Physics - Theses, Dissertations, and other Required Graduate Degree Essays

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Growth, characterization, and fabrication of GaAs core/shell and axial nanowire devices

Author: 
Date created: 
2016-09-21
Abstract: 

Semiconductor nanowires are promising candidates for the emerging nano-scale optoelectronics. They provide opportunities for novel axial and lateral designs with the possibility of improvement in the device performance and reduction in the size. An essential requirement for this research and development is the fundamental understanding of the electronic, electrical and optical properties of semiconductor nanowires. This thesis aims to address several critical factors that limit commercial integration of GaAs nanowire devices. The latter includes investigation of novel nanowire growth methods and understanding the charge transport properties in axial and radial structures. I grew gold-catalyzed GaAs nanowires via the vapour-liquid-solid mechanism using the metalorganic chemical vapor deposition technique. A thin GaP shell was used to passivate the sidewall surface states in GaAs nanowires. Electrical and optical measurements were carried out on the core/shell GaAs/GaP nanowires to demonstrate unpinning of the Fermi level by improvement in the nanowire resistivity and photoluminescence, respectively. Control of the surface recombination velocity in GaAs nanowires was also achieved using a thin lattice-matched InGaP passivating shell. This was determined through an enhancement of the minority carrier diffusion lengths in GaAs/InGaP nanowires measured using electron beam induced current technique. In addition, axial GaAs nanowire p-n junctions were fabricated to demonstrate a free-standing single nanowire photodetector. The degree of the p-n junction abruptness and the impact of the Au reservoir effect was studied by a numerical modeling of the corresponding electrostatic potential. This model was further verified using electron holography measurements. Radial GaAs nanowire p-n junctions combined with a novel growth technique lead to development of GaAs homostructure radial tunnel diodes. A lithography-free growth method took advantage of an array of Ga2O3 coated GaAs pedestals to electrically isolate nanowire devices from the substrate. Nano-probe measurements of radial GaAs nanowire p-n junctions indicated clear tunneling current-voltage properties.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Simon Watkins
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) Ph.D.

Reconstructing a Quark and Gluon Response at ATLAS

Author: 
Date created: 
2016-09-02
Abstract: 

Jets, collimated sprays of particles, are the most commonly produced objects in high energy subatomic collisions. Jets are the final state of colliding quarks and gluons. The fraction of a jet's energy that is measured by a calorimeter is called the response. Quark jets (jets initiated by quarks) and gluon jets (jets initiated by gluons) have a different response in the calorimeter. In this thesis the response of quark and gluon jets is reconstructed using jets in dijet and photon + jet events. To measure jet response in dijet events a method is developed to correct the energy of one jet in a dijet event so that it may be used as a reference object in the calibration procedure. The reconstructed dijet, quark and gluon responses are shown to agree with Monte Carlo simulation predictions within their uncertainties.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Michel Vetterli
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) M.Sc.

Experiments on the thermodynamics of information processing

Author: 
Date created: 
2016-09-22
Abstract: 

Information is often considered as an abstract entity, but it is always stored and processed by a physical medium. As such, it obeys all the restrictions and possibilities related to the laws of physics. In 1961, Rolf Landauer proposed the existence of a fundamental energetic cost associated with information processing: each time information is processed in a logically irreversible way, at least kTln2 of heat is released, on average, into the surrounding bath. This principle also resolves the long-standing threat to the second law of thermodynamics posed by Maxwell's demon. Although the Landauer principle has been widely accepted, it remained untested and controversial for more than half a century. The small amount of heat released as a result of logically irreversible operations was hard to detect in any conventional information-processing device. With recent technical and theoretical advances in micromanipulations, this became possible. In my graduate study, I used and calibrated a feedback trap to execute logical operations and measure the tiny energetic cost associated with them. I start this thesis with a brief review of stochastic thermodynamics and information theory, followed by my experimental approach. I present two feedback traps: one that I inherited and the other that I developed later in my studies. Both traps use the same real-time calibration method based on a recursive maximum likelihood algorithm. The calibrated trap was initially used to test the Landauer principle and show that erasing a symmetric one-bit memory requires kTln2 work on average, while no work is required for similar protocols with no net erasure. This experiment confirmed Landauer's hypothesis that information is physical. In my later work, I explored information in more complex environments. I experimentally studied erasure for a memory encoded in an asymmetric double-well potential. I found that the average work to erase can be below kTln2, as predicted by a recent theory. Surprisingly, erasure protocols that differ subtly give measurably different values for the asymptotic work, a result I explain by showing that one protocol is symmetric with respect to time reversal, while the other is not. The differences between the protocols help clarify the distinctions between thermodynamic and logical reversibility. I further explored the same phenomena divorced from Landauer's principle, where a system starts and ends in the same equilibrium state, and I show that arbitrarily slow transformations, produced by smooth deformations of a double-well potential, need not be reversible. Finally, I present my work towards a direct test of the form of the Shannon entropy function.

Document type: 
Thesis
File(s): 
Senior supervisor: 
John Bechhoefer
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) Ph.D.

Crystallization kinetics of a colloid-polymer system with depletion interaction

Date created: 
2016-04-22
Abstract: 

Compared to systems with only a steric interaction, a colloidal system with an attractive interaction, caused by addition of a polymer, exhibits a richer set of phases. Richer because more phases are possible; for example, one can actually see the coexistence of liquid, gas and crystal phases in equilibrium. In this thesis, we studied the crystallization of colloid particles in a colloid-polymer system. To understand how crystals nucleate within different phases, the samples investigated were in the three-phase region of the colloid-polymer phase diagram. Experiments were carried out in a microgravity environment on board the International Space Station (ISS). The samples were photographed as they crystallized and captured images were sent to Earth. Python scripts were developed to analyze data from the images. One key observation similar to that observed in a hard-sphere system was the formation of dendrites in one of the samples suggesting diffusion-limited growth.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Barbara Frisken
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) M.Sc.

Zero field optical study of phosphorus magnetic resonance in enriched silicon

Date created: 
2016-08-11
Abstract: 

Silicon-based qubits are one of the most promising technologies for the construction of a quantum computer. The nuclear spins of phosphorus donors in enriched silicon have among the longest coherence times of any solid-state system. In this thesis, I examine the phosphorus in silicon system in the regime of "zero" magnetic field. Laser spectroscopy and magnetic resonance are used to characterize the phosphorus in silicon system in this environment. I show the system can be hyperpolarized and has ~10 s coherence times. Additionally, the methods and apparatus developed for this study prove useful for the study of other similar but more exotic systems.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Michael Thewalt
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) M.Sc.

Analysis of novel ionenes via X-ray and neutron scattering

Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2016-08-23
Abstract: 

This thesis narrates the analysis of a novel series of polybenzimidazole-based ionenes via X-ray scattering, neutron scattering, and molecular dynamics simulation. The ionenes have been reported as stable hydroxide-conducting solid polymer electrolytes. Robust anionexchange membranes with strong conductivity are necessary for the wide application of anion-exchange membrane fuel cells, which are a compelling alternative to the internal combustion engine. Three length scales were observed: ion-polymer, polymer-polymer (and ion-ion), and the monomer length. No structure was visible above the monomer length, which is rare in high-performance ion-conducting membranes. In a preliminary molecular dynamics simulation, water molcules were observed forming bridges between anions, even at a low level of hydration.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Barbara Frisken
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) M.Sc.

TF-μSR Sensitivity to Charge Density Wave Order in 2H-NbSe2

Date created: 
2016-04-22
Abstract: 

The sensitivity of transverse-field muon spin rotation (TF-μSR) to static charge-density wave (CDW) order in the bulk of 2H-NbSe2 is demonstrated. In the presence of CDW order, the quadrupolar interaction of the 93Nb nuclei with the local electric-field gradient is modified, and this in turn affects the magnetic dipolar coupling of the positive muon to these nuclei. For a weak magnetic field applied parallel to the c-axis, we observe a small enhancement of the muon depolarization rate at temperatures below the established CDW phase transition. Aligning the applied field perpendicular to the c-axis, we observe a sensitivity to static CDW order in regions of the sample extending up to a temperature nearly 3 times the CDW transition temperature T CDW. The results suggest that the muon is mobile over the temperature range explored above the superconducting transition temperature Tc, but becomes trapped in the vicinity of defects.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Jeff Sonier
David Broun
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) M.Sc.

Interactions and symmetries of electronic systems with quadratic band touching

Date created: 
2016-02-04
Abstract: 

In this essay we study two-dimensional electronic systems that exhibit quadratic band touching (QBT) in their low energy dispersion. A lattice realization is shown to introduce the emergence of a QBT in the framework of the tight binding approximation. We then make a continuum limit, which allows for a more general approach on the level of low energy Hamiltonians. The basic concepts and the applied methods are introduced via a simplified problem, a system of spinless fermions. We use a renormalization group (RG) approach to examine the effects of short-range electron-electron interactions. Once it is found that the system is unstable toward ordered phases, we use the mean field approximation to investigate how the phases compete energetically. We then introduce the spin degree of freedom of the electrons and follow analogous calculations using the RG approach. A number of ordered phases are identified, and the flow equations of the susceptibilities are calculated. We find a symmetry of the action that is larger than what we start with and emerges under some plausible conditions.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Igor Herbut
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) M.Sc.

Dynamics of Trapped Ions Near the Linear-Zigzag Structural Phase Transition

Author: 
Date created: 
2015-12-14
Abstract: 

Laser-cooled ions held in a linear Paul trap with strong transverse confinement organize into a one-dimensional (1-D) linear crystal. If the transverse confinement is relaxed, the linear ion crystal undergoes a continuous, structural phase transition to a 2-D zigzag configuration. We study the dynamics near the critical point of the linear-zigzag transition. In the first part of this thesis, we study the spontaneous nucleation and dynamics of topological kink defects, formed as a result of a rapid quench across the linear-zigzag transition. The experimental results are compared to the Kibble-Zurek mechanism, which provides an intuitive model of defect formation and predicts a power-law scaling for the number of defectsformed as a function of transition quench rate. The second part of this thesis is focused on one of the key requirements for investigations of the near-transition dynamics in the quantum regime. To achieve an efficient ground state cooling of the zigzag vibrational mode, we demonstrate 3-D polarization-gradient cooling of strings of 1-4 trapped ions as an intermediate step between Doppler and sideband cooling, and study the polarization-gradient cooling rate and cooling limit as a function of the cooling beam intensity in and near the Lamb-Dicke regime. The results of this thesis pave the way towards our future experiments aimed at assessing the coherence time of a zigzag superposition state through measurements of tunneling oscillations.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Paul C. Haljan
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) Ph.D.

Spin pumping and spin transport in magnetic heterostructures

Author: 
Date created: 
2016-01-19
Abstract: 

High quality, ultrathin magnetic films were prepared by means of molecular beam epitaxy (MBE). Magnetization dynamics and anisotropies were studied by means of ferromagnetic resonance (FMR) in GaAs|Fe|Au(001) structures as a function of the Fe layer thickness, allowing the determination of bulk and interface properties. Spin transport was studied in GaAs|Fe|Au|Pd structures, where two interesting results were found: (1) The spin pumping induced damping showed an oscillatory dependence on the Au spacer layer thickness when this layer's thickness was less than the electron mean free path. This effect is attributed to the formation of quantum well states in the Au layer. (2) The spin pumping induced damping was quickly suppressed with the addition of the Au spacer layer as compared to GaAs|Fe|Pd samples. It is experimentally shown that this reduction is not related to the removal of magnetic proximity effect induced damping at the Fe|Pd interface. It is shown that the Pd layer can neither be treated as an ideal spin sink nor as a simple normal metal (diffusive spin scatterer) with respect to spin currents and that the reduction in damping is due to a reflection of spin currents at the Au|Pd interface. Magnetization dynamics were investigated in ferrimagnetic insulator Yttrium Iron Garnet (YIG, Y3Fe5O12). Ferromagnetic resonance was used to determine the spin pumping induced damping in YIG and YIG|Au|Fe|Au structures. In the YIG|Au|Fe|Au structures, the YIG acts as a spin pump and the Fe as a spin sink when the YIG layer undergoes ferromagnetic resonance. Comparing the damping in the YIG and YIG|Au|Fe|Au structures allows one to determine the efficiency of spin pumping at the YIG|Au interface given by the spin mixing conductance. It is found that the spin mixing conductance of as grown YIG films is ~10% of that typically found at metallic FM|NM interfaces. Surface treatment of the YIG films by Ar+ etching is able to improve the spin pumping efficiency, approaching closely to that obtained by first principle electron band calculations.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Bret Heinrich
Erol Girt
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) Ph.D.