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

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Optical characterization of the Si:Se+ spin-photon interface

Author: 
Date created: 
2018-08-27
Abstract: 

The combination of both matter qubits and photonic qubits presents a very promising method for generating entanglement between qubits in order to scale up both quantum computing and quantum communication platforms. Hosting both qubits in silicon would be a favourable approach as silicon has not only the most mature microelectronics industry but also the most mature photonics industry. Singly ionized selenium donors (Si:Se+) have recently been identified as a possible candidate. Si:Se+ possess the excellent coherence lifetimes of conventional donor spin qubits in silicon but additionally has photonic access to the spin states at a convenient wavelength, 2.9 um. The spin-photon interface of Si:Se+ has the potential to be the basis for an integrated, all silicon, quantum computer. For this work we made custom samples with the specific purpose of measuring the crucial optical properties that determine the viability of the Si:Se+ spin-photon interface as a basis for a quantum architecture. We present photoluminescence, absorption, hole burning, and magnetic resonance experiments towards the characterization of the Si:Se+ spin-photon interface. We determined a transition dipole moment of 1.96 +- 0.08 Debye, a lower bound for the zero-phonon line fraction of > 15.1 +- 0.3 %, and a lower bound for the radiative efficiency of > 0.75 +- 0.08 %. Further results of the peak and area conversion factors and the dependence of peak energy and linewidth on electrically neutral impurities are also presented.

Document type: 
Thesis
File(s): 
Supervisor(s): 
Stephanie Simmons
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) M.Sc.

Cascade of magnetic transitions in the frustrated antiferromagnetic CePtPb

Author: 
Date created: 
2018-08-08
Abstract: 

CePtPb is an antiferromagnetic, heavy-fermion, metallic compound that crystallizes in the ZrNiAl-type structure with space group P-62m, where the Ce-ions form a quasi-Kagome lattice in the ab-plane. Other compounds in this family with a quasi-Kagome magnetic lattice such as CePdAl and YbAgGe have shown a complex temperature-versus-magnetic field (T-H) phase diagram with multiple magnetically-ordered phases. In this thesis, a T-H phase diagram for single crystal CePtPb is constructed from electric resistivity and specific heat measurements. The constructed phase diagram also shows multiple magnetically-ordered phases as the Neel temperature T_N=0.9 K is suppressed continuously to T=0.4 K by applied field with an extrapolated zero-temperature critical field H_c=7 kOe. In zero-field, muon spin relaxation measurements show residual spin dynamics at 25 mK, consistent with the magnetic structure proposed for CePdAl, where 2/3 of the Ce-4f spins order antiferromagnetically and the other 1/3 of the Ce-4f spins remain fluctuating. From a power-law analysis of the electrical resistivity p=p_0+AT^n), neither Fermi-liquid (n=2) nor non-Fermi-liquid (n<2) behaviour have been observed down to T=0.4 K for H>=H_c. Instead, there is an anomalous evolution of n that increases from n=2.5 at H=H_c to n=4.1 at H=90 kOe, with a tendency towards saturation near the value n~4 for H>30 kOe. The phase diagram and the measurements are compared to CePdAl and YbAgGe.

Document type: 
Thesis
File(s): 
Supervisor(s): 
Jeff Sonier
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) M.Sc.

Slow dipole dynamics in organic charge transfer salts

Author: 
Date created: 
2018-08-20
Abstract: 

At temperatures around 60 K, measurements of the dielectric constant in the κ-(BEDT-TTF)_2 X family of organic charge transfer salts have indicated the emergence of glassy dynamics in a relaxor ferroelectric phase. We propose that an extended Hubbard model on a triangular lattice of dimers is a minimal model for these systems to capture glassy dynamics. Allowing for disorder, we use a strong coupling expansion to second order in the Hubbard interaction to obtain a low-energy effective model in terms of spin and dipole degrees of freedom. By focusing on classical terms in the effective model we obtain a model amenable to classical Monte Carlo simulations. We perform equilibrium and out-of-equilibrium Monte Carlo simulations and calculate an analog of the Edwards-Anderson order parameter for dipoles and the two-time auto-correlation function for charge degrees of freedom. For appropriate parameters we find evidence for aging dynamics and a non-zero Edwards-Anderson order parameter, implying the emergence of glassiness in the charge degrees of freedom at low temperatures, as would be expected for a relaxor ferroelectric.

Document type: 
Thesis
File(s): 
Supervisor(s): 
Malcolm Kennett
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) M.Sc.

Investigating a model lipid nanoparticle release system with 2H NMR and SAXS

Author: 
Date created: 
2018-04-04
Abstract: 

Lipid Nanoparticles (LNPs) are an attractive way of delivering of short interfering RNA (siRNA) for cancer therapeutics. Their release method relies on protonation of an ionisable amino-lipid (XTC2) in acidic endosomes. Hypothetically, the protonated XTC2 and anionic lipids in endosomal membranes interact to form non-lamellar phases, releasing the siRNA. In this project, a model release system consisting of XTC2 and anionic distearoylphosphatidylserine (DSPS-d70) at pH 4.7 was investigated with deuterium nuclear magnetic resonance (2H NMR) and small angle x-ray scattering (SAXS) to determine the lipid phases which form as a function of temperature and their structural parameters. Since cholesterol is an important structural component in LNPs, increasing amounts of cholesterol were added to the system to determine its effect. Non-lamellar phases were observed for each sample particularly at high-temperatures, though interestingly the specific phase observed by each technique was not always in complete agreement.

Document type: 
Thesis
File(s): 
Supervisor(s): 
Jenifer Thewalt
Nancy Forde
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) M.Sc.

Thermodynamic and transport properties of single crystal YbNi4Cd

Author: 
Date created: 
2018-07-20
Abstract: 

The single crystal growth and the physical properties of the rare-earth based ternary intermetallic compounds RNi4Cd (R = Y and Yb) will be presented. The powder X-ray diffraction measurement reveals that these compounds crystallize in the face-centered cubic (fcc), MgCu4Sn-type structure (space group F-43m). Magnetization, electrical resistivity, and specific heat measurements are used to study thermodynamic and transport properties of YbNi4Cd. The magnetic susceptibility shows that 4f electrons of Yb3+ ions are well localized. The electrical resistivity and specific heat measurements show antiferromagnetic ordering below T_N = 0.97 K. Applying a field along the [111] direction results in the suppression of T_N below 0.4 K at the critical field Hc ~ 4.5 kOe. No non-Fermi liquid behavior is observed in the vicinity of Hc. Above Hc, the magnetoresistivity shows an unconventional temperature dependence rho(T) = rho_0 + AT^n with n > 2, suggesting that an additional scattering mechanism in the resistivity needs to be considered. Based on the analysis of experimental results, we conclude that the Yb3+ moments and conduction electrons are weakly coupled. Despite the antiferromagnetic ordering below T_N, YbNi4Cd exhibits a large frustration parameter |theta_p/T_N| ~ 16, where the magnetic Yb3+ ions occupy the tetrahedra on the fcc lattice.

Document type: 
Thesis
File(s): 
Supervisor(s): 
Eundeok Mun
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) M.Sc.

Predictive models for chromatin folding: connecting sequence to structure

Author: 
Date created: 
2018-04-13
Abstract: 

The DNA packaged inside a nucleus shows complex structures stabilized by a host of DNA-bound factors. This combination of DNA and bound factors is known as chromatin. Both the distribution of bound factors and the contacts between different locations of the DNA can be now measured on a genome-wide scale. Nevertheless, to what extent is the likelihood of contact between sites in the genome encoded by the spatial sequence of bound factors? Current approaches at addressing this question primarily use simulations of heterogeneous polymers to generate structures using the locations of bound factors. In contrast, here we develop novel predictive models for connecting chromatin sequence to structure using statistical physics, information theory and machine learning. Since our methods do not require costly polymer simulations they can quickly predict the effect on structure due to changes in the distribution of bound factors. In addition, our methods are formulated in a manner that allows us to solve the inverse problem: namely, given just structural data, predict the likely sequence of bound factors. We show that the models developed can make biologically meaningful predictions, highlighting key features of the mechanisms through which the three-dimensional conformation of DNA is coordinated by the interactions between DNA-bound factors.

Document type: 
Thesis
File(s): 
Supervisor(s): 
Eldon Emberly
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) Ph.D.

Observation of critical spin dressing

Date created: 
2018-03-26
Abstract: 

It has long been proposed that spin dressing could be employed to realize a highly effective helium-3 nuclear precession co-magnetometer for a neutron electric dipole moment (nEDM) search. The proposal is to apply an intense, continuous, and far off-resonant oscillating magnetic field, called a dressing field, in such a way that the apparent Larmor precession frequencies of the helium-3 and the neutron are modified. Under appropriateconditions a desirable situation known as critical spin dressing (CSD) is anticipated: the neutron and the helium-3 nucleus (or more generally, any two spin species) are expected to behave as if they had the same gyromagnetic ratio and hence should precess at the same rate in a static magnetic field. Spin dressing has been studied in the context of the neutron, helium-3, and a variety of other systems. Critical spin dressing, however, has not previously been demonstrated. In this thesis I report the first experimental demonstration of pulsed CSD in which simultaneous spin dressing of 1H and 19F nuclei is achieved and studied. I also demonstrate that CSD can be performed using variety of different dressing field waveforms, a consideration that until now has received little or no attention. Examples of parameters studied include the role of phase and amplitude modulation on spin dressing. Of particular note is a significant improvement in reproducibility achieved by alternating the phase of successive cycles of the dressing field waveform by pi radians. Such innovations may prove useful in an eventual nEDM search where demands on precession stability are anticipated to be extreme. To enable my study of CSD I developed a simple and robust apparatus. The central innovation was the first use of Magneto-Impedance (MI) sensors to detect weak magnetic fields associated with the precession of nuclear magnetic moments. The thesis thus begins with summaries of experiments to characterise and validate the use of MI sensors for ultra-low field (ULF) nuclear magnetic resonance. I then describe a refined version of the ULF NMR apparatus, and the manner in which it is used to investigate CSD.

Document type: 
Thesis
File(s): 
Supervisor(s): 
Michael Hayden
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) Ph.D.

In-situ measurement of the jet energy scale and studies of jet structure at ATLAS

Date created: 
2017-12-07
Abstract: 

This thesis presents results for the determination of the ATLAS jet energy scale (JES) using the Missing $E_{\mathrm T}$ Projecting Fraction (MPF) method along with studies to better understand and validate the MPF. Hadronic jets are the most commonly observed objects in proton-proton collisions, and are therefore a part of most final states for processes which are studied at the Large Hadron Collider (LHC). The abundance of jets makes a precise knowledge of the JES essential to the success of the ATLAS physics program. This thesis uses the MPF in events where either a photon or a Z boson is produced back-to-back with a jet to provide an uncertainty on the response of the calorimeter which is below 1\% for jets between 30 GeV and 1 TeV. Studies measuring the impact of the underlying event on the MPF's ability to measure the response of the hadronic recoil are also presented, which validate the previously held assumption that the MPF is insensitive to these effects. In addition, studies into the relation between the measured recoil response and the desired jet response are presented. This includes measures of the flow of energy across the jet boundary during the showering process and the effect on the total measured response of low energy/low response particles near the fringe of the recoil. These measurements show up to a 10% difference between the jet response and the recoil response for jets reconstructed with the anti-k_t algorithm with midrange size parameters (0.4-0.7). These differences however show little dependence on physics modeling choices (less than 1%), on which the Monte Carlo jet calibration is based. These results put the MPF technique on a firmer ground, and they will reduce future JES uncertainties for jets with energies below 100 GeV.

Document type: 
Thesis
File(s): 
Supervisor(s): 
Michel Vetterli
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) Ph.D.

Origin of perpendicular magnetic anisotropy in Co/Ni multilayers and their use in STT-RAM

Author: 
Date created: 
2017-12-08
Abstract: 

Magnetic properties of (111)-textured SAF/Cu/FL multilayer film structures were optimized by varying individual layer thickness and sputtering conditions. The SAF is a synthetic antiferromagnet consisting of Co/Ni multilayers coupled antiferromagnetically across a Ru spacer layer, and the FL is a free layer consisting of a single Co/Ni multilayer. The Co and Ni thicknesses were varied to obtain larger perpendicular magnetic anisotropy. The perpendicular magnetic anisotropy, saturation magnetization, damping and zero-frequency line broadening of the Co/Ni multilayers strongly depend on the number of bilayers. With increasing Cu seed-layer thickness, the texture of the Co/Ni multilayers improves while the grain size and film roughness increase. The increase in grain size results in the reduction of the direct exchange coupling between magnetic grains, which enhances the coercivity of the SAF and the FL. Experimentally measured coercivities of the SAF and FL are compared with calculations obtained from a coherent rotation model. The effect of the role of the Co/Cu interface in the magnetoresistance, is also discussed. Spin-transfer-torque induced switching is investigated in 200 nm diameter circularly shaped, perpendicular magnetized nanopillars. The SAF layer is used as a reference layer to minimize the dipolar field on the free layer. The use of Pt and Pd was avoided to lower the spin-orbit scattering in magnetic layers and intrinsic damping in the free layer, and therefore, reduce the critical current required for spin-transfer-torque switching. In zero magnetic field the critical current required to switch the free layer from the parallel to antiparallel (antiparallel to parallel) alignment is 5.2 mA (4.9 mA). Given the volume of the free layer, VFL = 1.01×10-22 m3, the switching efficiency, Ic/(VFL 0Hc), is 5.28×1020 A/Tm3, twice as efficient as any previously reported device with a similar structure. Variation in perpendicular magnetic anisotropy of (111) textured Au/N×[Co/Ni]/Au films as a function of number of bilayer repeats N is studied. The experimental measurements show that the perpendicular magnetic anisotropy of Co/Ni multilayers first increases with N for N ≤ 10 and then moderately decreases for N> 10. The model we propose reveals that the decrease of the anisotropy for N < 10 is predominantly due to the reduction in the magnetoelastic and magnetocrystalline anisotropies. A moderate decrease in the perpendicular magnetic anisotropy for N > 10 is due to the reduction in the magnetocrystalline and the surface anisotropies.

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

Electrical transport in semiconductor nanowires

Author: 
Date created: 
2017-08-28
Abstract: 

Semiconductor nanowires are promising building blocks for future nanoscale electronic devices. A fundamental control of the impurity and free-carrier concentration as well as the understanding of charge injection and extraction is required. This thesis describes numerical and experimental studies on the electrical transport in semiconductor nanowires. We present a numerical study on geometric scaling of space-charge-limited current, which is often observed in semiconductor nanowires due to carrier depletion and reduced electrostatic screening. The model highlights the effects of the surroundings for nanowires and shows that the dielectric properties of the semiconductor have a negligible effect on the space-charge-limited transport for small dimensions. The results of numerical calculations agree with a simple capacitance formalism which assumes a uniform charge distribution along the nanowire, and experimental measurements for InAs nanowires confirm these results. We discuss the elemental composition and electrical transport characteristics of nominally-undoped and Ga-doped ZnO nanowires, a promising candidate for optoelectronic applications in the UV range. We estimate an upper limit of the Ga impurity concentration with atom-probe tomography and present the electrical transport characteristics measured with a nanoprobe technique and with lithographically-defined contacts allowing back-gated measurements. An increase in apparent resistivity by two orders of magnitude and drop in the effective carrier concentration and mobility was found. Little change in resistivity was observed with Ga doping, which indicates that the concentration of native or background dopants was higher than the Ga doping concentration. We investigate the electrical properties of undoped, Si-doped and Mg-doped InN nanowires directly on degenerate n-type and p-type Si substrates, with a nanoprobe technique. The resulting transport characteristics are weakly rectifying for InN grown on n+-Si with similar ratios for all InN dopant types. On p+-Si, Mg-doped InN nanowires show a strong rectification behaviour with opposite voltage polarity compared to n+-Si, while undoped and Si-doped nanowires show nearly symmetric transport. These characteristics are analyzed in terms of the properties of broken gap band offsets at the Si/InN heterojunction.

Document type: 
Thesis
File(s): 
Supervisor(s): 
Karen Kavanagh
Simon Watkins
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) Ph.D.