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

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Discovery of single top quark production

Author: 
Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

The top quark is by far the heaviest known fundamental particle with a mass nearing that of a gold atom. Because of this strikingly high mass, the top quark has several unique properties and might play an important role in electroweak symmetry breaking—the mechanism that gives all elementary particles mass. Creating top quarks requires access to very high energy collisions, and at present only the Tevatron collider at Fermilab is capable of reaching these energies. Until now, top quarks have only been observed produced in pairs via the strong interaction. At hadron colliders, it should also be possible to produce single top quarks via the electroweak interaction. Studies of single top quark production provide opportunities to measure the top quark spin, how top quarks mix with other quarks, and to look for new physics beyond the standard model. Because of these interesting properties, scientists have been looking for single top quarks for more than 15 years. This thesis presents the first discovery of single top quark production. An analysis is per- formed using 2.3 inverse fb of data recorded by the D0 detector at the Fermilab Tevatron Collider. Boosted decision trees are used to isolate single top signal from background, and the single top cross section is measured to be sigma( ppbar -> tb + X, tqb + X ) = 3.74 +0.95 −0.74 pb. Using the same analysis, a measurement of the amplitude of the CKM matrix element Vtb, governing how top and b quarks mix, is also performed. The measurement yields: | Vtb f_L1 | = 1.05+0.13 −0.12, where f_L1 is the left-handed Wtb coupling. The separation of signal from background is improved by combining the boosted decision trees with two other multivariate techniques. A new cross section measurement is performed, and the significance for the excess over the predicted background exceeds 5 standard deviations.

Document type: 
Thesis
File(s): 
Senior supervisor: 
D
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)

In-place bonding of III-V semiconductor heterostructures

Author: 
Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

Elastically strain-relaxed GaAs/In$_{0.08}$Ga$_{0.92}$As/GaAs heterostructures on GaAs(001) substrates were fabricated. Pseudomorphic heterostructures grown by MOCVD were patterned using conventional photolithography and a sacrificial AlAs layer was removed by selective etching. As etching proceeds, the structure is released from the substrate, elastic strain relaxation occurs and the strain-relaxed structures are weakly bonded in-place to the substrate. The bond between the structure and the substrate was then strengthened by annealing under conditions similar to those used for whole wafer bonding of GaAs. The strain, composition and thickness of the layers were determined using high resolution X-ray diffraction and the sample surface quality was examined using atomic force microscopy. The degree of strain relaxation of the InGaAs layer is determined by the relative thickness of the GaAs and InGaAs layers in agreement with a force balance model. The increase in the in-plane lattice parameter of the bonded structures as compared to GaAs is 0.25-0.44%.

Document type: 
Thesis
File(s): 
Senior supervisor: 
P
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (M.Sc.)

The electroluminescence and scanning tunneling microscopy of single molecules

Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

The scanning tunneling microscopy (STM) of single molecules has become a prominent experimental method in the field of molecular electronics. It has been found that in STM experiments, when an electric current flows through a single molecule, the molecule may luminesce. This electroluminescence, in conjunction with traditional STM data, provides a potentially important additional degree of freedom for understanding nanoscale systems. This thesis describes exploratory theoretical work on the newly emerging phenomenon of molecular electroluminescence, and its relationship to the scanning tunneling microscopy of single molecules. A local electrode framework is progressively developed, in order to explain single-molecule electroluminescence data as well as simulating STM current-voltage characteristics and topographic maps for molecules on complex substrates. The molecule Zn(II)-etioporphyrin I is chosen for detailed analysis and comparison with experiment. Electron transport is studied using Landauer theory that relates electric current to the transmission probability for an electron to scatter through the molecule. The theoretical approach utilizes tight binding and extended Huckel approaches for the electrodes and molecule, a charge-conserving scheme to self-consistently model the influence of electric fields and electric currents on the molecular energy level structure, and Fermi's golden rule in calculating electroluminescence. A single coherent framework is ultimately achieved that explains for the first time both the electric current data and molecular electroluminescence in a molecular system and elucidates the physics underlying a rich and previously puzzling array of interlinked optical and transport phenomena.

Document type: 
Thesis
File(s): 
Senior supervisor: 
G
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)

Using boosted decision trees for tau identification in the ATLAS experiment

Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

The ATLAS detector will begin taking data from $p$-$p$ collisions in 2009. This experiment will allow for many different physics measurements and searches. The production of tau leptons at the LHC is a key signature of the decay of both the standard model Higgs (via H $\rightarrow \tau \tau$) and SUSY particles. Taus have a short lifetime ($c \tau= 87$ $\mu$m) and decay hadronically ~65\% of the time. Many QCD interactions produce similar hadronic showers and have cross-sections about 1 billion times larger than tau production. Multivariate techniques are therefore often used to distinguish taus from this background. Boosted Decision Trees (BDTs) are a machine-learning technique for developing cut-based discriminants which can significantly aid in extracting small signal samples from overwhelming backgrounds. In this study, BDTs are used for tau identification for the ATLAS experiment. They are a fast, flexible alternative to existing discriminants with comparable or better performance.

Document type: 
Thesis
Senior supervisor: 
D
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (M.Sc.)

Anisotropies and spin dynamics in ultrathin magnetic multilayer structures

Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

High quality magnetic films were prepared by Molecular Beam Epitaxy (MBE) using Thermal Deposition (TD) and Pulse Laser Deposition (PLD) techniques. Ferromagnetic Resonance (FMR) and Mossbauer studies have shown that the Fe films prepared by PLD exhibited a more intermixed interface lattice structure than those prepared by TD. Dramatic decrease of the inplane interface uniaxial anisotropy for the PLD films compared to those prepared by TD has shown that the in-plane uniaxial anisotropy is caused by magnetoelasticity driven by the Fe/GaAs(001) interface lattice shear. Magnetization dynamics of the ultrathin Fe/Au,Ag/Fe films was studied using Time-Resolved Magneto-Optical Kerr Effect (TRMOKE) and FMR in the frequency range from 1 to 73 GHz. The Gilbert damping was studied in the Au/Fe/GaAs(001) structures as a function of the Fe and Au layer thickness, respectively. The observed increase in magnetic damping in the Fe film covered with thick Au capping layers was explained by spin pumping at the Fe/Au interface accompanied by spin relaxation and diffusion of the accumulated spin density in the Au layer. The spin diffusion length in Au was found to be 34 nm at room temperature. Significant increase of the Gilbert damping was observed in the Au/Fe/GaAs structures with decreasing Fe film thickness. Its origin lies in the additional damping at the Fe/GaAs interface. Direct detection of the spin current propagating across the Ag spacer in Fe/Ag,Au/Fe/GaAs(001) structures was carried out with stroboscopic TRMOKE measurements. The Fe layer grown on GaAs served as a spin pumping source and the Fe layer grown on the Au,Ag spacer was used as a probe for detection of the spin current propagating across the Au and Ag spacers. The experimental results were interpreted using self-consistent solution of the Landau Lifshitz Gilbert (LLG) equations of motion with the spin diffusion equation for the accumulated spin density in the Au and Ag spacers. The spin diffusion length in Ag was found to be 150 nm.

Document type: 
Thesis
Senior supervisor: 
B
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)

Nonlinear-optical and terahertz investigations of complex oxides

Author: 
Date created: 
2008
Abstract: 

Ultrafast pulsed-laser sources offer many benefits for experimental condensed-matter physics. This thesis describes a variety of ways in which their high field strength and extreme time resolution aid in the study of complex oxide materials. The design, construction and characterization of a visible-pump, terahertz-probe spectrometer is described. This instrument is applied to the study of photoexcited carriers in Sr2CuO2Cl2, an undoped, insulating cuprate. Ultrafast laser pulses are used to excite photocarriers in the antiferromagnetic insulating lattice. The low-frequency dynamical conductivity of the resulting nonequilibrium state is then measured with time-domain terahertz spectroscopy. Photoconductivity appears promptly, followed by a non-exponential decay on picosecond timescales. In this first direct measurement of the low-energy conductivity after photoexcitation, the estimated peak mobility is just 0.3 cm2/V·s, much lower than the Hall mobility found by other workers in chemically doped systems with similar carrier concentrations. Possible physical mechanisms behind this discrepancy are discussed. The spectrometer is then used to determine the photoexcited carrier relaxation time in a proton-damaged GaAs photoconductive antenna. A frequency-domain model for antenna behaviour is verified, demonstrating that the response of an antenna fabricated from an arbitrary substrate can be used directly for material characterization. Calculations and modelling are presented which show how a resonantly-enhanced optical frequency comb can efficiently generate terahertz pulses while preserving the stabilized comb structure. This could be a useful tool for high-precision comb spectroscopy or coherent excitation of structural modes. Several issues concerning the realization of such a source are described. Finally, the nonlinear-optical response of the pyrochlore oxide Cd2Re2O7 is used to characterize the structural phase transition it undergoes at 200 K, which has an unusual tensor character. Optical second harmonic generation with polarization sensitivity is able to resolve an ambiguity in the low-temperature crystal structure, assigning the I¯4m2 space group while verifying an auxiliary condition on the structure that is implied by the order parameter symmetry. The temperature-dependence of the order parameter is consistent with thermal occupation of a Goldstone mode that results from the Eu order parameter symmetry. This methodology may be applied more widely in characterizing ordered states in matter.

Document type: 
Thesis
File(s): 
Senior supervisor: 
J
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)

QCD topology and lattice perturbation theory from Monte Carlo simulations with improved staggered fermions

Author: 
Date created: 
2005
Abstract: 

The staggered quark formulation is one of many ways to include fermions on the lattice. Dynamical simulations are now routinely done with improved staggered quark actions which are more efficient than other popular formalisms. In this thesis two research works on improved staggered fermions are presented. A systematic study of the staggered Dirac operator's spectral properties is first presented. It is a long standing belief that staggered fermions do not feel gauge field topology because of the lack of zero eigenvalues of the operator at finite lattice spacing. The existence of fermionic zero modes in topological nontrivial background gauge fields is required by the Atiyah-Singer index theorem. In this study we observe that eigenmodes with very small eigenvalue and large chirality appear if improved staggered operators are used. These small eigenmodes can be identified as the "zero modes" associated with the topology of the gauge fields. We have also compared the distribution of the remaining nonchiral modes with the predictions of Random Matrix Theory. Satisfactory agreement is obtained. In the second project perturbative expansions of Wilson loops are computed in full QCD from Monte Carlo simulations with improved staggered fermions. This approach provides a much simpler alternative to diagrammatic perturbation theory, and has previously been shown to be successful in reproducing the perturbation series in pure gauge theory. This method is applied here for the first time to unquenched QCD. Twisted boundary conditions are used to eliminate effects of zero momentum modes and to suppress tunneling between the degenerate Z3 vaccua. A new simulation algorithm, the rational hybrid Monte Carlo algorithm, with no finite step size error is also employed. This is the first time this algorithm has been used in a numerical application. Results are in excellent agreement with analytic perturbation theory; this provides an important cross-check of the perturbation theory input to a recent determination of the strong coupling am(MZ) by the HPQCD collaboration.

Document type: 
Thesis
File(s): 
Department: 
Department of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)

Magnetization dynamics in ultrathin magnetic films

Author: 
Date created: 
2008
Abstract: 

Ultrathin magnetic multilayer structures are prepared by Molecular Beam Epitaxy (MBE) on GaAs(001) substrate. Growth is monitored and characterized by Reflection High Energy Electron Diffraction (RHEED), Auger Electron Spectroscopy (AES), X-Ray Photoelectron Spectroscopy (XPS) and Scanning Tunneling Microscopy (STM). The interface of Fe/GaAs system and its influence on the static magnetic properties in magnetic ultrathin films were studied by means of FMR and Mossbauer spectroscopy. Studies were performed on high quality single crystalline magnetic films. It will be shown that in ultrathin magnetic films magnetic properties are governed by a combination of interface and bulk effects. Static and dynamic magnetic properties of magnetic single and double layers were studied using Ferromagnetic Resonance (FMR) and Time Resolved Magneto-optic Kerr Effect (TRMOKE) in a wide range of microwave frequencies. Magnetic damping was studied in Au/Fe/GaAs structures as a function of the thickness of magnetic Fe layer and as a function of the capping Au layer. Spin currents generated by spin pump/spin sink effect structures were extensively investigated in magnetic doublelayer. Pure spin current driven dynamics in magnetic doublelayers was observed using TRMOKE technique. The spin diffusion and spin accumulation analysis was applied to explain spin momentum propagation in the normal metal. Spin diffusion length in Au was measured via direct observation of magnetic moment transfer. Spin pump/spin sink theory was tested for antiparallel driving of two magnetic layers of the magnetic doublelayer structure. This extreme condition was achieved via patterning sample into a coplanar waveguide. Results are in good agreement with simulations based on theory, which includes spin pump/spin sink model together with diffusive transport of accumulated spin momentum through the non-magnetic layer. Au/Fe/Au/Fe/GaAs magnetic double layer structure was used to study spin pump/spin sink effect in case of non-collinear orientation of magnetic moments. It will be shown that only component of spin momentum which is transverse to static magnetization contributes to non-local dynamics.

Document type: 
Thesis
File(s): 
Senior supervisor: 
B
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)

Positron-neutrino correlation measurements in the beta decay of magneto-optically trapped 38mK atoms

Author: 
Date created: 
2008
Abstract: 

This thesis describes the measurement of the angular correlation between the positron and the neutrino emitted in the beta decay of the isomer 38mK. This is a superallowed transition between nuclear states of the same spin and parity (0+), which is known to result primarily from the vector component of the weak interaction. The angular correlation involves two parameters. In the Standard Model of the weak interaction these have the values a=1 and b=0. Any meaningful deviation from this result can be interpreted as evidence for the existence of a scalar component in the weak interaction. The fundamentally new method that was used involved selectively confining neutral atoms of the isomer in a magneto-optical trap located between two detectors, one to measure the energy and direction of the positron and the other to detect the 38Ar nuclei that recoil with a momentum pR=-(pe+pv). The 38mK atoms were produced using the TRIUMF/ISAC facility. The trap provided a pure, cold, compact source essential to avoid distortion of the recoil momenta. For those events in which the positron was detected, the recoil momenta were deduced by measuring the time of flight from the trap to the recoil detector. About 500,000 positron-recoil coincident events were recorded. When the analysis, based on detailed Monte Carlo simulations, was restricted to positrons with kinetic energy >2.5 MeV, it showed that the angular correlation could be characterized by a "reduced" correlation parameter ã=0.9988±0.0028(stat)±0.0034(syst) (68% CL) where ã=a/(1+0.1503b). This measurement is consistent with the Standard Model and is 33% more restrictive than the only comparable previous measurement for such a transition. In the most general form, the strength of a possible scalar interaction can be specified in terms of two complex numbers, L and R, which define, respectively, the coupling to left- and right-handed neutrinos. This experiment did not usefully restrict the value of Re(L) (or b). Other experiments do provide rather strict limits on Re(L). If these are combined with the result of the present experiment one obtains the most restrictive direct limits available on Re(R), Im(R) and Im(L).

Document type: 
Thesis
File(s): 
Senior supervisor: 
P
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)

OMVPE growth and characterization of carbon doped InAs

Date created: 
2008
Abstract: 

Carbon-doped InAs samples grown by organometallic vapor phase epitaxy were studied by Raman and IR spectroscopy. Local vibrational modes (LVMs) related to isolated substitutional carbon acceptors, carbon acceptor-hydrogen complexes, and dicarbon centers were detected in samples doped with two isotopes of carbon. Energies of the observed carbon-hydrogen modes are in close agreement with carbon acceptor-hydrogen modes in GaAs and InP, and are consistent with hydrogen occupying a bond-center position in the complex. No sign of substitutional carbon donors was observed. N-type conductivity of carbon-doped InAs was explained by the presence of dicarbon centers that are believed to be deep donors. The stretch mode of this complex was detected at 1832 cm-1 by Raman spectroscopy in as-grown and annealed samples. Annealing measurements performed on heavily carbon-doped InAs samples confirmed that the n-type conductivity observed for this material is due to the formation of dicarbon defects. The structural, electrical, and optical properties of n-type carbon-doped InAs samples annealed at temperatures of 400°C and higher were studied using Raman and infrared spectroscopy, and X-ray diffraction. Based on the expected energy level of dicarbon donors in GaAs, we predicted that the di- carbon defects in InAs should introduce a resonant level close to or slightly above the conduction band minimum.

Document type: 
Thesis
File(s): 
Senior supervisor: 
S
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)