Since the discovery of high-temperature superconductors, there has been a worldwide effort towards the development of processes for fabricating coated conductors for power applications. Most of these processes are based on vapour phase deposition techniques that have relatively low growth rates. A high-rate processing route was proposed based on the observation of high growth rate of (RE)Ba2Cu3O7- δ superconducting compounds (RE = rareearth element) from a flux supersaturated with one or more RE elements by liquid phase epitaxy (LPE). LPE has been successfully used to grow YBCO thick films with both c- and a,b-orientations on (110) NdGaO3 substrates and pure c-oriented films on YBCO seeded (100) MgO and (100) SrTiO3 under carefully controlled growth temperature and undercooling. The film growth mode (c- or a,b-oriented) is determined by the growth rate, which is directly related to the level of RE supersaturation that could be controlled by the undercooling used along with the amount of total RE solubility in the solution. The LPE grown films were highly epitaxial and biaxially aligned with good in-plane and out-of-plane textures. YBCO thick films grown on NdGaO3 by LPE showed high Tc of ~92 K and zero-field Jc at 77 K of 2.5x105 A/cm2. The initial growth of YBCO was found to be a multi-nucleation process. However, above a critical film thickness, dislocations started to form as a lattice-misfit stress relieving mechanism that led to step formation and spiral growth around dislocation cores. The growth kinetics from an unstirred solution was found to obey a

The critical, currents and magnetic fields required to destroy superconductivity have been measured for thin films of indium and tin in the thickness range of 585 Å to 3600 Å, The measurements were made in the region close to the transition temperature, T[subscript c] . The critical current measurements on a 585 Å thick indium film are the first reported which combine the use of a compensated geometry avoiding the difficulties associated with specimen edges, and fast rising current pulses in which the transition is not obscured by specimen heating. The fast current pulses used had a rise time of 7 nanoseconds. It was found that the temperature dependence of the critical currents in the region near the transition temperature, 0 ⋜ ΔT ⋜ 0.15°K is in. agreement with the Ginzburg-Landau theory. For a number of films the critical currents were measured using pulses having a rise time of 1.2 microseconds. The critical currents have been found to vary linear 1 with the film thickness, in agreement with the G-L theory. Measurements with fast pulses seem to indicate the existence of a transition delay of about 7 nanoseconds, independent of the current amplitudes. The transition from the superconducting to the normal state immediately following the transition delay appears to be very fast. For the critical field data, the temperature dependence is in accordance with the G-L theory in the range 0⋜ΔT ⋜ 0.3°K for both indium and tin films. The effective penetration depth calculated in the manner of Ittner, and Douglass and Blumberg was found to be dependent on the thickness and the mean free path. The critical magnetic fields were found to be inversely proportional to the film thickness in agreement with G-L theory. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A superconducting thin film tunnel junction (Sn-Sn0₂ -Sn) of total thickness 4000 Å, area 7 x 10ˉ⁴ cm² and normal (4.2 K) resistance 77 mΩ was prepared on a glass substrate. When cooled to 1.2 K the junction was biased at 0.3 mV where, the Josephson supercurrent having been suppressed with a magnetic field, the junction dynamic resistance had its maximum value of 9.3Ω . The junction was then bombarded with 5.1 MeV alpha particles and the resulting pulses induced in the tunneling current were observed to have amplitudes up to 19 times the preamplifier-dominated rms output noise level. For purposes of analysis, it was assumed that the induced current pulse had the form i(t) = i₀ exp(-t/Ƭ), t ≥ 0. With this form of the current pulse and the known transfer function of the transmission line-amplifier system, it was calculated that for all pulses T = (1.38±.33)xl0ˉ⁷ sec and that for the largest amplitude pulses, corresponding to an energy loss ΔE⍺ ≤2.75 MeV, i₀ lay in the range 20 ≤ i₀ ≤ 26 μA with a most probable value of 22 μA. With this value of i₀ and ΔE⍺ = 2.75 MeV, an upper limit of 8.2 x 10ˉ³ eV has been assigned to the value of w(Sn), the average energy expended by the alpha particle to excite a quasiparticle pair in superconducting tin at 1.2 K. A tentative theory of the superconducting tunnel junction charged particle detector is given and the cryogenic and electronic apparatus required for the measurements are described. Details related to thin film junction fabrication technology and interpretation of dc experimental results are discussed in four appendices. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A study has been made of some energy and momentum properties associated with the electrons of superconducting indium. In the experiments, an electric current was induced in the indium sample and measurements were made to detect any effects of an increasing super-state electron density on the current. By lowering the sample's temperature appropriately, the electron density could be controlled. The indium sample studied was in the form of a thin film, constructed by vacuum evaporation onto a glass substrate. Two indium wires connected the ends of the thin film to a copper wafer, forming a complete electric circuit. This circuit was electrically isolated and measurements of the current through the thin film were made with a search-coil coupled to the current's magnetic field. A relationship between the current changes observed and some momentum properties of superstate electrons was then established. The assumption that all "virtual pairs" in a superconductor have a common momentum proved consistent with the experimental results. Theoretical calculations are given which suggest that only those normal electrons with a preferred momentum can take part in increasing the superconductive electron density. This implies that a superstate electron system may effect the transformation of internal energy into work. However, the experiments carried out have not been sufficiently sensitive to show this conclusively. The inference stated above is based on the experimental results in conjunction with the required agreement of theoretical calculations with accepted theory. The indium-copper junction resistance had a resistance value which was markedly temperature dependent in the region below 3.4°K (the transition temperature of pure indium). A variation of resistance between the extremes 9.4 x 10⁻¹º Ω at 3.1°K and 8 x 10⁻¹º Ω at 1.2°K was found. The cause of this large resistance change is ascribed to various effects but probably the most important one is the diffusion of copper impurity into the indium. Arguments in favor of this explanation are given. A number of suggestions are also included which may be helpful to the design of experiments similar to the one reported in this thesis. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Intercalation of graphite with alkali metals has previously been shown to, in some cases, produce superconducting compounds from the two non-superconducting components. The use of graphene as a basis to continue this research offers new possibilities as confinement of intercalant species is reduced from bulk graphite. Papers comprised of exfoliated graphene flakes were doped with Li, Cs, K and Ca atoms via vapour transport methods in order to investigate superconducting properties. While Li, Cs and K-doping showed no signs of a superconducting transition as low as 1.8 K, observed through magnetic measurements, Ca-doped graphene became superconducting below 6.4 K – a lower transition temperature than Ca-doped graphite, TC = 11.5 K. The carrier concentration could also be changed using composite papers made from graphene and various proportions of insulating boron nitride flakes, allowing TC to be varied. Optical reflectivity spectra were used to determine the level of doping present in each compound, directly calculated from their estimated plasmon energy. Ca-doped graphene paper exhibited a 20% lower carrier concentration than Ca-intercalated graphite, offering an explanation for the lower value of TC. To allow insight into the partial doping of graphene papers, samples were exposed to air and monitored via dynamic x-ray diffraction techniques and optical analysis during degradation. With prolonged reaction in air, the carrier concentration was found to drop monotonically, while the interlayer separations contracted as intercalant species vacated the structure, leaving an arrangement of flakes similar to that of the initial, un-doped, graphene paper. The range of carrier concentrations observed suggests that doping of graphene flakes is non-discrete, thus implying tunable TC.

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Thesis advisor: Kenneth S. Burch / In condensed matter physics we study the behavior of crystals at finite density and low temperatures. By tuning and breaking the various materials, symmetries, and the topology of a crystal one can bring about brand new quantum phases of matter. These new phases of matter in turn produce emergent quasiparticles such as the cooper pair in superconductivity, the spinon in magnetic systems, and the Fermi arcs in Weyl semimetals. Of particular interest are systems in which superconductivity interacts with topology. These systems have been theoretically predicted to produce anyonic quasiparticles which may be used as qubits in a future fault-tolerant quantum computer. However, these ideas usually require the use of the superconducting proximity effect to inject cooper pairs into the topological system. This in turn requires interfacing two different materials which not only requires extremely clean interfaces, but also matching Fermi surfaces, comparable Fermi velocities, and more. The ideal candidate for topological superconductivity would therefore be a material that is both superconducting and topologically non-trivial. One promising candidate is the iron-based superconductor FeTe(1−x)Sex, specifically at the FeTe0.55Se0.45 (FTS) doping which also has non-trivial topology. In this dissertation, we address the fabrication of pristine interfaces using a new tool as well as new probes into the topology of FTS. In Chapter II we discuss the motivation, construction, and use of the “cleanroomin-a-glovebox”. This tool places an entire nanofabrication workflow into an inert argon atmosphere which has allowed us access to study a myriad of new materials and systems. A delightful offshoot of this glovebox is that it is a useful tool in training new scientists in fabrication techniques. The photolithography, Physical Vapor Deposition (PVD), and characterization tools in the glovebox are designed to be easy to use and thus afford new users a low-risk method of learning new techniques. In chapter III we discuss a specific example of a new quantum phase of matter e.g. topological superconductivity in FTS. There, I discuss the fabrication requirements to probe this elusive phase as well as the unique measurement technique used to provide evidence that FTS is a higher-order topological superconductor. The characterization of FTS continues in Chapter IV where we reveal some exciting new results in the FTS system. These new results are direct evidence for the topological nature of FTS, a feat which has only been shown in Angle-Resolved Photo Emission Spectroscopy (ARPES) and Scanning Tunneling Microscopy (STM). Chapter V concludes the dissertation with a summary of Chapters II, III, and IV. In addition, we give suggestions for future experiments to investigate the FTS system further as well as suggestions for insightful teaching programs with the cleanroom-in-a-glovebox. / Thesis (PhD) — Boston College, 2021. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

This dissertation is the beginning of the development of a microscopic theory of the Knight shift. The Knight shift experiment has been used in superconductivity research throughout history, however, a complete understanding of the Knight shift in conventional as well as unconventional superconductors does not yet exist. Motivated by the results of a literature review, which discusses Knight shift anomalies in multiple superconducting materials, this research studies a new model of the Knight shift, which involves the processes involved in nuclear magnetic resonance measurements in metals. The result of this study is a microscopic model of nuclear magnetic resonance in metals. The spins of the spin-1/2 local nucleus and its surrounding orbital electrons interact with the arbitrary constant ${\bf B}_0$ and perpendicular time-oscillatory magnetic inductions ${\bf B}_1(t)$ and with each other via an anisotropic hyperfine interaction. An Anderson-like Hamiltonian describes the excitations of the relevant occupied local orbital electrons into the conduction bands, each described by an anisotropic effective mass with corresponding Landau orbits and an anisotropic spin ${\bf g}$ tensor. Local orbital electron correlation effects are included using the mean-field decoupling procedure of Lacroix. The metallic contributions to the Knight shift resonance frequency and linewidth shifts are evaluated to leading orders in the hyperfine and Anderson excitation interactions. While respectively proportional to $(B_1/B_0)^2$ and a constant for weak $B_0 > > B_1$, both shifts are shown to depend strongly upon ${\bf B}_0$ when a Landau level is near the Fermi energy.

We take a model for Ferromagnetic Superconductors based on a variational energy functional, and search for radially symmetric minimizers. First we define what it means for a solution to the Euler-Lagrange equations to be admissible, before relating these admissible solutions to an appropriate function space. We then use a variational approach to prove the existence of minimizers. Since it is not clear at first whether or not the energy is bounded below, the direct method of the calculus of variations does not apply. Instead, we first prove existence in a case where the energy is bounded below, namely when the Zeeman coupling constant g vanishes. We then use the implicit function theorem to prove the existence of physically relevant minimizers for small values of g. / Thesis / Master of Science (MSc)

In this thesis, we use Muon Spin Rotation, Relaxation, and Resonance (μSR) as a probe for three frustrated pyrochlore systems; Gd2ScNbO7 (GSNO), Nd2ScNbO7 (NSNO) and Sm2Ti2O7 (STO), as well as the type-I superconductor BeAu. We grew all of the pyrochlore samples at McMaster using the Optical Floating Zone method. We make use of Direct Current (DC) and Alternating Current (AC) susceptibility, powder x-ray diffraction and Laue x-ray diffraction to characterize our samples. We make use of AC susceptibility measurements to explore the dynamics of the classical spin-ice Dy2Ti2O7 (DTO) and find that the system acts as a supercooled magnetic liquid, analogous to glassforming dielectric liquids. We find GSNO is a dense spin-glass based on our μSR and AC susceptibility measurements. NSNO is a moment fragmentation candidate where spin-ice, as well as all-in all-out magnetic ordering, are observed simultaneously. Our μSR measurements on this material show a strong similarity to another moment fragmentation candidate, Nd2Zr2O7, suggesting NSNO may be in a similar state. STO is a closely related compound that fully orders into a magnetic state which we study using μSR. We find subtle evidence of this magnetic transition along with persistent spin dynamics which we suggest has a common, but as of yet unexplained, origin as other frustrated pyrochlores measured in μSR. Finally, we use μSR to measure the temperature dependence of the critical field in the type-I superconductor BeAu. Using an ellipsoid of BeAu and a pressure cell, we study the magnetic properties of the sample under pressure. / Thesis / Candidate in Philosophy / In this thesis, we use Muon Spin Rotation, Relaxation, and Resonance (μSR) as a probe for three frustrated pyrochlore systems; Gd2ScNbO7 (GSNO), Nd2ScNbO7 (NSNO) and Sm2Ti2O7 (STO), as well as the type-I superconductor BeAu. We grew all of the pyrochlore samples at McMaster using the Optical Floating Zone method. We make use of Direct Current (DC) and Alternating Current (AC) susceptibility, powder x-ray diffraction and Laue x-ray diffraction to characterize our samples. We make use of AC susceptibility measurements to explore the dynamics of the classical spin-ice Dy2Ti2O7 (DTO) and find that the system acts as a supercooled magnetic liquid, analogous to glassforming dielectric liquids. We find GSNO is a dense spin-glass based on our μSR and AC susceptibility measurements. NSNO is a moment fragmentation candidate where spin-ice, as well as all-in all-out magnetic ordering, are observed simultaneously. Our μSR measurements on this material show a strong similarity to another moment fragmentation candidate, Nd2Zr2O7, suggesting NSNO may be in a similar state. STO is a closely related compound that fully orders into a magnetic state which we study using μSR. We find subtle evidence of this magnetic transition along with persistent spin dynamics which we suggest has a common, but as of yet unexplained, origin as other frustrated pyrochlores measured in μSR. Finally, we use μSR to measure the temperature dependence of the critical field in the type-I superconductor BeAu. Using an ellipsoid of BeAu and a pressure cell, we study the magnetic properties of the sample under pressure.