Physics and Astronomy Honors ProjectsCopyright (c) 2014 Macalester College All rights reserved.
http://digitalcommons.macalester.edu/physics_honors
Recent documents in Physics and Astronomy Honors Projectsen-usWed, 29 Jan 2014 08:35:29 PST3600Neutral Gas Dynamics of the Extreme Star-Forming Dwarf Galaxy Leo P
http://digitalcommons.macalester.edu/physics_honors/16
http://digitalcommons.macalester.edu/physics_honors/16Thu, 23 Jan 2014 11:17:19 PST
We present Very Large Array and Giant Metrewave Radio Telescope HI observations of Leo P, a low-mass star-forming dwarf galaxy discovered by the ALFALFA survey. This low-mass (M_{HI} = 9.5 × 10^{5} Solar masses) system has a nebular oxygen abundance of 3% Solar, making it among the most metal-deficient galaxies and a benchmark for models of galaxy evolution. Leo P’s proximity (d = 1.75 Mpc) allows our HI imaging to probe spatial scales of 160 pc at 1.8 kms^{−1} velocity resolution. We present morphological and dynamical analysis that offers one of the most complete views of the interstellar medium in an extremely lowmass dwarf galaxy to date.
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Elijah Bernstein-CooperUltrafast Time-Resolved Spectroscopy of Graphene after Mid-Infrared Excitation
http://digitalcommons.macalester.edu/physics_honors/15
http://digitalcommons.macalester.edu/physics_honors/15Thu, 23 Jan 2014 11:17:18 PST
In this time-resolved photoconductivity experiment we photo-excite graphene with femtosecond pulses of mid-infrared light and measure the resulting change in transmission of terahertz frequency radiation. In single-layer CVD graphene films we observe an increase in terahertz transmission following photoexcitation, reflecting either a light-induced decrease in conductivity or gain at terahertz frequencies. Previous timeresolved terahertz transmission experiments in CVD graphene have reported either gain or a decrease in loss. Gain has been attributed to photo-excitation of electrons from the valence band of graphene to the conduction band leading to a population inversion, while decrease in loss has been attributed to carrier heating due to photo-excitation. Our work reproduces earlier reports of increased terahertz transmission following photoexcitation by near-infrared and mid-infrared light, but we also observe this phenomenon after photo-excitation with long-wavelength infrared light where interband excitation is not possible, nullifying the hypothesis of interband gain, and instead supporting the model of decreased conductivity due to heating of the electron gas.
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Jacob SteinThe effects of ice arch formation on currents in Nares Strait during the springs of 2008 and 2009
http://digitalcommons.macalester.edu/physics_honors/14
http://digitalcommons.macalester.edu/physics_honors/14Tue, 21 May 2013 14:12:44 PDT
In Nares Strait between Greenland and Ellesmere Island, ice arches hold back ice during winter and spring, preventing sea ice flux into Baffin Bay and freshwater flux (in the form of ice) into the North Atlantic. Freshwater flux influences the formation of deep water and affects the Meridional Overturning Circulation, with ramifications for global climate. We analyzed current velocity data from a moored array of ADCPs and model wind data at approximately 80.5º N to understand the effects that an ice arch at the southern end of Nares Strait had on currents in 2008, and the difference in 2009 when the southern ice arch did not form. We found much stronger currents near the surface in 2009 during the mobile ice regime. In 2008, we found small correlation with winds in spite of the landfast ice cover, and in 2009 we found correlation with winds on daily, weekly, and biweekly timescales at different depths.
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Allison E. EinolfProperties of Hydrogenated Amorphous Silicon-Germanium Alloys Deposited by Dual Target Reactive Magnetron Sputtering
http://digitalcommons.macalester.edu/physics_honors/13
http://digitalcommons.macalester.edu/physics_honors/13Thu, 10 May 2012 11:34:15 PDT
Hydrogenated amorphous silicon-germanium alloy thin films (a-Si_{1-x}Ge_{x}:H) were deposited using reactive magnetron sputtering. Dual targets of silicon and germanium were sputtered in an argon + hydrogen atmosphere using RF excitation. Films with x = 0.4 were deposited as a function of substrate temperature and hydrogen partial pressure, and were evaluated by dark and photoconductivity, infrared absorption, and optical transmission. Photosensitivity reached a maximum value of about 4500 between 150 and 200°C. Using the stretching modes in the region of 2000 cm^{-1}, the hydrogen bonding was characterized in terms of the preferential attachment ratio (PA), which represents the ratio between H bonded to Si and H bonded to Ge. The PA shows a systematic increase with increasing temperature, generally independent of hydrogen partial pressure. The interplay between thermodynamic and kinetics effects in determining PA and film quality is discussed.
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Samuel J. LevangRate of Gravitational Capture of Dark Matter Particles
http://digitalcommons.macalester.edu/physics_honors/12
http://digitalcommons.macalester.edu/physics_honors/12Mon, 17 Oct 2011 13:48:37 PDT
Nonbaryonic dark matter, specifically weakly interacting massive particles (WIMPs) can become bound within the Solar System due to three-body interactions with Jupiter and the Sun. Numerical simulation and analytical methods were used to determine a time-averaged cross section for the capture process. This cross section of bound particles as found by simulated two-dimensional WIMP trajectories, along with the partial flux of WIMPs based on a Maxwell-Boltzmann velocity distribution, can be used as a step towards understanding the current dearth of WIMP detection by finding the WIMPs' rate of capture. As an intermediary step,a two-dimensional toy Solar System was considered. For the two-dimensional simulation, this rate of capture is found to be R = ERjvjC, with Rj the radius of Jupiter's orbit, vj the orbital velocity of Jupiter, p, the dark matter density at the location of the Solar System, m, WIMP mass, and thc dimensionless constant C determined to be, C = 2.2942 x lop5. Similar methods were used in a three-dimensional simulation, which resulted in a rate of capture of R = ER?u,C, with the dimensionless constant C, determined to be, C = 2.3806 x lou6. The phase space distribution of captured WIMPs was also determined through numerical simulation. The rate of capture is one part of the calculation to determine the steady state distribution of bound dark matter particles in the Solar System.
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Allison KennedyHiggs Boson Decay into Branons
http://digitalcommons.macalester.edu/physics_honors/11
http://digitalcommons.macalester.edu/physics_honors/11Tue, 08 Dec 2009 11:59:23 PST
The elusive Higgs boson is currently best understood in light of its decay products. A number of its decay modes into Standard Model particles have already been studied rigorously in anticipation of the upcoming LHC data. We look at an extension of the Standard Model involving scalar branon particles that effectively reflect the motion of a four-dimensional brane into flat five-dimensional space-time. Using tools of quantum field theory and differential geometry, we look at the specific cases of Higgs decay into two massive and into four massless branons. We compare the branching ratio of the Higgs decaying into branons with the established major decay modes in order to determine to what extent the decay into branons, if they exist, significantly complicates the search for the Higgs boson.
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Jeffrey GustafsonSupersymmetry and Shape Invariance: Soliton Excitation Spectra in Anti-de-Sitter Space
http://digitalcommons.macalester.edu/physics_honors/10
http://digitalcommons.macalester.edu/physics_honors/10Thu, 07 May 2009 06:42:13 PDT
Solitons are an important part of field theory, that feature discrete vacua. In this paper, we start with a specific soliton potential to study in depth. Utilizing perturbation theory, in both real and imaginary space, we study the scalar potentials in Minkowski and AdS space. Using supersymmetry and shape invariance, we analytically solve the soliton normal mode equations and determine their frequency spectra. Furthermore, reversing the methods used, we are able to work backwards and find more soliton potentials that are in the same family as our original one.
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Matthew WebsterSupersymmetric Solitons in Anti-de-Sitter Space
http://digitalcommons.macalester.edu/physics_honors/9
http://digitalcommons.macalester.edu/physics_honors/9Tue, 05 May 2009 07:24:55 PDT
This research explores the behavior of solitons in supersymmetric scalar ﬁeld theories. Both the ﬂat space and Anti-de-Sitter (AdS ) space cases are considered. Soliton solutions are determined analytically for the ﬂat-space case, and the excitation spectrum is calculated using supersymmetry and shape invariance. General forms of these ﬂat space soliton solutions are used to generate potentials in AdS (1+1) space, though the models constructed in this manner were not found to be supersymmetric. Finally, we solve the equation of motion for solitons in supersymmetric ﬁeld theories in AdS (1+1) space, and ﬁnd representative numeric solutions for various regions in parameter space.
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Jennifer L. BarnesPlanck and the Integrated Sachs-Wolfe Effect: A New Constraint on Dark Energy?
http://digitalcommons.macalester.edu/physics_honors/8
http://digitalcommons.macalester.edu/physics_honors/8Tue, 21 Apr 2009 06:07:20 PDT
This capstone develops tools usable for the detection of the Integrated Sachs-Wolfe Effect within the data products of the Planck satellite, scheduled to be launched in late April 2009. This project prepared for ﬂight data using simulations of the cosmic microwave background as well as of galaxy surveys. Using these simulated maps as a base, I developed tools to ﬁnd the power spectrum of the cross correlation in order to detect the ISW effect. With this detection and the corresponding error bars, I effected a comparison with theoretical models. This comparison constrained the value of ΩDE (dark energy density) put into simulations, along with the conﬁdence interval. The products of this investigation will be directly portable to the forthcoming ﬂight data from the Planck satellite.
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Jillian ScudderEffect of Nitrogen and Oxygen on the Growth of RF-Sputtered ZnO Films
http://digitalcommons.macalester.edu/physics_honors/7
http://digitalcommons.macalester.edu/physics_honors/7Fri, 03 Oct 2008 05:51:11 PDT
This honors project examines the effect of the addition of nitrogen and oxygen on the electrical and optical properties of RF-sputtered ZnO films. Nitrogen has been reported to act as an acceptor in ZnO. However, producing low resistivity, stable p-type ZnO films is in general a difficult task. In this work, films having dark resistivities between 10(-3) and 10 (8) ohm cm have been deposited, depending on gas and target composition. Seebeck and Hall Effect measurements give inconclusive results about the majority carrier type of the films. However, the more resistive films exhibit a surprising photoconductive effect.
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Kyle BraamPhotoconductivity in Amorphous Silicon Germanium Alloys at Low Temperature
http://digitalcommons.macalester.edu/physics_honors/5
http://digitalcommons.macalester.edu/physics_honors/5Wed, 17 Sep 2008 09:31:37 PDT
Hydrogenated amorphous silicon germanium (a-SiGe:H) has important applications in solar cell technology. In order to maximize efficiency of solar cells, the semiconductor materials used in them must have a low defect density. Photoconductivity is sensitive to defect density, so measurements and modeling of photoconductivity are important diagnostics for a-SiGe:H. In this work, the photoconductivity of thin film samples of a-SiGe:H were measured at carrier generation rates for 10(14) to 10 (17) cm(-3)s(-1) and temperatures from 80K to 310K. The experimental data was compared to two numerical models published in the literature. One model, by Shen and Wagner(1), used a simple model for recombination and transport and was reproduced here in (see Appendix) in order to fit the acquired data. The total density of states and distribution in the band gap was varied. However, the model predicted a greater decrease in photoconductivity at low temperatures than was observed in the experimental data for all cases. The second model by Merazga et al (2), which included recombination of electrons and holes and transport in the band tail states, appears to be in better agreement with the data.
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Hallie C. BoyerParticle-in-Cell and Monte Carlo Modeling of Plasma Probe Characteristics
http://digitalcommons.macalester.edu/physics_honors/4
http://digitalcommons.macalester.edu/physics_honors/4Tue, 10 Jun 2008 11:42:15 PDT
A one-dimensional particle-in-cell (PIC) algorithm is applied to a planar plasma probe in contact with a weakly ionized plasma. The algorithm couples Poisson's equation and Newton's laws of motion to calculate the electric field and the forces on the charged particles. A new set of boundary conditions is developed to simulate the semi-infinite neutral plasma. Collisions between the charged particles and the background gas are incorporated using a Monte Carlo collision algorithm. Changes in the electrostatic sheath in response to changes in the voltage and the plasma density are consistent with previous experimental and theoretical results.
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Jonas HiltropSoliton Solutions in Anti-de-Sitter Space
http://digitalcommons.macalester.edu/physics_honors/3
http://digitalcommons.macalester.edu/physics_honors/3Fri, 08 Jun 2007 08:52:13 PDT
Solitons exist in field theories with discrete vacua in flat two-dimensional Minkowski space. In our work, we are able for the first time to find analytic soliton solutions for specific values of the parameters for a generalization of the Ginzburg-Landau model in Anti-de-Sitter space. Using perturbation theory we show solutions continue to exist for parameter values close to these specific values. Further, acceptable numeric solutions exist for general values of the parameters. All of this supports the conclusion that soliton solutions to the equation of motion in Anti-de-Sitter 1+1 space exist for all values of the given parameters.
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Daniel P. SchroederSolitons in AdS_2
http://digitalcommons.macalester.edu/physics_honors/2
http://digitalcommons.macalester.edu/physics_honors/2Thu, 24 May 2007 06:38:25 PDT
The dynamics of the center of mass motion of a soliton embedded in a curved (1+1)-dimensional Anti-de-Sitter (AdS) space as well as a finite number of its low frequency excitations is modeled using the method of non-linear realizations. The variations of the field representing the center of mass coordinate and of the time coordinate under isometry group transformations are determined by use of the coset method. The covariantly transforming Maurer-Cartan one forms are constructed as the building blocks from which an invariant Lagrangian is constructed. This Lagrangian can be expanded in terms of a scale which physically represents the inverse width of the soliton. Considering the simplest form of the invariant Lagrangian by keeping only the universal lowest order term in the expansion, the classical Euler-Lagrange equations of motion are obtained from the action by the least action principle. The solution of the equation of motion describes the motion of a point particle in AdS space, as expected. The Noether method is used to determine the conserved charges associated with the isometries of the model. These conserved charges are the equivalents of energy and momentum in flat Minkowski space. The dual, conformally invariant model is determined, and the relation between the two models is studied as a first attempt to elucidate the AdS-CFT correspondence for this system. A supersymmetric extension of the model is constructed by the introduction of additional Grassmann coordinates.
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Kassahun HaileyesusSpectral Classifications of Companion Stars in Herbig AeBe Binary Systems
http://digitalcommons.macalester.edu/physics_honors/1
http://digitalcommons.macalester.edu/physics_honors/1Tue, 01 May 2007 05:49:14 PDT
Herbig AeBe (HAeBe) binary systems are good environments for the study of pre-main sequence stellar evolution in companion stars whose mass may be significantly lower than that of the primary star. Measurements of the spectral type and surface gravity of the companion star in the system allow it to be placed on the H-R diagram, where theoretical evolutionary model tracks can then constrain its mass and age, and comparisons can be made between these low mass stars and those formed without the presence of a high mass star. We present two different ways to measure effective temperature (Teff) and estimate surface gravity (log g) from the spectra of late type stars, depending on the spectral resolution (R=18000, R=6000 or 1700). At high resolution, detailed model fits to the shapes of Na lines at 2.21 microns and the (2-0) 12CO bandhead at 2.29 microns provide an accurate way to measure effective temperature (sigma = 190 K) and surface gravity (sigma = 0.82 cm s-2), in addition to allowing for values of vsini, veiling, and radial velocity to be estimated. At medium resolution, the equivalent widths of 10 of the strongest absorption lines present in the NIR spectrum were measured to determine Teff at a lower accuracy. Preliminary results show that these techniques are useful for characterizing late-type companions. Analysis of two stars is discussed, more data is needed to address statistical questions about the nature of HAeBe companions and better calibrate classification techniques.
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Anne C. Sweet