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

## Dissipation and control in microscopic nonequilibrium systems

Author:
Date created:
2020-11-24
Abstract:

Quantifying the flow of energy, entropy, and information within and through nonequilibrium systems remains a central challenge in understanding the microscopic physics of biological systems. Over the past two and a half decades, parallel developments in the fields of theoretical stochastic thermodynamics and single-molecule experiments have made tremendous steps towards this end, advancing our understanding of the fundamental physical limitations and constraints faced by biological systems in vivo. Central in this focus are molecular machines: nanoscale protein complexes which interconvert between different forms of energy to perform useful functions to the cell. While single-molecule experiments on molecular machines have predicted impressively high efficiencies, much is still unknown about their performance in vivo. In this thesis we build upon these primitives, largely by making use of near-equilibrium phenomenological models to simplify and make tractable the problem of quantifying dissipation in molecular machines and predicting the operational modes which are imperative to minimizing their dissipation. By exploring the relevance of near-equilibrium models in the experimental investigation of a DNA hairpin, we find that such an approach can provide utility in understanding the strategies to reduce dissipation in nonequilibrium processes. However, single-molecule manipulations are significantly separated from the in vivo dynamics of molecular machines, and thus for the remainder of the thesis we expand upon this approach in various ways, generalizing the existing theoretical framework to more closely parallel the dynamics of molecular machines. By incorporating the inter-system feedback present in molecular machines, we find that familiar intuitions about how excess work and entropy production are related break down. Finally, we derive a phenomenological expression for the energy flows communicated within the components of a mechanochemical molecular machine. Ultimately, our analysis shows that intersystem feedback can lead to nonvanishing energy flows which are the manifestation of a Maxwell demon in the molecular machine itself.

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

## In situ observations of hexagonal boron nitride growth on Cu (110)

Author:
Date created:
2020-08-05
Abstract:

Hexagonal boron nitride (h-BN) was grown on top of (0001) textured polycrystalline Ru or single crystalline Cu (110) inside a Low Energy Electron Microscope (LEEM). Ru samples were prepared via radiofrequency magnetron sputtering on Si (100), Cu (110) single crystals were prepared ex situ via mechanical and electro-chemical polishing. Samples were exposed to borazine precursor at temperatures of 600–750°C and pressures of 0.5 - 10×10−7 Torr. Exposure of Ru substrates resulted in rapid yet small grained h-BN growth covering the entire surface. Exposure of Cu (110) single crystals resulted in the nucleation of well-aligned trigonal h-BN islands when Cu (110) showed a particular hydrogen-induced surface reconstruction. These islands merged to ribbons along surface steps, and into larger, more irregularly shaped features. A ring in the low energy electron diffraction (LEED) pattern was observed with a preferential orientation aligned along Cu (0 1) directions of the underlying substrate. A second Cu (110) single crystal was prepared via multiple sputter-anneal cycles using argon and hydrogen ions resulting in an unreconstructed surface. Exposing this single crystal to borazine also resulted in trigonal h-BN islands, yet these islands did not merge nor appear to align with surface steps. Nucleation was determined by defects and decreased with increasing temperature. LEED patterns revealed two preferential orientations, each aligned with Cu (1 1) directions. Growth dynamic plots suggested a modification to the growth model for self-limited monolayer growth, as islands did not merge. Extracted growth rates did not depend on the substrate temperature in the range tested, but increased with pressure. On the other hand, the maximum coverage increased with temperature, but did not depend on pressure. Annealing this Cu (110) single crystal in H2 atmosphere resulted in a weak (2 x 1) reconstruction. These surfaces had a lower defect density, and borazine exposure at 700–750°C resulted in larger h-BN islands, revealing preferred dendritic growth along the Cu (0 1) and (1 0) directions. This resulted in T-shaped islands early on in the exposure, which later filled out into triangular shapes, indicating a strong influence of the substrate. Dark-field LEEM revealed that neighboring islands did not merge, regardless of whether they had the same orientation or not.

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

## Test of fermi liquid theory with terahertz conductivity measurements of MnSi

Author:
Date created:
2020-07-13
Abstract:

I present terahertz time-domain spectroscopy measurements of the dynamical conductivity of MnSi, which I compare to Fermi liquid theory at low temperatures and low frequencies. I also describe a new methodology for terahertz time-domain data analysis, developed to perform this comparison, which has higher sensitivity to fit quality than earlier methods. Within the extended Drude model framework, the conductivity scattering rate exhibits quadratic dependence on both frequency and temperature, as expected in Fermi liquid theory. However, the joint dependence of the scattering rate on frequency and temperature deviates from the standard functional form associated with Fermi liquid theory, as observed previously in other materials. I find better agreement with two alternative models, which are also motivated by Fermi liquid theory but that rely on slightly different assumptions. These observations offer a way to reconcile Fermi liquid theory with the observed conductivity of real materials.

Document type:
Thesis
File(s):
Supervisor(s):
J. Steven Dodge
Department:
Science: Department of Physics
Thesis type:
(Thesis) Ph.D.

## Physical properties of RE3TMSb5 (RE = La, Ce; TM = Ti, Zr, Hf)

Author:
Date created:
2019-12-12
Abstract:

Single crystals of RE3TMSb5 (RE = La, Ce; TM = Ti, Zr, Hf) have been grown by Sn flux and characterized by magnetization, electrical resistivity, and specific heat measurements. Powder X-ray diffraction analysis indicates that the title compounds crystallize into the hexagonal Hf5Sn3Cu-type structure (P63/mcm). The physical property measurements for Ce-containing compounds clearly indicate an antiferromagnetic ordering around 5 K. The effective magnetic moment estimated from magnetic susceptibility measurements is close to the theoretical value, indicating the 4f-electrons of Ce3+ ions are well localized. Magnetization isotherms at T = 2 K show anisotropic behaviour between H||ab and H||c. The temperature-dependent electrical resistivity follows a typical Kondo lattice behavior associated with thermal population of crystalline electric field (CEF) levels. The specific heat measurement for Ce-containing compounds reveals a large Cm/T value at low temperatures, which is much bigger than that of La-containing samples. At high temperatures, the CEF energy level scheme is analyzed by fitting to the Schottky peak observed in specific heat, from which the energy splitting levels between the three doublet states are found to be 165 and 380 K for all three Ce-containing samples. The resistivity measurements for all La-containing samples indicate an anomalous broad peak structure at high temperatures. We also investigate the previously reported superconductivity observed in La3TiSb5 and Ce3TiSb5 below 4 K. Our resistivity and specific heat measurements show that the superconductivity is not an intrinsic property of the single crystals, and is attributed to residual Sn flux.

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

## Search for charged Higgs bosons in tau-lepton final states with 139 inverse femtobarns of proton-proton collision data recorded at a centre of mass energy of 13 TeV with the ATLAS detector

Author:
Date created:
2019-11-20
Abstract:

The Higgs boson, with a measured mass of approximately $125~GeV$, has been studied extensively since its discovery in 2012 at the Large Hadron Collider. This discovery opens the question of whether the Higgs boson of the Standard Model (SM) is the only scalar particle of Nature or it belongs to a larger scalar sector, as predicted in many Beyond the Standard Model (BSM) theories. Therefore, observation of charged Higgs bosons would indicate new physics. This thesis presents results of a search for a charged Higgs boson in the mass range $80~GeV$ to $3~TeV$, through tau-lepton final states. The search is performed using proton-proton collisions data at $\sqrt{s}=13~\mbox{TeV}$, collected with the ATLAS experiment, during 2015 to 2018. The final results are interpreted in the context of the Minimal SuperSymmetric Standard Model (MSSM) benchmark scenarios. In these scenarios, charged Higgs bosons coupling to tau-lepton are enhanced for some parts of the search phase space, thus increasing the chance of their discovery. No significant excess of events above the expected background from the Standard Model processes is observed. Therefore, upper limits on the charged Higgs boson production cross section times its branching ratio to tau-lepton and its associated neutrino are set at a 95\% Confidence Level. The results are also interpreted in the context of the hMSSM and $m^{mod -}_{h}$ benchmark scenarios of the MSSM. Due to the enhancement of the charged Higgs boson coupling to tau-leptons at high values of the $\tan\beta$ parameter of the MSSM, it is possible to exclude the high $\tan\beta$ region in the $M_{H^{\pm}}$--$\tan\beta$ parameter space. In this work, $\tan\beta$ values around 60 are excluded up to a charged Higgs boson mass of $1400~\GeV$. Furthermore, in the low mass region, below $170~GeV$, all values of $\tan\beta$ in range 1--60 are excluded at 95\% confidence level.

Document type:
Thesis
File(s):
Supervisor(s):
Dugan O'Neil
Department:
Science: Department of Physics
Thesis type:
(Thesis) Ph.D.

## Mathematical modelling of electrokinetic phenomona in soft nanopores

Author:
Date created:
2020-04-15
Abstract:

In and around porous systems with at least one characteristic dimension below 100 nm, solid/liquid interfaces play a key role in surface-charge-governed transport, separation processes and energy storage devices. Nanopores with well-defined geometry and chemical characteristics have emerged as valuable tools to unravel interactions between external and induced electric fields and the underlying transport, in the presence of embedded charges. In this thesis, theoretical and numerical investigations of electrokinetic effects in soft cylindrical nanochannels with uniformly distributed surface charges are carried out within continuum mean-field approximations. The aim is to provide a theoretical framework through which one can access a comprehensive understanding of the coupling between electrokinetic transport, double-layer charging and wall deformations in nanochannels embedded in soft polymeric membranes. In the first part of the thesis, numerical calculations using the coupled continuum mean-field equations are conducted to quantify ion and fluid transport in a finite, cylindrical and rigid nanochannel connected to cylindrical electrolytic reservoirs. Results of these calculations, verified by experiments, serve as a guide for theoretical investigations in later components of the thesis. Subsequently, the transport of protons and water in a long, negatively charged channel is studied from a theoretical point of view. A theoretical model is developed that describes nonlinear coupling between wall deformation and water and proton flows in a charged, eformable nanochannel whose viscoelasticity is governed by the linear Kelvin-Voigt model. In addition to focusing on transport phenomena in an open nanochannel, we direct attention to the equilibrium structure of the electric double layers. This was achieved by considering a physical situation where the charged channel is finite and sealed at both ends by metal electrodes under external voltage bias. Size-modified mean-field equations were used to account for finite ion sizes, subject to a self-consistent electroneutrality condition which demands that the net amount of charge on both electrode surfaces balances. Equilibrium ion distributions and differential capacitance curves are presented and analysed. Motivated by electroactuators, the last part of the thesis added deformations of the channel walls to the closed-channel system modelling.

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

## Valleytronics of quantum dots of topological materials

Author:
Date created:
2020-04-08
Abstract:

The local minima (maxima) in the conduction (valence) band of crystalline materials are referred to as valleys. Similar to the role of spin in spintronics, the manipulation of the electron's valley degree of freedom may lead to technological applications of the new field of research called valleytronics. Those crystalline solids that have two or more degenerate but well separated valleys in their band structure are considered to be potential valleytronic systems. This thesis presents a theoretical investigation of the valley degree of freedom of electrons in quantum dots of two-dimensional topological materials such as monolayer and bilayer graphene and monolayer bismuthene on SiC. To this end, a method for the calculation of the valley polarization of electrons induced by the electric current flowing through nanostructures was developed in this thesis. The method is based on a projection technique applied to states calculated by solving the Lippmann-Schwinger equation within Landauer-Büttiker theory. Applying the proposed method, this thesis addresses several valleytronic problems of current interest, including: the valley currents, valley polarization, and non-local resistances of four-terminal bilayer graphene quantum dots in the insulating regime, a valley filtering mechanism in monolayer graphene quantum dots decorated by double lines of hydrogen atoms, and the valley polarization of the edge and bulk states in quantum dots of monolayer bismuthene on SiC, a candidate for a high-temperature two-dimensional topological insulator.

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

## Confocal microscopy for T centres in silicon

Author:
Date created:
2020-04-07
Abstract:

Spin defects in silicon boast long lifetimes and potential scalability with existing nanofabrication foundry processes. Paired with silicon photonics, optically active spin defects offer a path to scalable optically interfaced quantum technologies, such as quantum communication networks and optically coupled qubits. The T centre in silicon is a paramagnetic radiation damage centre that is optically active in the telecommunication O-band, making it a strong candidate for spin-photon interfaces. Certain single-photon based quantum technologies rely on the production of indistinguishable photons, a characteristic which may be found from an optical centre’s zero-phonon line. In this study we measure the zero-phonon line fraction of the T centre in silicon-28 at 4.2 K to be 22.9 ± 0.2%. Isolating optical defects in silicon is difficult due to the relatively low radiative efficiencies of silicon-based emitters and silicon’s large refractive index (n ≈ 3.5), trapping light by total internal reflection. Estimates using bound exciton ground state lifetime measurements from previous studies suggest isolation and measurement of single T centres is possible by confocal microscopy. We develop a confocal microscope system designed for measuring photoluminescence from cryogenically cooled silicon and characterize its resolution performance in reflection and above-band photoluminescence. Silicon photonic ‘micropuck’ structures were designed and fabricated to increase collection efficiency from single T centres into a microscope objective.

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

## Potential mapping of growth sequence in semiconductor NW p-n junctions

Author:
Date created:
2019-12-19
Abstract:

Semiconducting nanowire-based clean energy technologies are promising in terms of efficiency and material consumption. The vapor-solid-liquid (VLS) mechanism using gold (Au) as the catalyst allows for the growth of various compounds. Although dopant impurity control and interface sharpness define the operating principles of these new clean energy technologies, growing and characterizing them is challenging. This complexity increases if one takes into account sequencing and/or binary and ternary compound growth, for instance p- to n-type growth and vice-versa. Measurement of the electrostatic potential gradients due to dopant activation of the various interfaces is of primary importance, since this leads to the understanding of VLS growth parameters that yield working devices. We have confirmed the presence of narrow, degenerately-doped axial silicon nanowire (SiNW) p-n junctions via off-axis electron holography. SiNWs were grown via the VLS - Au catalyst, using silane (SiH 4 ), diborane (B 2 H 6 ) and phosphine (PH 3 ) as the precursors, and hydrochloric acid (HCl) to styabilize the growth. Two types of growth were carried out and in each case we explored growth with both n/p and p/n sequences. In the first type, we abruptly switched the dopant precursors at the desired junction location, and in the second type we slowed the growth rate at the junction to allow the dopants to readily leave the Au catalyst. We demonstrate degenerately-doped p/n and n/p NW segments with abrupt potential profiles of 1.0 ± 0.1 and 0.9 ± 0.1 V, and depletion region widths as narrow as 13 ± 1 nm via EH. The results presented here show that the direct VLS growth of degenerately-doped axial SiNW p-n junctions is feasible, an essential step in the fabrication of more complex SiNW-based devices for electronics and solar energy. Electron Holographic Tomography was used to obtain 3-dimensional reconstructions of the morphology and electrostatic potential gradient of axial GaInP/InP nanowire tunnel diodes. Crystal growth was carried out in two opposite growth sequences: GaInP:Zn/InP:S and InP:Sn/GaInP:Zn, using Zn as the p-type dopant in the GaInP, but with changes to the n-type dopant (S or Sn) in the InP. Secondary electron and electron beam induced current images obtained using scanning electron microscopy indicated the presence of p-n junctions in both cases and current-voltage characteristics measured via lithographic contacts showed the negative differential resistance, characteristic of band-to-band tunneling, for both diodes. EHT measurements confirmed a short depletion width in both cases (21 ± 3 nm), but different built-in potentials, V bi , of 1.0 V for the p-type (Zn) to n-type (S) transition, and iii0.4 V for both were lower than the expected 1.5 V for these junctions, if degenerately-doped. Charging induced by the electron beam was evident in phase images which showed non-linearity in the surrounding vacuum, most severe in the case of the nanowire grounded at the p-type Au contact. We attribute their lower V bi to asymmetric secondary electron emission, beam-induced current biasing and poor grounding contacts.

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

## Probing primordial magnetic fields with the cosmic microwave background

Author:
Date created:
2020-01-21
Abstract:

A primordial magnetic field (PMF) present before recombination can leave specific signatures in the cosmic microwave background (CMB) fluctuations. Of particular importance is its contribution to the B-mode polarization power spectrum. Indeed, vortical modes sourced by the PMF can dominate the B-mode power spectrum on small scales, as they survive damping up to a small fraction of the Silk length. Therefore, measurements of the B-mode polarization at high-l, such as the one recently performed by the South Pole Telescope (SPT), have the potential to provide stringent constraints on the PMF. We use the publicly released SPT B-mode polarization spectrum (2015 and 2019), along with the temperature and polarization data from the Planck satellite, to derive constraints on the magnitude B1Mpc, the spectral index nB and the energy scale at which the PMF was generated. We find that after marginalizing nB, Planck data constrains the magnetic amplitude to B1Mpc < 3.3 nG at 95% confidence level (CL), the SPT measurement improves the constraint to B1Mpc < 1.5 nG. The magnetic spectral index, nB, and the time of the generation of the PMF are unconstrained. For a nearly scale-invariant PMF, predicted by the simplest inflationary magnetogenesis models, the bound from Planck+SPT is B1Mpc < 1.2 nG at 95% CL for a non-helical PMF and B1Mpc < 1.1 nG for a maximally helical PMF. The bound from Planck data alone is B1Mpc < 1.7 nG at 95% CL when considering a maximally helical field. For a non-helical PMF with a spectral index of nB =2, expected for fields generated in post-inflationary phase transitions, the 95% CL bound is B1Mpc < 0.002 nG, corresponding to the magnetic fraction of the radiation density Ω_Bγ < 10-3 or the effective field Beff < 100 nG. We find that accounting for the helicity weakens the CMB constraints on PMF, allowing to have more magnetic power available on the 1Mpc comoving scale relevant to the formation of galactic magnetic fields. The patches for the Boltzmann code CAMB and the Markov Chain Monte Carlo engine CosmoMC, incorporating the PMF effects on CMB, are made publicly available.

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