Performance and energy consumption of large-scale computing infrastructures could be improved significantly by adoption of superconducting electronics. Rapid single flux quantum (RSFQ) circuits with logic based on the vortex state in Type II superconductors were first demonstrated several decades ago and as of now, research focusing on developing modern superconducting electronic components operating on the level of a single Abrikosov vortex is ongoing, where RSFQ serves as a convenient reference point. The main focus of this project was one crucial aspect for such components, namely reliable manipulation of vortex dynamics under transport current. A set of realistic and relevant micron scale geometries of superconducting thin films with a circular vortex trap were simulated by obtaining numerical solutions of time-dependent Ginzburg-Landau equations in two spatial dimensions. Within the scope of this work, successful manipulation of vortex dynamics means being able to perform each of the following actions on-demand: introduce one vortex into the device, pin it to the trap, remove it from the device. Current and time-dependent behaviour of vortices in different device geometries was studied and an important role of seemingly small changes in geometric parameters was established. Certain geometric configurations were discovered to be inherently more favourable for deterministic control of vortex dynamics, while others were identified as inherently unfavourable. Practically conceivable methods to separate between the two were introduced. Repeated sequences of "write" (trapping a vortex) and "erase" (removing a vortex) operations with square waves of transport current tailored to a particular device geometry were simulated as a demonstration of successful vortex manipulation. Findings of this thesis are expected to improve the success rate of physically conducted experiments within the area of superconducting vortex-based electronics.

This report presents the redesign of the Ultra-Low Energy Implanter (LEION) at the Tandem Laboratory. The primary objectives were to evaluate the current configuration of LEION, identify potential improvements through simulations, and enhance the control software, SIMBA. The studyinvolved detailed simulations of ion trajectories within the LEION system using the SIMION software, focusing on the steering plates and the sample holder. Key findings highlighted limitationsin the current setup, including the steering plates configuration. Which were addressed througha proposed new design that would solve many of LEION’s current shortcomings by incorporating an additional electromagnetic lens into the system. Upgrades to the SIMBA interface aimed tointegrate additional functionalities, streamline beam alignment, and consolidate control systemsto improve user accessibility. The redesigned system demonstrated improved performance insimulations, providing a more even ion distribution and enhanced usability. This project offerssignificant insights into optimising low-energy ion implantation systems and presents a frameworkfor future developments in this field.

This thesis describes experimental investigation of thin films made of diluted Pt 1-x Ni x ferromagnet alloy and Nb-Pt 1-x -Nb Josephson junctions. Such Hybrid Superconductor-Ferromagnet (S-F) Structures are of significant interest because of the new physics involved and possible applications in low temperature and spintronic devices. In many cases, such devices require components with small monodomain ferromagnetic layers, which requires development of specific nano-fabrication techniques. Pt 1-x Ni x alloy is used as the ferromagnet layer due to very good solubility of the two components which results in homogeneous diluted ferromagnet. Systematic analysis of both chemical composition, and ferromagnetic properties of Pt 1-x Ni x thin films for Ni concentrations ranging between 0 and ~70 at.% is performed. The energy-dispersive X-ray spectroscopy (EDS) technique is employed to study chemical composition of Pt 1-x Ni x thin films. To eliminate possible errors during EDS characterization, EDS is used with different electron beam energies, different electron beam incident angles and on the free standing Pt 0.59 Ni 0.41 flakes. Ferromagnetic properties of Pt 1-x Ni x thin films are analyzed by studying the anomalous Hall effect. The Curie temperature of Pt 1-x Ni x films decreases in a non-linear manner with the Ni concentration and has the onset at ~27 at.% of Ni. It is observed that the critical concentration of Ni is lower and the Curie temperature is higher than it had been observed early for the bulk PtNi alloys. The 3D Focused ion Beam Nanosculpturing is used to fabricate nanoscale S-F-S Josephson junctions providing the uniform, monodomain structure of the ferromagnet layer within the junction. The detailed studies of S-F-S Josephson junctions are carried out depending on the size of junction, thickness and composition of the ferromagnet layer. The obtained Fraunhofer modulation of the critical current as a function of in-plane magnetic field serves as evidence for uniformity of the junction properties and monodomain structure of ferromagnet layer. The junction critical current density decreases in spin glass state with increasing Ni concentration. In the ferromagnetic state the maximum current density of the junction starts to increase. The latter is attributed to switching into the pi state as a function of Ni concentration. Simultaneously it is observed that the critical current can completely disappear presumably as the result of stray fields from the F layer in contact leads. The Josephson junction is used as a phase sensitive detector for analysis of vortex states in mesoscopic superconductors. By changing the bias current at constant magnetic field the vortices can be manipulated and the system can be switched between two consecutive vortex states. A mesoscopic superconductor can thus act as a memory cell in which the junction is used both for reading and writing information (vortex).

<p>Infrared intersubband absorption of III-nitride materials has been studied rigorously due to its broad potential applications into optoelectronic devices. III-nitrides have advantages of large conduction band offset, large longitudinal-optical phonon energy, and fast intersubband relaxation time. These special characteristics make nitrides promising materials for intersubband devices in the near-infrared range. However, the existence of challenges from these materials delays the progress towards the realization of high performance nitride intersubband devices. In this document, we discuss the challenges of III-nitrides and our efforts towards high intersubband transitions strength of different nitrides, in particular non-polar m-plane AlGaN/GaN, non-polar m-plane near strain-balanced (In)AlGaN/InGaN, and polar lattice-matched InAlN/GaN. Samples are characterized by multiple methods including atomic force microscopy, high-resolution x-ray diffraction, high-resolution (scanning) transmission electron microscopy, and Fourier transform infrared spectroscopy.</p> <p>Polar c-plane AlGaN/GaN exhibits good agreement between experimental and predicted results for the intersubband transition energy. However, the lattice strain between layers caused by the lattice mismatch between materials leads to a large number of defects, affecting the vertical transport and resulting in low-quality devices. Lattice-matched InAlN/GaN was suggested as an alternative to eliminate this lattice strain, thus providing a better quality material for devices. We discuss the challenges of growing homogeneous InAlN alloys that persist after exploring a wide range of growth conditions. Additionally, the non-polar mplane AlGaN/GaN is also being investigated. Low Al-composition m-plane AlGaN/GaN experimental intersubband absorption shows good agreement with the theoretical results. As the Al composition exceeds 60%, however, the m-plane AlGaN alloy becomes kinetically unstable during plasma-assisted molecular beam epitaxy growth, resulting in unique nanostructures that affect the intersubband transition energy and linewidth. For the first time, we reported the ISBA energy of near strain-balanced non-polar m-plane (In)AlGaN/InGaN heterostructures in the mid-infrared range with narrow linewidths comparable to tdth-half-max published in the literature for non-polar m-plane AlGaN/GaN superlattices. Additionally, we propose polar near lattice-matched Sc0.15Al0.85N/GaN as an alternative to c-plane lattice-matched InAlN/GaN. </p>

Syftet med denna avhandling är att bidra till forskningen av befintliga TMD:er (på Engelska transition metal dichalcogenide-TMD) som visar laddningstäthetsvågor och supraledning vid låga temperaturer (som 2H-TaS2). 2H-TaS2 är också känt för att visa supraledning vid 2K. Dessutom kommer en betydande del av denna avhandling att ägnas åt analysen av den elektronstrukturen nära Ferminivån av Cu-interkalerad TiSe2 och speciellt dess laddningstäthets-beteende vid temperaturer under 200K. Medan de teoretiska modellerna överlåts till teoretikerna, är följande sidor tillägnad att ge ett kvalitativt perspektiv på materialen. Avhandlingen är uppdelad i fyra huvudavsnitt: grundläggande begrepp, experiment-ella tekniker, tidigare rön och dataanalys. Det första avsnittet syftar till att introducera de viktigaste relevanta begreppen för att spåra de många möjliga fenomen som händer i bulk-TMD, speciellt Fe-dopade TaS2 i 2H-fasen och Cu-interkalerade TiSe2 i 1T fas. Elektronisk dispersion i fasta ämnen kommer att diskuteras på ett inledande och fenomenologiskt sätt utan rigorösa härledningar och ska hjälpa läsaren att förstå kapitlen därefter. / Motivation: The family of transition metal dichalcogenides (TMDs) has captured the fascination of researchers worldwide due to their remarkable properties and vast potential for various applications. These 2D materials exhibit a wide range of electronic, optical, and mechanical characteristics, making them incredibly versatile. From semiconductors to superconductors, TMDs offer a rich playground for exploration in condensed matter physics and materials science. Their unique properties are paving the way for breakthroughs in electronics, optoelectronics, energy storage, and beyond. As we delve deeper into the world of TMDs, we uncover new opportunities to revolutionize technology and enhance our understanding of the fundamental principles governing the behavior of matter. Joining the journey of discovery within the TMD family promises exciting challenges and the potential to contribute to the forefront of scientific and technological advancement.  The aim of this thesis is to add to the canon of existing TMDs that display charge density waves and superconductivity at low temperatures (like 2H-TaS2). 2H-TaS2 is also known to display superconductivity at 2K. Additionally, a substantial part of this thesis will be dedicated to the analysis of the electronic structure near the Fermi level of Cu-intercalated TiSe2 and especially its charge-density behaviour at temperatures below 200K. While the theoretical models are left to the theoreticians, the following pages are dedicated to giving a qualitative perspective on the materials. Thesis Outline: The primary goal of this thesis is to provide an introduction to both widely utilized and cutting-edge experimental setups employed by physicists worldwide. This will enable the acquisition of practical experience, facilitating the mastery of best practices and analysis techniques within the realm of experimental condensed matter physics. The thesis is split into four main sections: fundamental concepts, experimental techniques, previous findings and data analysis. The first section is occupied to introduce the main relevant concepts to trace the many possible phenomena happening in bulk TMDs, specifically Fe-doped TaS2 in the 2H phase and Cu-intercalated TiSe2 in the 1T phase. Subjects such as electronic dispersion in solids will be discussed in a rather introductory and phenomenological manner without rigorous derivations and shall aid the reader in understanding the chapters thereafter.

Nonlocal equations for coupled heat and mass transport are developed within the Green-Kubo formalism. Nonlocal thermal transport in Lennard-Jones solids is computed to establish the existence of semi-ballistic transport. Deviations from the diffusive theory are shown by comparing the Fourier transform of the response function from the nonlocal theory to that of the diffusive one. It is shown that the deviations from the local theory correspond to acoustic phonons, whose frequency dependence gives rise to the observed deviations from the local theory.

<p>In this thesis, we investigate the properties of a model of an anti-phase modulated d-wave superconductor, particularly in the presence of a magnetic field. This so-called model of $\pi$-striped superconductor has been proposed to describe the decoupling between Cu-O planes in $1/8$ doped La$_{2-x}$Ba$_{x}$CuO$_{4}$. The d-wave superconducting order parameter in a $\pi$-striped superconductor oscillates spatially with period 8 and zero average value. Unlike a uniform d-wave superconductor, this model has non-zero density of states at zero energy and exhibits an extended Fermi surface. Within Bogoliubov-de Gennes theory, we study the mixed state of this model and compare it to the case of a uniform d-wave superconductor. We find a periodic structure of the low-energy density of states, with a period that is proportional to $B$, corresponding to Landau levels that are a coherent mixture of particles and holes. These results are also discussed in the context of experiments which observe quantum oscillations in the cuprates.</p> <p>Furthermore, within Bogoliubov-de Gennes theory, a semiclassical approximation is used to study quantum oscillations and to determine the Fermi surface area associated with these oscillations in this model. The Fermi surface is reconstructed via Andreev-Bragg scattering, and the semiclassical motion is along these Fermi surface sections as well as between them via magnetic breakdown. Oscillations periodic in 1/B are found in both the positions and widths of the lowest Landau levels. The area corresponding to these quantum oscillations for intermediate pairing interaction is similar to that reported for experimental measurements in the cuprates. A comparison is made of this theory to data for quantum oscillations in the specific heat measured by Riggs et al.</p> / Doctor of Philosophy (PhD)

<p>Yb2Ti2O7 is one member of a series of magnetic compounds with the pyrochlore lattice structure. For specific types of single-ion anisotropy and exchange interactions, the geometry of the pyrochlore lattice frustrates near-neighbor interactions and coaxes a wide variety of unusual magnetic ground states from such compounds. Yb2Ti2O7 is unique among these compounds in that the source of the frustration is not immediately obvious when one considers the combination of single-ion anisotropy (XY-like) and the nature of the exchange interactions (ferromagnetic) present therein. A conventional magnetic transition was indeed initially expected based on the observation of specific heat anomaly near 200mK. However, many studies produced no signs of long-range magnetic order below this temperature. Intriguingly, above the transition, evidence for unusual two-dimensional correlations came in the form of rods of magnetic diffuse neutron scattering. This thesis contains four articles that detail the results of several neutron scattering studies on Yb2Ti2O7. The goal of these studies was to determine the nature of the static and dynamic spin correlations throughout the magnetic field vs. temperature phase diagram of Yb2Ti2O7.</p> <p>We first performed a time-of-flight neutron scattering experiment on a single crystal of Yb2Ti2O7, which we prepared using the optical floating zone method. This initial study provided a comprehensive survey of the phase diagram, including the previously unexplored response to a magnetic field. We found that the rods of diffuse scattering change qualitatively upon cooling below the temperature of the reported specific heat anomaly, showing signs for the development of short-range three-dimensional correlations. Additionally, we discovered that a relatively small magnetic field applied along the [110] direction could remove the diffuse scattering entirely, and produce sharp spin wave excitations in the inelastic channel, indicating long range spin correlations.</p> <p>We further quantified the temperature dependence of the diffuse scattering in zero-field using a triple-axis neutron spectrometer. The crossover from two-dimensional correlations to short-range three-dimensional correlations was found to begin at 400mK and reach completion near the temperature of the specific heat anomaly, ∼200mK. Our measurements of the low temperature specific heat of several single crystal samples, as well as a powder sample, revealed that significant sample-dependence of the magnetic properties exists. The single crystal samples were shown to have broader features in the specific heat at relatively low temperatures compared to the powder samples, pointing to some amount of structural disorder in the single crystals.</p> <p>To understand the nature of the structural defects in the single crystals, we compared the structure of a crushed single crystal of Yb2Ti2O7 to that of a powder sample using neutron powder diffraction. The major conclusion of that work was that the single crystal is non-stoichiometric, containing 2.3% excess ytterbium on the (non-magnetic) titanium sublattice. The introduction of additional magnetic moments into the system is expected to be the cause of the sample-dependence of the specific heat anomaly.</p> <p>Finally, we fit the spin wave dispersions in the field-polarized state, as measured by time-of-flight inelastic neutron scattering, to an effective spin-1/2 anisotropic exchange Hamiltonian. The microscopic parameters extracted from these fits place Yb2Ti2O7 close to exotic Quantum Spin Liquid phases predicted for the anisotropic spin-1/2 pryochlore model. The exchange parameters also reveal that the source of the frustration in Yb2Ti2O7 comes from the “quantum spin ice” nature of its exchange interactions.</p> / Doctor of Philosophy (PhD)

<p>The most important results in this thesis are those concerned with the levelling of a stepped film’s height profile. Films are prepared such that their height profiles are well described by a Heaviside step function and to a good approximation, they are invariant in one dimension. The temporal dependence of the levelling gives rheological information about the molecules making up the stepped films. For the range of heights that is much larger that the typical size of molecules making up the film, we use classical hydrodynamics to model the flows in these stepped films. Having measured the temporal and geometric dependence of the energy dissipation in time, we find that the hydrodynamic models are in excellent agreement.</p> / Doctor of Philosophy (PhD)

Experimental investigations of the optical properties of InGaN/GaN quantum dots are presented. A pulsed laser is used to perform time-integrated and time-resolved microphotoluminescence, photoluminescence excitation, and polarisation-resolved spectroscopy of single InGaN quantum dots under a non-linear excitation regime. The first micro-photoluminescence results from InGaN/GaN quantum dots grown on a non-polar crystal plane (11^-<sub style='position: relative;left: -.4em;'>2</sub>0) are presented. Time-resolved studies reveal an order of magnitude increase in the oscillator strength of the exciton transition when compared to InGaN quantum dots grown on the polar (0001) plane, suggesting a significantly reduced internal electric field in non-polar InGaN quantum dots. Polarisation resolved spectroscopy of non-polar InGaN quantum dots reveals 100% linearly polarised emission for many quantum dots. For quantum dot emissions with a polarisation degree less than unity, a fine structure splitting between two orthogonal polarisation axes can be resolved in an optical setup with a simple top-down excitation geometry. A statistical investigation into the origins of spectral diffusion in polar InGaN quantum dots is presented, and spectral diffusion is attributed to charge carriers trapped at threading dislocations, and itinerant and trapped carriers in the underlying quantum well layer which forms during the growth procedure. Incorporating quantum dots into the intrinsic region of a p-i-n diode structure and applying a reverse bias is suggested as a method to reduce spectral diffusion. Coherent control of the excited state exciton in a non-polar InGaN quantum dot is experimentally demonstrated by observation of Rabi rotation between the excited state exciton and the crystal ground state. The exciton ground state photoluminescence is used as an indirect measurement of the excited state population.