory-Huggins χ parameter by extrapolating the perturbation theory to the limit of a hypothetical system of infinitely long chains. The first order perturbation theory yields an accurate estimation of χ to first order in α. Going to second order, however, turns out to be more involved and an unambiguous determination of the coefficient of α2 term is not possible at the moment. Lastly, we test the predictions of a renormalized one-loop theory of fluctuations using two coarse-grained models of symmetric polymer blends at the critical composition. It is found that the theory accurately describes the correlation effect for relatively small values of χN. In addition, the universality assumption of coarse-grained models is examined and we find results that are supportive of it.
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    Coupled quantum systems in inflationary cosmology.
    (2010-08) Gumrukcuoglu, Ahmet Emir
    The studies presented in this thesis describe applications of quantum field theory in a time dependent background. Two distinct problems are addressed in the framework of inflationary cosmology. The strict predictions of inflation are mostly in agreement with the Cosmic Microwave Background observations. In the recent years, large scale anomalies in the data motivated a series of analyses leading to a detection of broken statistical isotropy. Assuming that this effect is sourced by early time cosmology, I discuss the phenomenology of inflationary models extended to anisotropic backgrounds. Due to lack of rotational invariance, these models generically involve a system of coupled quantum fields. This leads to a tensor-scalar correlation function, which is a characteristic signature of these models. Another open question in cosmology involves the transition from inflation to the Hot Big Bang cosmology. In the presence of supersymmetric flat directions, the formation of the thermal radiation may undergo a dramatic delay, provided that these directions decay only perturbatively. In the scope of a toy model and a realistic example, both involving two flat directions, I discuss the nonperturbative decay that rapidly depletes the flat directions. If realized, this process can dramatically affect the previous assumptions on the thermalization scale. Due to the vast number of degrees of freedom, this problem generically involves coupled quantum fields. The decay of the flat directions gets contributions from both the diagonal (nonadiabatic evolution of frequency eigenvalues) and nondiagonal (nonadiabatic evolution of frequency eigenstates) effects. An additional characteristic effect of coupled quantization is the rotation of light eigenstates to heavy ones, which do not get produced in a diagonal system.
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    Critical Vortices in 2+1 dimensions and stochastic background of gravitational waves from cosmic strings.
    (2012-02) Olmez, Serkay
    This thesis consists of two parts. In the first part we consider vortices which are topological defects that emerge upon spontaneously broken local symmetries in 2 + 1 dimensions. We discuss the renormalization of the central charge and the mass of the N = 2 supersymmetric Abelian vortices. At the classical level the mass of the vortex is equal to its central charge, which is referred to as Bogomol’nyi - Prasad - Sommerfield (BPS) saturation. At the quantum level both the mass and the central charge get corrections. We show that the mass and the central charge of the vortex get the same nonvanishing quantum corrections, which preserves BPS saturation at the quantum level. In the second part, we study the stochastic background of gravitational waves (SBGW) generated by kinks and cusps on cosmic string loops. Cosmic strings, which can be considered as 2+1 dimensional vortices extended along the additional dimension, are one dimensional topological defects predicted by a large class of unified theories as remnants of spontaneously broken local or global symmetries. Grand unified theories have gauge symmetries which are eventually spontaneously broken down to the symmetry of the Standard Model, and certain class of these phase transitions are expected to produce cosmic strings. The interactions of cosmic strings result in cusps or kinks on them which decay by radiating gravitational waves. In this study, we find that kinks contribute at the same order as cusps to the SBGW, and discuss the accessibility of the total background due to kinks as well as cusps to current and planned gravitational wave detectors, as well as to the big bang nucleosynthesis (BBN), the cosmic microwave background (CMB), and pulsar timing constraints. Furthermore we consider anisotropies in the SBGW arising from random fluctuations in the number of sources. Such anisotropies are analogous to the anisotropies observed in the CMB radiation and would carry additional information about the gravitational-wave sources that generated them.
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    Cross-correlation searches for persistent gravitational waves with Advanced LIGO and noise studies for current and future ground-based gravitational-wave detectors.
    (2018-05) Meyers, Patrick
    Over the last three years, the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) has detected signals from colliding black holes and a signal from colliding neutron stars. These detections ushered in a new era of gravitational-wave (GW) astrophysics and multimessenger astronomy that allows us to probe new regions of the universe. One of the next frontiers for gravitational-wave astronomy is the detection and characterization of the stochastic GW background (SGWB). A measurement of the SGWB from unresolved compact binary systems could come as Advanced LIGO reaches design sensitivity, and future detectors will be important for digging beyond that astrophysical background towards trying to measure signals from relic gravitational waves produced in the early universe. In this dissertation, I present cross-correlation-based searches for a SGWB and other persistent sources of GWs. I introduce and use a new method for setting limits on the strain amplitude of a potential source of GWs in the directions of Scorpius X-1, the galactic center, and Supernova 1987a in the frequency band from 20-1726 Hz. I also set limits on persistent, broadband point sources of GWs across the whole sky. Finally, I show how we can implement data analysis techniques to improve the Advanced LIGO detector sensitivity to persistent sources of GWs. Improving sensitivity of current detectors and planning for future detectors is vital to the effort to measure and understand the SGWB. This will requires a better understanding of the noise sources that limit sensitivity, especially at lower frequencies. To this end, I outline a method for estimating and modeling correlated magnetic noise between spatially separated GW detectors. I also present results from a 3D seismometer array deployed at the Homestake Mine, aimed at characterizing seismic and Newtonian noise for future GW detectors. I estimate the fundamental Rayleigh-wave eigenfunction, and then use it in a seismic radiometer algorithm to separate different components of the seismic field that contribute differently to the Newtonian noise. Finally, I present estimates of the Newtonian noise as a function of depth in the frequency band from 0.5-5 Hz based on results from the seismic radiometer.
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    Design, construction, and assessment of a neutron shield for CDMS test facilities.
    (2011-08) Radpour, Roxanne
    The Cryogenic Dark Matter Search (CDMS) experiment is an effort to detect dark matter in the form of weakly interacting massive particles (WIMPs). CDMS utilizes cryogenic semiconductor detectors in order to accomplish this. At the University of Minnesota the testing and characterization of these detectors is one of the main responsibilities. To further improve characterization of the detectors, it is important to reduce the number of background events from reaching the detector as best as possible. This is especially significant in the case of neutrons since they produce nuclear recoils in the crystal similar to what is expected of a WIMP; these signals would be indistinguishable from each other in the data. Thus, the project was to build a neutron shield made of high-density polyethylene which would surround the dilution refrigerator where the detector was operated. Data was taken with the detector exposed to a Cf- 252 (neutron) source with and without the shield in order to determine the shielding capability of the polyethylene. The data was analyzed and the results demonstrated a consistent shielding effect on events appearing as nuclear recoils, suggesting that the neutron shield is fulfilling its desired purpose.
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    Design, implementation, and calibration of physics Half-Wave Plate polarimetry for the E and B Experiment
    (2014-10) Klein, Jeffrey Michael
    The E and B Experiment (EBEX) is a balloon-borne telescope designed to measure the polarization of the Cosmic Microwave Background (CMB) and dust foregrounds at 10' scales and three frequency bands of 150 GHz, 250 GHz, and 410 GHz in order to detect or constrain B-mode polarization. Results may provide evidence to support the theory of cosmological inflation, or constrain specific models.EBEX's polarization measurement capability is implemented via continuously-rotating Half-Wave Plate (HWP) polarimetry. We discuss the design and implementation of the polarimetry hardware for the E and B Experiment (EBEX). In order to achieve low-temperature rotation of our 15 cm, 635 g achromatic HWP stack, we implement a unique application of a Superconducting Magnetic Bearing (SMB), building off an earlier prototype. We discuss design constraints, detail our implementation, and present results of tests of power dissipation, rotation speed stability, dynamic stability, and operational lifetime. We find power dissipation of 15 mW in our LDB configuration, and achieve successful operation of the system in both a 2009 test flight and a 2012 Long Duration (LDB) flight.We design and carry out calibration tests to verify our ability to measure polarized signals. We develop a data analysis pipeline to extract polarization measurements from the chopped polarized signals we use in calibration; we verify and optimize the performance of this pipeline with a simulation. We find that a thorough understanding of the time constants of EBEX's bolometric sensors is essential to measure polarization. We develop methods to measure and remove the effects of these time constants. Tests of polarization rotation across our bands verify predictions of rotation due to our achromatic HWP 5-stack. Polarized beam scans allow us to set an absolute calibration for EBEX with a standard deviation of 1.5 degrees.
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    Determining optimum imaging parameters for SWIFT: application to superparamagnetic iron oxides and magnetized objects.
    (2011-06) O’Connell, Robert Daniel
    A relatively new pulse sequence in MRI known as SWIFT, sweep imaging with Fourier transformation, has been shown to effectively image spins with both short and long transverse and longitudinal relaxation rates. It is desirable to have equations that accurately describe the signal of spins when excited by SWIFT; however the Bloch equations are not easily solvable for the SWIFT sequence for all relaxation rates and flip angles. The purpose of this work is to determine a set of optimization equations for the SWIFT sequence through comparison to the Ernst energy equations via a Bloch simulator. An innovative contrast technique is also developed. The optimization equations are then tested experimentally and applied to imaging of superparamagnetic iron oxides. Susceptibility artifacts distort images around metal objects. In SWIFT images the susceptibility artifacts manifest as signal voids surrounded by pileup artifacts. This work develops predictive equations for the pileup artifacts around metallic spheres. A technique called ROC, radial off-resonance correction, is developed to reconstruct distorted images by utilizing the pileup predictive equations in post-processing.