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<p dir="ltr">Antiferromagnetic spintronics promises next-generation information processing devices with ultra-fast speeds and ultra-low power consumption. Inspired by the recent demonstration of signatures of Tunnel Magnetoresistance (TMR) in non-colinear chiral antiferromagnets of the Mn₃X family, we study the thermal stability of such magnets in both low and high barrier limits. A stochastic Landau-Lifshitz-Gilbert (s-LLG) based numerical assessment of the dynamics reveals that strong exchange fields in Mn₃Sn could lead to thermally-driven rapid fluctuations of the order parameter, viz., octupole moment. However, distinct Random Telegraph Noise (RTN)-like signals distinguish the high barrier limit from the low barrier limit - suggesting different physical phenomena in the two regimes. To that end, the correlation time for thermal fluctuations has been explored analytically following an approach inspired by Langer's theory in the high barrier limit and dephasing mechanisms in the low barrier limit. It has been shown that the dynamics in chiral antiferromagnetic nanoparticles in both regimes are an order of magnitude faster than easy plane ferromagnetic particles. The thermal instability of chiral antiferromagnets could lead to picosecond-scale random number generation in probabilistic bits -- paving the path toward ultra-fast probabilistic computation. </p>

We perform microscopic mean-field studies of topological order in a two-dimensional topological superconductor in the Bogoliubov-de Gennes (BdG) formalism. By adopting a two-dimensional s-wave topological superconductivity (TSC) model on a minimal tight-binding system, we solve the BdG equations self-consistently to obtain not only the superconducting order parameter, but also the Hartree potential. By computing the Thouless, Kohmoto, Nightingale, and den Nijs (TKNN) number and investigating the bulk-boundary correspondence, we study the nature of Abelian and non-Abelian TSC in terms of self-consistent solutions to the BdG equations. In particular, we examine the effects of temperature and a single non-magnetic impurity deposited in the centre of the system and how they vary depending on topology. We find that the non-Abelian phase exhibits signs of unconventional superconductivity, and by examining the behaviour of this phase under both low and high Zeeman field conditions, we show that the magnitude of the Zeeman field largely dictates the susceptibility of the system to temperature. Furthermore, we investigate the possible interplay of charge density waves (CDW) and TSC. By self-consistently solving for the mean fields, we show that TSC and topological CDW are degenerate ground states---with the same excitation spectrum in the presence of surfaces---and thus can coexist in the Abelian phase. The effects of a non-magnetic impurity, which tends to pin the phase of charge density modulations, are examined in the context of topological CDW.

This thesis investigates a number of transition-metal coordination polymers and iron-pnictide superconductors through the use of high magnetic fields, low temperatures, and on occasion, high pressures. The thesis will begin by describing my development of the proximity detector dynamic susceptometer, a novel technique that can be used for magnetometery and transport measurements in high magnetic fields. This technique is highly compact and has no moving parts, making it suitable for use in pressure cells, hence opening the way for a variety of new experiments. Through high-field magnetometery and other measurements, I will demonstrate that the pressure can be used to directly control the magnetic properties of the polymeric magnet CuF₂(H₂O)₂(pyrazine). In particular, I observe a transition from quasi-two-dimensional to quasi-one-dimensional antiferromagnetism at 9~kbar, driven by the rotation of the Jahn-Teller axis. I will then present a series of measurements on two coordination polymers, showing how a small chemical difference can lead to drastically different magnetic properties. I show that [Cu(pyrazine)H₂O(glycine)₂]ClO₄ is an excellent spin-chain, while the sister compound [Cu(pyrazine)(glycine)]ClO₄ is a dimerised material that shows a spin-gap and is disordered down to very low temperatures, but then undergoes a field-induced phase transition to an ordered phase. I will also describe a series of pulsed-field measurements of the upper critical field of the iron-based superconductors NaFe_1-xCo_xAs across the whole of the doping phase diagram. It is shown that paramagnetic pair-breaking effects dominate the critical field when the field is parallel to the crystal planes. In the parent compound the paramagnetic limit is equal to that expected from BCS theory, but becomes significantly enhanced above the BCS limit upon doping. It is shown that the multi-band nature of the superconductivity leads to a convex curvature in the evolution of the critical field as the temperature is reduced.

This work is focused on the parent compounds of the Fe-chalcogenide superconductors. For this purpose poly- and single-crystalline forms of tetragonal β-FexSe, Fe1+yTe, Fe1+yTe1-xSex and Fe(1+y)-xMxTe (M = Ni, Co) have been prepared. Second focal points of this study are the low-temperature structural phase transitions and physical property changes in tetragonal Fe1+yTe which are induced by composition, external pressure, and cationic substitution.

Thesis (PhD (Physics))--University of Stellenbosch, 2002. / Includes bibliography. / ENGLISH ABSTRACT: High temperature superconductivity is an important topic in contemporary solid state physics, and an area of very active research. Due to it’s potential for application in low temperature electronic devices, the material has attracted the attention of researchers in the electronic engineering and material science fields alike. Moreover, from a fundamental point of view, several questions remain unanswered, related to the origin of superconductivity of this class of materials and the nature of quantised magnetic flux present in magnetised samples. In this work, flux creep phenomena in a thin superconducting YBa₂Cu₃O₇ film deposited by pulsed laser deposition, is investigated near the critical temperature 0 ≤ Tc – T ≤ 10 K. Creep activation energy U0 and critical current density jc were determined as a function of temperature close to Tc, providing important data to a problem of high-Tc superconductivity which is still a matter of debate. In particular it is still an open question whether restoring the temperature in a creep freezing experiment in fact restores the film to it's original state before the freezing. The most important novel results concern the regime of critical fluctuations in the vicinity Tc - T < 1 K. We studied the isothermal relaxation of trapped magnetic flux, and determined that the long time decay follows a power law, where the exponent is inversely proportional to the creep activation energy. The temperature dependence of the critical current density jc(T) of the YBa₂Cu₃O₇ film close to Tc was obtained during warming runs. It was determined that jc(T) follows a square root dependence on T to high accuracy in the range 0.2 ≤ Tc – T ≤ 1.5 K. During flux creep experiments an interesting phenomenon called creep freezing related to the strong temperature dependence of the relaxation rate was observed. A pronounced slowing of relaxation with only a small drop in temperature from a starting temperature close to Tc was detected. Experiments were conducted by initiating an isothermal flux decay run. At a certain point the temperature was slightly lowered, and the flux decay stopped within experimental accuracy. When the temperature was restored to the initial value, the flux decay resumed at the previous rate before cooling. An argument based on vortex drift velocity was employed to explain the phenomenon qualitatively. During the course of this investigation, a pulsed laser deposition (PLD) system was designed and built from scratch. PLD involves the interaction of a focussed laser pulse with a multielemental solid target material. Material ablated from the target forms a fast moving plume consisting of atomic and molecular particles, directed away from the target, and towards a usually heated substrate on which the particles condense layer by layer to form a thin film. The substrate temperature and background gas are carefully controlled to be conductive to the growth of a desired phase of the multi-elemental compound. The PLD system proved to be quite versatile in the range of materials that could be deposited. It was used to deposit thin films of different materials, most notable were good quality superconducting YBa₂Cu₃O₇, thermochromic VO2, and magnetoresistive LaxCa1-xMnO3. Metallic Au and Ag layers were also successfully deposited on YBa2Cu3O7 thin films, to serve as protective coatings. The critical temperatures of the best superconducting films were 90 K as determined by resistivity measurement. The optimal deposition conditions to deposit high quality superconducting YBa₂Cu₃O₇ thin films was found to be: deposition temperature 780°C, laser energy density 2-3 J/cm2, oxygen partial pressure 0.2 mbar, and target-substrate distance 35 mm. This yields film with Tc ~ 90 K. It was found that deposition temperature plays the predominant role in determining the quality of YBa₂Cu₃O₇ thin films deposited by PLD. / AFRIKAANSE OPSOMMING: Hoë temperatuur supergeleiding is tans ’n aktuele onderwerp van vastetoestandfisika en dit is ’n gebied van baie aktiewe navorsing. Weens die potensiaal vir toepassings van hoë temperatuur supergeleiers in elektronika, het dié klas materiale die aandag van fisici and elektronici getrek. Verskeie fundamentele vraagstukke bly steeds onbeantwoord, veral met betrekking tot die oorsprong van supergeleiding in hierdie materiale en die gedrag van gekwantiseerde magnetiese vloed (“vortekse”) in gemagnetiseerde monsters. In hierdie werk word diffusie van vortekse in dun supergeleidende YBa₂Cu₃O₇ films ondersoek naby die kritieke temperatuur (0 ≤ Tc - T ≤ 10 K). Die temperatuur afhanklikheid van die diffusie aktiveringsenergie U0 en die kritieke stroomdigtheid jc word bepaal naby Tc. Dit verskaf belangrike inligting tot probleme in hoë temperatuur supergeleiding wat tans nog onbeantwoord bly. In die besonder is dit steeds ’n ope vraag of die herstel van die aanvanklike temperatuur in ’n vloedstollings eksperiment waarlik die film tot die oorspronklike toestand herstel. Die belangrikste nuwe resultate hou verband met die gebied van kritieke fluktuasies van die orde parameter in die omgewing 0 < Tc - T < 1 K. Ons het die isotermiese ontspanning van vortekse verstrik in die kristalstruktuur bestudeer, en bepaal dat die lang tydsverval ’n magsverwantskap handhaaf, waar die eksponent omgekeerd eweredig is aan U0. Die temperatuur afhanklikheid van die kritieke stroomdigtheid jc(T) van die YBa₂Cu₃O₇ film naby Tc is bepaal tydens verhittingslopies. Daar is bevind dat naby Tc, jc ’n vierkantswortel verband met T volg, tot hoë noukeurigheid in die gebied 0.2 ≤ Tc – T ≤ 1.5 K. Gedurende vorteksdiffusie eksperimente is ’n interessante verskynsel naamlik vloedstolling (“flux freezing”) waargeneem. Dit hou verband met die sterk temperatuur afhanklikheid van die vervaltempo van die magnetiese moment van ’n gemagnetiseerde film. ’n Skerp daling van die vervaltempo, weens slegs ’n klein temperatuurdaling vanaf die begin temperatuur naby Tc, is waargeneem. Gedurende eksperimente is daar aanvanklik ’n isotermiese vloedontspanning teweeg gebring. Op ’n sekere tydstip is die temperatuur effens verlaag, waarby die vloedontspanning tot stilstand gekom het binne grense van waarneming. Wanneer die temperatuur weer herstel is na die oorspronklike, het die vloedontspanning voortgegaan teen die tempo voor die temperatuurverlaging. ’n Verklaring wat gebaseer is op vorteks dryfsnelheid was aan die hand gedoen om hierdie gedrag te verklaar. ’n Groot komponent van die projek was om die dun YBa₂Cu₃O₇ films self te vervaardig. Tydens hierdie ondersoek, is ’n gepulseerde laser deposisie (“PLD”) sisteem eiehandig ontwerp en gebou. PLD behels die interaksie van ’n gefokuseerde laser puls met ’n teiken bestaande uit ’n multi-element vastestofverbinding. Materiaal wat verdamp (“ablate”) word, vorm ’n snelbewegende pluim bestaande uit atomiese en molekulêre deeltjies. Dit beweeg vanaf die teiken na ’n verhitte substraat, waarop die deeltjies kondenseer om laag vir laag ’n dun film te vorm. Die substraat temperatuur en agtergrond gas word sorgvuldig beheer om die groei van die verlangde fase van die multi-element verbinding teweeg te bring. Die PLD sisteem is baie veeldoelig ten opsigte van die verskeidenheid materiale wat suksesvol neergeslaan kan word. Dit was aangewend om verskillende materiale neer te slaan, onder andere supergeleidende YBa₂Cu₃O₇, termochromiese VO2, en magnetoresistiewe LaxCa1-xMnO3. Geleidende Au en Ag lagies is ook suksesvol neergeslaan op YBa₂Cu₃O₇ dun films, om te dien as beskermingslagies. Die kritieke temperatuur van die beste supergeleidende films was 90 K soos bepaal deur weerstandsmetings. Die optimale neerslaan toestand vir hoë kwaliteit YBa₂Cu₃O₇ dun films was: substraat temperatuur 780°C, laser energiedigtheid 2 - 3 J/cm2, suurstofdruk 0.2 mbar, en teiken-substraat afstand 35 mm. Daar is bevind dat die substraat temperatuur die deurslaggewende rol speel tydens die neerslaan proses om die kwaliteit van die supergeleidende films te bepaal.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2007. / A major challenge in the design and operation of High Temperature Superconducting (HTS) devices is the fabrication of reproducible Josephson junctions with good IcRn products. One objective of this thesis was to fabricate successfully HTS step-edge junctions. This objective necessitated a critical evaluation of the available facilities to provide much needed improvements. These improvements included a newly optimised photolithography process, the incorporation of a three-gridded extraction system into the in-house argon ion mill as well as alterations to the Pulsed Laser Deposition (PLD) system to improve thin film quality. These process modifications finally allowed for the fabrication of novel PrBa2Cu3O7−δ buffered step-edge junctions. These junctions were tested for dc and ac Josephson effects and displayed IcRn products of 1.5 mV at 55 K as well as well-defined Shapiro steps. A second objective was to introduce a high quality thin film deposition system that could produce smooth superconducting films for use in filters and multilayer technology. An Inverted Cylindrical Magnetron system was built and optimised to grow YBa2Cu3O7−δ thin films on MgO (001) substrates. A complete optimisation process of these films are presented by utilising several growth and electrical characterisation methods such as XRD, RBS and AFM.

Thesis (MScEng)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Rapid Single Flux Quantum (RSFQ) cells are key in the design of complex and applicable RSFQ electronic circuits. These cells are low-level circuit elements that are used repeatedly to build larger, applicable RSFQ circuitry. Making these cells simple to layout and manufacture, but reliable for extensive use demands a careful development process for RSFQ cells. Cell functionality is verified through simulations, thereafter the cell is laid out in special software packages. Inductance of on-chip superconductor structures is extracted through careful modelling with numerical field solver software. A cell library has been developed by incorporating existing or published cells after further analysis and optimization, as well as developing new cells. Cells that have been adapted into the library include the Josephson transmission line (JTL), Splitter, Merger, D-Flip Flop (DFF), T-Flip Flop (TFF), NOT, AND, OR and XOR, DC-SFQ and SFQ-DC and PTL Driver and Receivers. New cells include NOR, NAND and XNOR. The cells were designed for the IPHT’s RSFQ1D 1kA/cmª and Hypres’ 4.5kA/cmª processes. The cells in the library have good bias current operating margins obtained through simulations (> ±26%). All cells have all the parameters listed in the thesis including extracted inductance values. In order to have a complete and verified RSFQ cell library, cells have been sent for fabrication at IPHT and Hypres facilities. These cells can now be tested on-chip, in the laboratory, to establish functionality and practical bias current margins. All test signal patterns and bias currents required for testing are defined to allow co-workers or collaborators to test the cells. / AFRIKAANSE OPSOMMING: "Rapid Single Flux Quantum" (RSFQ) selle is van sleutelbelang in die ontwerp van komplekse en toepaslike RSFQ elektroniese stroombane. Hierdie selle is laevlak stroombaanelemente wat herhaaldelik gebruik word om groter RSFQ bane mee te bou. Versigtige ontwikkeling is nodig om hierdie selle eenvoudig vir uitleg en vervaardiging te hou terwyl dit ook betroubaar is vir wye gebruik. Selfunksionaliteit word geverifieer deur middel van simulasies, waarna selle vir vervaardiging uitgelê word in spesiale sagtewarepakette. Induktansie van supergeleierstrukture op vervaardigde skyfies word deur versigtige modellering met behulp van numeriese veldoplossingsagteware onttrek. In hierdie tesis is ’n selbiblioteek ontwerp deur bestaande (gepubliseerde) selle verder te analiseer en optimeer, en deur nuwe selle te ontwerp om die biblioteek volledig te maak. Selle wat aangepas is vir hierdie biblioteek sluit die Josephson-Transmissielyn (JTL), Verdeler, Samevoeger, DWipkring (DFF), T-Wipkring (TFF), NIE, EN, OF en XOF, asook die DC-SFQ en SFQ-DC selle en Passiewe Transmissielyn (PTL) drywers en ontvangers in. Nuwe selle sluit die NOF, NEN en XNOF hekke in. Die selle is ontwerp en uitgelˆe vir beide IPHT se RSFQ1D 1kA/cmª en Hypres se4.5kA/cmª prosesse. Die selle in die biblioteek toon goeie voorspanningstroom-werksmarges, soos verkry deur simulasie (> ±26%). Parameters en berekende induktansies vir alle selle word in die tesis gelys vir naslaandoeleindes. Vir die daarstel van ’n volledige en geverifieerde RSFQ selbiblioteek is selontwerpe vir vervaardiging na IPHT en Hypres gestuur. Aangesien vervaardiging slegs een maal per jaar by IPHT gedoen word, is die skyfies egter nog nie beskikbaar nie. Na vervaardiging kan die skyfies egter getoets word om selfunksionaliteit in die laboratorium te meet. Ten einde hierdie toetsing vir enige medewerker te vergemaklik, word alle toetsparameters soos voorspanningstroom en intreeseinpatrone in die tesis gedefinieer.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2005. / A process was developed to design and manufacture a dc SQUID magnetometer. Superconductor theory is given as a foundation to explain the Josephson junction. This knowledge is applied to explain the ideal and practical dc SQUID. The design of the dc SQUID is done with inductance calculation formulas. Each step of the manufacturing process is discussed in detail. Many improvements have been made to the process to make each step reproducible. The steps in the manufacturing process that were done in-house are: manufacturing an YBa2Cu3O7−d pellet, depositing the thin film with the pulsed laser deposition process, creating a mask with the UV lithography process, wet etching the circuit and depositing silver contact pads with thermal evaporation. The device is packaged on a printed circuit board device holder and shielded with a mu-metal shield. A test setup is developed to test the final device. Each step in the manufacturing process was successful. The final device did not work, due to oxidisation of the YBa2Cu3O7−d thin film.

Thesis (MEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / The Josephson effect of high-temperature superconducting nanobridge structures is studied worldwide. Until now, nanobridges are generally fabricated with focused ion beam milling on planar thin films. These nanobridges are employed as weak links in superconducting quantum interference devices (SQUIDs) and used in superconducting flux flow transistors and devices. This project had two main objectives: to improve the sidewall angle of photoresist lines, with the aid of atomic force microscopy (AFM) nanolithography; and to fabricate hightemperature Josephson junctions by constricting superconductive YBCO lines on MgO substrates with AFM nanolithography. The entire fabrication process is explained including photolithography, deposition of the YBCO thin films with pulsed laser deposition (PLD), nanolithography and wet etching. Although the testing of the junctions did not show any Josephson behaviour, it could be demonstrated that nanobridge structures can successfully be created by AFM nanolithography. The entire fabrication process has been demonstrated in detail for the benefit of future research.