Douglas Andrew Bonn
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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
Electrical conduction becomes non-local when an inhomogeneous electronic distribution is induced with a spatial variation shorter than the mean free path (MFP) between momentum-relaxing scattering processes. Two important methods of inducing such a distribution are via the size and skin effects. In the size effect, one or more dimensions of a medium are reduced below the MFP. The scattering of electrons from the medium's boundaries then induces an inhomogeneous electronic distribution under an applied direct current. In the skin effect, the exponential decay of alternating electromagnetic fields as they propagate into the medium gives rise to a so-called skin layer. The electronic distribution within the skin layer becomes inhomogeneous as the skin depth falls below the MFP. Here we study the size and skin effects in PdCoO₂, both experimentally and theoretically. While previous theoretical treatments of non-local electrical conductivity have assumed a free-electron dispersion, we observe that the anisotropic Fermi surface (FS) in PdCoO₂ results in behaviour that is incompatible with this assumption. Measurements of the size effect in PdCoO₂ revealed two novel phenomena, both of which are symmetry-forbidden for local conduction: anisotropy in the in-plane longitudinal resistivity, and a non-zero transverse resistivity at zero magnetic field. We developed a theory of the size effect for arbitrary FS geometry and used it to reproduce the key features of these measurements. Motivated by recent interest in the possibility that electrons in solids may behave viscously as a result of frequent internal momentum-conserving scattering, we developed a generalized theory of the skin effect, taking into account separate rates of momentum-conserving and momentum-relaxing scattering for arbitrary FS geometry. For an isotropic FS, our theory encompasses several known limiting behaviours. For anisotropic FSs, we explored geometries which lead to changes in the scaling of the surface impedance. By applying bolometric broadband microwave spectroscopy, we studied the skin effect in PdCoO₂ for three different directions of electromagnetic propagation. Using symmetry-based arguments, we determined that our measurements were neither in the local nor purely viscous regime. We argued instead that the data demonstrate a novel, predominantly ballistic effect as a result of the faceted FS.
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In this work the use of interactive simulations in inquiry activities designed for the tutorials of a large enrollment introductory electromagnetism course is investigated. Interactive simulations are educational tools that allow students to uncover the rules that govern a simulated physics phenomenon through a process of scientific inquiry. This thesis reports the results of a study that compared a series of three collaborative and scaffolded inductive inquiry activities where students invented a rule for a certain physics phenomenon from observations they either generated themselves from a simulation or that were instead provided directly to them as a set of so-called "contrasting cases''. Contrasting cases are particularly beneficial since they directly highlight important rule features when compared to one another. While contrasting cases thus provide more support to students when inventing their rule, the simulation-based activities can provide students with more of an opportunity to practice and develop valuable exploration skills. Results from the study demonstrate that generating observations from a simulation is less likely to lead students to invent the correct rule and generally leads to poorer immediate conceptual learning outcomes as compared to using a set of contrasting cases. In terms of exploration outcomes, students using contrasting cases also reported comparatively more observations to support their rule. However, differences between conditions on exploration outcomes were minimized when students used simulations that were less open-ended and possessed clear visual cues that guided students in investigating relevant domain features. Finally, the above student outcomes were also compared across two final assessment inquiry activities where students worked individually; in the first, students invented the target physics rule by generating their own cases from a simulation, while in the other they used a set contrasting cases. Results show that students that had previously practiced inventing with contrasting cases performed the same or better on all outcomes compared to those that practiced inventing by generating observations from a simulation.
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Topological materials have been at the forefront of condensed matter physics research over the past few decades. Characterised by electronic bands with non-trivial topological invariants, topological materials exhibit a number of interesting electronic properties, such as conducting chiral boundary states and linear electronic dispersions, and have been theorised for use in a variety of applications ranging from spintronic devices to quantum computing. Recently, topological semimetals were discovered, where the bulk electronic bands are understood in the framework of the high-energy relativistic Dirac equation and its conditional variations, the Weyl and Majorana equations. Furthermore, the vast permutations of material compounds available results in a nearly infinite sandbox for researchers to study, which has resulted in topological semimetals that have no high-energy analogue. One of such material classifications is the nodal-line semimetal, characterised by linear electronic band crossings that form lines or loops in momentum space. These nodal-line semimetals also exhibit exotic surface states, named drumhead states, which are an interesting and exciting new state with promises in high-temperature superconductivity and quantum computation. A large effort is being placed to find materials that can be used to study the fundamental properties of these materials and their resultant surface states. Scanning tunnelling microscopy (STM) provides a perfect tool to study the topological properties of materials, able to atomically resolve the surface structure and also provide insight into scattering selection rules, which are deeply dependent on the band topology. Two topological materials were studied using STM in this thesis: the topological nodal-line semimetal ZrSiTe and the topological insulator (Bi$_x$Sb$_{1-x}$)$_2$Te$_3$. ZrSiTe was studied with an emphasis on the quasiparticle scattering characteristics, measured using Fourier-transform scanning tunnelling spectroscopy. Two main scattering features are examined, one relating to the nodal line, and the other arising from the drumhead surface state. These studies mark the first time a drumhead state has been observed using a real space measurement. (Bi$_x$Sb$_{1-x}$)$_2$Te$_3$ was studied with an emphasis on the nano-scale transport characteristics, measured using 4-probe STM and scanning tunnelling potentiometry. Effects of step edges and domain boundaries on the local resistance are studied for a fractional substitution of $x = 0.19$.
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At the atomic scale, surfaces exhibit a rich variety of physical phenomena that can be probed using a scanning tunnelling microscope (STM). The STM measures the quantum tunnelling of electrons between a metallic tip and conducting sample and can be used to characterize the nanoscale surface. This thesis presents STM measurements taken at low-temperature in ultra-high vacuum, which are used to characterize two different nanoscale environments: the two-dimensional surface states of Ag(111) and Cu(111) and the magnetic moments of iron and cobalt atoms deposited on a thin-film of magnesium oxide.Fourier-transform scanning tunnelling spectroscopy (FT-STS) analysis of quasiparticle interference, created by impurity scattering on the surfaces of the noble metals Ag(111) and Cu(111), is used to compare the most common modes of acquiring FT-STS data and shows, through both experiment and simulations, that artifact features can arise that depend on how the STM tip height is stabilized throughout the course of the measurement. Such artifact features are similar to those arising from physical processes in the sample and are susceptible to misinterpretation in the analysis of FT-STS in a wide range of important materials. A prescription for characterizing and avoiding these artifacts is proposed, which details how to check for artifacts using measurement acquisition modes that do not depend on tip height as a function of lateral position and careful selection of the tunnelling energy.In a separate set of experiments a spin resonance technique is coupled to an STM to probe the spin states of individual iron atoms on a magnesium oxide bilayer. The magnetic interaction between the iron atoms and surrounding spin centres shows an inverse-cubic distance dependence at distances greater than one nanometre. This distance-dependence demonstrates that the spins are coupled via a magnetic dipole-dipole interaction. By characterizing this interaction and combining it with atomic manipulation techniques a new form of nanoscale magnetometry is invented. This nanoscale magnetometer can be combined with trilateration to probe the spin structure of individual atoms and nanoscale structures. The information gained characterizing these new forms of magnetic sensing sets the stage for the study of complex magnetic systems like molecular magnets.
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Many undergraduate labs engage students in experimentation without developing critical thinking or scientific reasoning skills, especially about measurement and data. In this thesis, I present a pedagogical framework for developing students' critical thinking behaviours in a first-year undergraduate physics lab. The main critical thinking behaviours assessed were for students to reflect on their data collection and analyses, iterate to improve their measurements and methods, and evaluate the experiments and theoretical models. The pedagogy uses structured comparisons between measurements and models, with a critical focus on understanding measurement and uncertainty at a conceptual level and applying the concepts to quantitative analysis of data. Implementation involved scaffolded instructions and support for reflection and iteration that was dynamically faded throughout the course. Through analysis of students' written lab materials, I evaluated their engagement in reflection, iteration, and evaluation, comparing to a previous iteration of the course that did not include the critical thinking scaffolding. Students in the new course structure not only transferred the previously scaffolded reflection and iteration behaviours to unscaffolded experiments, but also spontaneously evaluated theoretical models, which was never explicitly structured. While the previous version of the course supported students in data analysis at a procedural, 'plug-and-chug' level, the new course structure significantly improved students' critical thinking behaviours, shifted students into more expert-like epistemological frames, and improved their motivation and attitudes about experimental physics.
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Identifying the pairing symmetry is a crucial step towards uncovering the superconducting mechanism. The pairing symmetry and interactions leading to pairing in the iron-based high-temperature superconductors are under debate. In this thesis work, the pairing symmetry of LiFeAs, a stoichiometric superconductor in the iron-based family, is studied by scanning tunneling microscopy. The tunneling conductance spectrum in a defect-free region shows two nodeless superconducting gaps. In addition, a dip-hump above-gap structure was observed, indicating coupling between the superconducting carriers and bosonic modes. Defect bound states were measured for iron-site defects. The bound states are pinned to the gap edge of the small superconducting gap, consistent with theoretical predictions for a sign-changing pairing symmetry. Finally, the observed Bogoliubov quasiparticle interference associated with scattering from defects provides compelling evidence for an s+- pairing symmetry in LiFeAs.
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High-temperature superconductivity (high-Tc) was discovered in 1986 in copper-oxide materials, and since that time the goal of understanding high-Tc has driven the advancement of theoretical and experimental condensed matter physics. Despite the concerted efforts of some of the brightest minds in physics over the past 26 years, there is still no microscopic understanding of these materials. One of the main problems is an uncertainty as to whether Fermi liquid theory, which has been the foundation of our understanding of conventional metals for over 50 years, can be used to describe the strange pseudo-metallic properties of the cuprates. This thesis studies the resistivity of the high-Tc superconductor YBa₂Cu₃O₆+x (YBCO) in magnetic fields up to 70 Tesla. These resistivity measurements show oscillatory behaviour as a function of magnetic field, which is a clear signature of a Fermi surface. The development of an advanced technique (based on a genetic algorithm) for analyzing the oscillatory resistance is presented, and the Fermi surface of YBa₂Cu₃O₆.₅₉ is determined with great precision by analyzing the field, angle, and temperature dependences of the oscillations. Analysis of the data shows that the electronic g factor, related to the strength of quasiparticle spin magnetic moment, does not experience strong renormalization in YBCO, in contrast with previous experimental studies. This lack of renormalization has important implications for theoretical descriptions of YBCO. A full description of the shape of the Fermi surface of YBCO is presented, and measurements of YBCO with different oxygen concentrations give the evolution of the Fermi surface with hole doping. A novel technique for fine-tuning the hole doping in YBCO is presented in the context of a Hall coefficient experiment. The result is a detailed doping dependence of the Hall coefficient, indicating that the Fermi surface seen in quantum oscillation experiments is influenced by some type of electronic order---such as charge and spin stripe order---competing with superconductivity near 1/8th hole doping. The behaviour of the superconducting vortex lattice in a magnetic field is analyzed as a function of temperature, and this behaviour also indicates that something is competing with superconductivity near 1/8th doping.
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Master's Student Supervision
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
Topological materials are a newly discovered class of matter where protected gapless states arise within the material band structure. Of these materials is PtSn₄, a topological semimetal with experimentally confirmed Dirac nodal arcs in its band structure. It exhibits extremely large magnetoresistance and a high residual resistivity ratio which is indicative of efficient electron transport. There are many material characteristics responsible for excellent carrier mobility such as a topological surface state or low density defects to name a few. Since materials can exhibit high mobility without being topological, it is unknown how much of the assumed high mobility of PtSn₄ is dependent on its topological features versus something else entirely.Scanning tunneling microscopy and spectroscopy is a surface sensitive technique that allows for the imaging of a conductive surface at an atomic resolution via quantum tunneling. It is an excellent tool for studying imperfect crystal lattices due to defects, characterizing cleaving terraces of materials and probing both occupied and unoccupied electron states.In this thesis, scanning tunneling microscopy and spectroscopy is used to explore PtSn₄ in detail. The structure of the pristine lattice is defined and the three different possible terrace steps are identified using differing spectroscopies as well as comparing terrace step edge heights to distances within the unit cell. One of the main goals was to explore the intrinsic defect density of the material and provide clarity to the cause of the high residual resistivity ratio. Five different types of defects are documented in both topographic and spectroscopic detail. The lattice site origins of each defect is discussed in hopes to provide a strong basis for future exploration.
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The microwave magnetic responses of realistic, strongly-demagnetized hexagonal platelet samples were studied using numerical simulations in COMSOL Multiphysics®, in the context of cavity bolometric and cavity perturbation experiments. The effects of sample conductivity and edge rounding on this response are investigated in detail. Emphasis was placed on the specific sample geometries, material conductivities, and measurement frequencies of previously-performed cavity bolometric broadband measurements of palladium cobaltate (PdCoO₂) samples. Simulations of samples with finite conductivity in three dimensions presented numerical difficulties, which prevented the direct determination of the microwave responses of realistic hexagonal samples to sufficient accuracies. Therefore, a two-step approach was used to arrive at experimentally relevant results. As a first step, perfectly conducting, sharp-edged hexagonal platelet samples were simulated with geometry-necessitated three-dimensional models, as this case allowed the response within the sample bulk to be completely ignored; this in turn greatly reduced model complexities and enabled the response to be determined to high precision. As a second step, two-dimensional axisymmetric models were used to accurately simulate the responses of disks with both finite skin depths and finite edge radii.Comparing the responses of realistic disks to those of perfectly conducting, sharp-edged disks made it possible to estimate the relative reduction in sample power dissipation as a function of both the skin depth and of sample edge radius. By direct analogy between the disk and hexagonal samples, these two threads were woven to provide estimates of the power dissipation of the PdCoO₂ samples that were studied in the broadband experiments.
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This thesis describes the design, construction and fabrication of a complete ultra-high vacuum (UHV) Dilution refrigerator based scanning tunneling microscope (STM). Data taken at a base temperature of 114 mK is presented and electrical, mechanical and vacuum design features are described for both the STM and the UHV system. Topographic images and spectroscopy on Au(111), graphene and other materials are presented to detail the performance of the STM. Techniques involving coherence and finite element analysis are used to address acousto-mechanical interaction between the STM and an acoustic room mode. The design and fabrication of an electron beam heater sample plate and complete UHV sputtering and annealing stage are presented.
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In this thesis, the microwave electrodynamic properties of stoichiometric FeSe are measured by a cavity perturbation technique based on a 940 MHz loop-gap resonator. Measurements surprisingly show, for the first time, that c-axis conductivity in stoichiometric FeSe superconductors exhibits a broad peak in its temperature-dependence, a phenomenon which was only observed for in-plane electrodynamics in the cuprates. This result implies that the charge transfer between FeSe-planes is enhanced by the development of long transport quasiparticle lifetimes below Tc.
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This thesis details the specification, design and characterization of an ultra-low vibration facility and aspects of the design and performance of an ultra-high-vacuum dilution temperature scanning tunneling microscope (STM) housed within this facility. The basic principles of vibration isolation and STM are introduced. Existing ultra-low-vibration facilities and dilution temperature scanning tunneling microscopy experiments are reviewed. A specification for the vibration isolation performance of the facility is developed based on a simple model of the vibrational mechanics of a STM head. The experimental techniques of accelerometery and microphony are introduced. A survey of the acoustic and vibrational conditions at the site of the facility prior to its construction leads into a detailed description of the facility design. This is followed by an experimental characterization of the facility performance. Acoustic transmission functions of double-walled acoustic isolation vaults are reported; the dominant ambient sounds inside these vaults are found to coincide substantially with the acoustic modes of the vaults. Massive pneumatically supported concrete inertia blocks are found to perform approximately as ideal 2nd order damped spring mass systems below 10-15 Hz. Above these frequencies, acoustic forces are found to cause additional motion of the pneumatically supported stages. It is found that these systems must be carefully adjusted and monitored to ensure low resonant frequencies are maintained. Inertia blocks optimized for flexural resonant frequencies above 200Hz are presented; these vibrations are found to be poorly damped and to degrade isolation performance at the flexural resonance frequencies. Experiments mounted on light-load pneumatic isolators on top of the inertia blocks are found to be very susceptible to acoustic forces and as a result exhibit non-ideal isolation behavior above approximately 7 Hz. The design of a rigid STM head for use in the ultra-high-vacuum dilution refrigerator experiment is detailed and an overview of the supporting experimental system is given. The results of preliminary commissioning of the microscope are given and poorly damped vibrations of the dilution refrigerator structure at ~20 Hz are found to be the dominant contribution to the noise in the tunneling current signal when the instrument is operated at dilution temperature.
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The microwave surface resistance, Rs(w,T), and the magnetic penetrationdepth, ⋋(T), of a rectangular Tl₂Ba₂CuO₆+δ (Tl-2201) single crystal measuredalong its different crystallographic axes are reported. The measurementsof the surface resistance as a function of frequency were made using aprecise broadband bolometric technique, and a loop-gap resonator was employedto measure the temperature dependence of the magnetic penetrationdepth.Disentangling the in- and out-of-plane components of both microwaveproperties was accomplished by comparing the measurement results obtainedfor different orientations of the sample with respect to the appliedmagnetic field, allowing us to report, for the first time, the c-axis componentsof ∆⋋(T), and Rs(w,T).In particular, our results show a quadratic temperature dependence of ∆⋋c(T) in Tl-2201 which is similar to that in other anisotropic cupratessuch as BSCCO, and YBCO. Furthermore, in the case of the surface resistance,a sign change in the curvature of Rcs(w) is observed. The originof this behavior is not yet understood. The ab-plane components of bothmicrowave properties behave similarly to those reported on other dopings ofthis material.The measurements of Rs(w,T) and ∆⋋(T) allow us to determine thecomplex conductivity of this material. Having Tc of 43 K, the sample studiedhere is in the middle of the overdoped side of the superconducting dome,where very few studies have been made. This particular sample possessesrelatively low quasiparticle scattering rates making the interpretation of themeasurement results more straightforward. The reliability of the results,current limitations, and further potential progress are also discussed.
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Invention activities are discovery-learning activities that ask students to invent a solution to a problem before being taught the expert solution. The combination of invention activities and tell-and-practice methods has been shown to lead to better student learning and performance on transfer tasks, as compared to tell-and-practice methods alone. A computer-based interactive learning environment, called the Invention Support Environment (ISE), was built using Cognitive Tutor Authoring Tools to improve the in-class use of invention activities, and act as a research tool for studying the effects of the activities. The system was designed to support three levels of metacognitive scaffolding, using domain-general prompts. It also features a platform for creating new invention tasks within the system, requiring little to no programming experience. The ISE was used to evaluate how domain-general scaffolding of invention activities can best support acquisition of domain knowledge and scientific reasoning skills. Five invention activities in statistics and data-analysis domains were given to 134 undergraduate students in a physics lab course at the University of British Columbia. Students either received guidance in the form of faded metacognitive scaffolding or unguided inventions. It was found that faded metacognitive scaffolding did not improve learning of invention skills compared to unguided inventions. Faded metacognitive scaffolding was found to improve understanding of domain equations, as seen through higher performance on debugging items in a statistics diagnostic. Future experimental design and ISE improvements are discussed.
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