A superconducting nanowire single-photon detector (SNSPD) is an emerging, and today commercially available technology, for photon-counting and quantum cryptography. Yet, the photon detection event is not fully understood and current modeling efforts require substantial computational resources which motivates studies of simpler models.  This thesis introduces a model for vortex dynamics in thin-layered superconductors, such as SNSPDs, using a simplified approach, which leads to a 2D Coulomb gas model where the vortices are modeled as electrostatic charges. The model is carefully constructed from the method of images to describe a wire with open boundary conditions and an applied supercurrent. Subsequently, equilibrium and non-equilibrium properties are sampled with the Metropolis-Hastings algorithm and further analyzed and discussed. The suggested model is shown to be effective and successfully reproduces expected SNSPD behavior; most importantly critical behavior and voltage pulses which are directly measured during detection events. In conclusion, a 2D Coulomb gas model can be a preferred alternative for modeling vortex dynamics in SNSPDs at a small computational cost, motivating further development and studies. / Supraledande nanotråd-enfotondetektorer (SNSPD:er) är en framväxande och idag kommersiellt tillgänglig teknologi som används för räknande av fotoner samt inom kvantkryptografi. Ändå är fotondetektionshändelsen inte helt förstådd och de nuvarande modelleringar kräver substantiell datorkraft vilket motiverar studier av enklare modeller. Det här examensarbetet introducerar en model för vortexdynamik i tunnskiktade supraledare såsom SNSPD:er genom ett förenklat tillvägagångssätt som leder till en 2D Coulombgas-modell där ett vortex modelleras som en elektrisk laddning. Modellen är noggrant konstruerad med öppna randvillkor och den så kallade frysta spegelbildsmetoden samt en pålagd superström. Då samlas mätvärden in på jämvikts- samt icke-jämviktsegenskaper hos systemet som vidare analyseras, jämförs och diskuteras. Den föreslagna modellen visas vara effektiv och reproducerar framgångsrikt förväntat beteende hos SNSPD:er; framför allt kritiskt beteende och spänningstoppar som direkt uppmäts i en fysisk detektionshändelse. Sammanfattningsvis, kan en 2D Coulombgas-modell vara ett föredraget alternativ för att modellera vortexdynamik hos SNSPD:er för en liten beräkningskostnad, vilket motiverar fler studier av detta.

Theoretical aspects of multicomponent superconductivity and systemswith competing interactions are studied using Monte Carlo techniques.Motivated by recent experimental and theoretical results of complex struc-ture formation of vortices in multicomponent systems, possible vortex struc-ture formations due to vortex interactions that are not purely attractive orrepulsive are considered. Vortex structures such as clusters, superclusters,hierarchical structure formation, stripes, gossamer patters, glassy phases, aswell as checkerboard lattices and loops are demonstrated to be possible.The order of the superconducting phase transition is considered for multi-component lattice London superconductors. The phase transition is demon-strated to be either rst-order or continuous depending on the strength of asymmetry-breaking Josephson intercomponent interaction. It is argued thatthe rst-order phase transition is caused by a vortex phase separation due toa uctuation-induced attractive interaction between vortex lines. / <p>QC 20151117</p>

No description available.

Continued pursuit of better superconducting devices and an understanding of how the focused ion beam evolves in a complex material are the primary motivations behind this work. The materials of interest are MgB2 and Co-doped Ba122. Superconducting properties of MgB2 were discovered in 2001. It is the first superconductor recognized as a multigap superconductor. Owing to its high Tc of ~39K, electronic circuits based on this material are expected to operate at a much higher temperature (~25 K) than low-temperature superconductors, using compact cryocoolers. Co-doped Ba122 is also a multigap superconductor which belongs to Fe-based superconductor (FeSC) family. The undoped Ba122 compound is a metal exhibiting antiferromagnetism which coexists with superconducting phase up to a certain doping level. The optimally electron-doped BaFe2As2 exhibits the transition temperature Tc of ~21 K which corresponds to the top of the “dome” in the phase diagram. While the Fe-based SC may not signify a particular advance in terms of practical applications, many unique aspects make them worth studying. In particular, the superconducting gap symmetry and structures which appear to be quite different from family to family and not yet fully understood. We report on investigating the normal-state, and superconducting properties of Co-doped BaFe2As2 and MgB2 thin films irradiated at room temperature using a 30-keV focused He+ ion beam in helium ion microscope (HIM). R-T measurement was carried out to extract the dose dependence for Tc and resistivity p0 of the irradiated region. We observed an increase in p0 and a decrease in Tc down to complete suppression of superconductivity for both materials, although the trend of the changes was quite different. In addition, for Ba122, the data for ΔTc ⁄ Tc0 versus measured change in resistivity favors s± over s++ symmetry. Using TRIM software, the projected range and the damage density distribution of the He+ ions were tracked in the samples. Single track irradiation sites for MgB2 sample were characterized using FIB extraction/TEM. The TEM micrographs reveal the subsurface damage density contours that evolve with increasing dose. The Josephson effect is a unique phenomenon that gives direct access to the phase difference �� of the macroscopic wave functions that describe the superconducting state. Josephson junction is also appealing for engineering application in superconducting electronics. Having found the dose at which complete suppression of Tc occurs from the first part of the study, a fabrication process was developed to produce planar Josephson junctions from MgB2 and Co-doped Ba122. The Josephson coupling across the barrier for both materials was observed. MgB2 Josephson junctions showed resistivity shunted junction (RSJ) I-V curve with excellent uniformity and reproducibility. We have also demonstrated tens of planar MgB2 Josephson junctions operating coherently in series arrays. 60 Josephson junction series arrays successfully developed with less than 4% spread in critical current at 12 K. Under microwave radiation, flat giant Shapiro steps up to 150 μA width appear at voltages Vn=NnΦ0f, where N is the number of junction in the array, �� is an integer representing Shapiro step index, and f is the applied microwave frequency. The uniformity and close spacing of JJs in the arrays are significantly better than MgB2 multi-junction devices made by other techniques. It has been a huge success in showing the feasibility of this technology for pursuing superconducting digital electronics, Josephson voltage standards and arbitrary function generators in particular, in MgB2 with ≥ 20K operating temperature. / Physics

Thesis (PhD)-- Stellenbosch University, 2013. / ENGLISH ABSTRACT: Superconducting electronics can make the Square Kilometre Array (SKA) a better instrument. The largest radio telescope in the world will consist of several arrays, the largest of which, consisting of more than 3000 dishes, will be situated primarily in South Africa. The ambitions of the SKA are grand and their realisation requires technology that does not exist today. Current plans see signals in the band of interest ampli ed, channelised, mixed down and then digitised. An all-digital frontend could simplify receiver structure and improve its performance. Semiconductor (analog-to-digital converters) ADCs continue to make great progress and will likely nd applications in the SKA, but superconductor ADCs bene t from higher clock speeds and quantum accurate quantisation. We propose a superconducting softwarede ned radio frontend. The key component of such a frontend is a superconducting ash ADC. We show that employing such an ADC, even a small- to moderately-sized one, will signi cantly improve the instantaneous bandwidth observable by the SKA, yet retain adequate signal-to-noise ratio so as to achieve a net improvement in sensitivity. This improvement could approach factor 2 when compared to conventional technologies (at least for continuum observations). We analyse key components of such an ADC analytically, numerically and experimentally and conclude that fabrication of such an ADC for SKA purposes is certainly possible and useful. Simultaneously, we address the power requirements of high-performance computing (HPC). HPC on a hitherto unprecedented scale is a necessity for processing the vast raw data output of the SKA. Utilising the ultra-low-energy switching events of superconducting switches (certain Josephson junctions), we develop rst demonstrators of the promising eSFQ logic family, achieving experimentally veri ed shift-registers and deserialisers with sub-aJ/bit energy requirements. We also propose and show by simulation how to expand the applicability of the eSFQ design concept to arbitrary (unclocked) gates. / AFRIKAANSE OPSOMMING: Supergeleier-elektronika kan 'n beter instrument maak van die \Square Kilometre Array" (SKA). Die wêreld se grootse radioteleskoop sal bestaan uit etlike skikkings, waarvan die grootste - met meer as 3 000 skottels - hoofsaaklik in Suid-Afrika gesetel sal wees. Die SKA is ambisieus en vereis tegnologie wat nog nie vandag bestaan nie. Volgens huidige planne sal seine in die band van belang versterk, gekanalisieer, afgemeng en dan versyfer word. 'n Heel-digitale kopstuk sal die ontvangerstruktuur kan vereenvoudig en sy prestasie kan verbeter. Halfgeleier analoog-na-digital omsetters (ADOs) verbeter voortdurend en sal waarskynlik toepassings in die SKA vind, maar supergeleier ADOs trek voordeel uit hoër klok spoed en kwantumakkurate kwantisering. Ons stel 'n supergeleier sagteware-gede nieerde radio kopstuk voor. Die sleutelkomponent van so 'n kopstuk is 'n supergeleier \ ash" ADO. Ons toon hoe die gebruik van so 'n ADO, selfs een van klein tot matige bisgrootte, die oombliklike bandwydte waarneembaar deur die SKA aansienlik sal verbeter en 'n voldoende sein-tot-ruis verhouding sal behou, en gevolglik 'n netto verbetering in sensitiwiteit sal bereik. Hierdie verbetering kan, vergeleke met konvensionele tegnologie, 'n faktor van 2 nader (ten minste vir kontinuum waarnemings). Ons analiseer belangrike komponente van so 'n ADO analities, numeries and eksperimenteel en lei af dat die vervaardiging van so 'n ADO vir SKA doeleindes beide moontlik en nuttig is. Terselfdertyd spreek ons die drywingsverkwisting van Hoë-verrigting rekenaars aan. Sulke rekenaars van 'n tot dusver ongekende skaal is 'n noodsaaklikheid vir die verwerking van die enorme rou data uitset van die SKA. Deur die gebruik van die ultra-lae-energie skakels van supergeleier skakelaars (sekere Josephson-vlakke), ontwikkel ons die eerste demonstratiewe hekke van die veelbelowende eSFQ logiese familie, en toon eksperimenteel bevestigte skuifregisters en deserieëliseerders met sub-aJ/bis energievereistes. Ons stel verder voor en wys met simulasies hoe om die toepaslikheid van die eSFQ ontwerpkonsep na arbitr^ere (ongeklokte) hekke uit te brei.

No description available.

Nous présentons une contribution à la modélisation tridimensionnelle des phénomènes électromagnétiques et thermiques couplés dans les matériaux supraconducteurs à haute température critique. La méthode des volumes finis est adoptée comme méthode de résolution des équations aux dérivées partielles caractéristiques aux phénomènes physiques traités. Le couplage électromagnétique thermique est assuré par un algorithme alterné. L’ensemble des modèles mathématico-numériques ainsi développés et implémentés sous Matlab, sont appliqués pour étudié le comportement des supraconducteurs dans le cadre des paliers magnétiques et pour étudier le comportement des supraconducteurs durant le processus d’aimantation. Les résultats à caractère magnétique et ceux à caractère thermique sont largement présentés. La validité du travail proposé est atteinte par comparaison des résultats ainsi obtenus à ceux donnés par l’expérimentation. / We present a contribution for three-dimensional modeling of coupled electromagnetic and thermal phenomena in high temperature superconductor. The control volume method is used for the resolution of the partial derivative equations characterising of the treated physical phenomena. The electromagnetic and thermal coupling is ensured by an alternate algorithm. All mathematical and numerical models thus developed and implemented in Matlab software, are used for the simulation. The results in magnetic term and those in thermal term are largely presented. The validity of the suggested work is reached by the comparison of the results so obtained to those given by the experiment.

Die untersuchten Ferekristalle sind neuartige Verwachsungs-Schichtverbindungen aus m Monolagen von Niobdiselenid (NbSe2), die wiederholt mit n atomaren Bilagen von Bleiselenid (PbSe) oder Zinnselenid (SnSe) geschichtet sind. Niobdiselenid als Volumenmaterial besitzt eine Schichtstruktur und ist ein Supraleiter. Aufgrund ihrer gezielt einstellbaren atomar geschichteten Struktur können Ferekristalle als Modellsysteme für geschichtete Supraleiter dienen. In dieser Arbeit werden ihre strukturellen und elektrischen Eigenschaften untersucht. Mittels Transmissionselektronenmikroskopie wird ihre turbostratisch ungeordnete, nanokristalline Struktur nachgewiesen. Die atomare Struktur innerhalb der einzelnen Schichten ist ähnlich wie in den Volumenmaterialien NbSe2, PbSe und SnSe, wobei die kristallographischen c-Achsen parallel zur Stapelrichtung der Ferekristalle zeigen. Eine quantitative Analyse unter Verwendung eines Zwei-Schicht-Modells für den spezifischen Widerstand, Hall-Koeffizienten und Magnetwiderstand liefert ähnliche Ladungsträgersorten, -dichten und –beweglichkeiten in den NbSe2-Schichten, wie sie für isolierte Einzellagen von NbSe2 berichtet wurden. Diese unterscheiden sich von denen des Volumenmaterials NbSe2. Erstmals wurde ein Übergang der Ferekristalle in den supraleitenden Zustand nachgewiesen. Die Sprungtemperaturen sind dabei in etwa auf die Hälfte der Sprungtemperaturen der jeweiligen nicht turbostratisch ungeordneten Misfit-Schichtverbindungen reduziert. Diese Reduzierung kann der turbostratischen Unordnung der Ferekristalle zugeordnet werden. Das Verhältnis zwischen der schichtsenkrechten Ginzburg-Landau-Kohärenzlänge und dem Abstand zwischen den supraleitenden Schichten ist bei den Ferekristallen kleiner als bei den nicht ungeordneten Misfit-Schichtverbindungen, was Ferekristalle zu vielversprechenden Kandidaten für (quasi-)zweidimensionale Supraleiter macht. / The investigated ferecrystals are novel layered intergrowth compounds consisting of m monolayers of niobium diselenide (NbSe2) stacked repeatedly with n atomic bilayers of lead selenide (PbSe) or tin selenide (SnSe). Bulk NbSe2 is a layered compound showing superconductivity. Due to their artificially atomic-scale layered structure, which is tunable on the atomic scale, ferecrystals can serve as model systems for layered superconductors. In this study, their structural and electrical properties are investigated. Using transmission electron microscopy their turbostratically disordered, nanocrystalline structure is revealed. The atomic structure within the individual layers is similar as for bulk NbSe2, PbSe and SnSe, with the crystallographic c-axes parallel to the stacking direction in the ferecrystals. A quantitative analysis using a two-layer model fit for the electrical resistivity, Hall coefficient and magnetoresistance yields a similar carrier type, density and mobility in the NbSe2 layers as reported for isolated NbSe2 monolayers. These values differ from those of bulk NbSe2. For the first time, a normal-to-superconducting transition has been detected in ferecrystals. The transition temperatures of the ferecrystals are reduced to about a half of those of analogous non-disordered misfit layer compounds. This reduction in transition temperature can be correlated to the turbostratic disorder in ferecrystals. The ratio between the cross-plane Ginzburg-Landau coherence length and the cross-plane distance between the NbSe2 layers for the ferecrystals is lower than for non-disordered misfit layer compounds, making ferecrystals promising candidates for (quasi-)two-dimensional superconductors.

Matériaux dans lesquels les électrons responsables des propriétés de basse énergie son soumis à fortes corrélations sont aujourd'hui très étudiés à la recherche de nouvelles phases émergentes aux propriétés surprenantes et/ou utiles.Les supraconducteurs à base de fer (IBSC) sont maintenant considérés dans cette classe de composés. En utilissant des techniques multi-corps nécessaires pour le traitement théorique de ces corrélations (théorie du champ moyen de spin esclave - SSMFT et théorie du champ moyen dynamique - DMFT - en conjonction avec la théorie du fonctionnelle de la densité, DFT), dans cette thèse, j'etudie plusieurs propriétés d'IBSC.D’abord, j'analyse les composés très dopés de la famille de IBSC, qui montrent expérimentalement certains comportements typiques des ``fermions lourds'', des composés typiquement des terres rares ou des actinides, où des électrons extrêmement corrélés coexistent avec des électrons moins corrélés. En particulier je me concentre sur la chaleur spécifique et le pouvoir thermoélectrique et je montre comment ces propriétés peuvent être comprises dans le paradigme récemment développé ``métaux de Hund''. En effet, l’échange intra-atomique (le ``couplage de Hund'') est responsable de ces matériaux à éléments métal de transition en montrant la physique des fermions lourds. Je montre aussi que les caractéristiques typiquement fermions-lourds du spectre d’excitation, connues car les singularités de Van Hove sont bien capturées par notre modélisation au sein de DFT+SSMFT. J'utilise ensuite DMFT dans un modèle afin d'étudier l'impact direct des singularités de Van Hove sur la force des corrélations.Dans une seconde partie, je montre comment FeSe, le IBSC actuellement le plus étudié, se trouve également dans une phase métal de Hund, mais il est amené à la frontière de cette phase par la pression. Cette frontière est liée à une augmentation de la compressibilité électronique qui est positivement corrélée à l’augmentation de la supraconductivité trouvée dans les expériences.Je réalise une étude analogue sur le détenteur du record pour la température supraconductrice critique la plus élevée, la monocouche FeSe où je trouve également une compressibilité augmentée. Cela appuie la récente proposition selon laquelle la frontière du métal de Hund favorise la supraconductivité à haute température.Enfin, j'étudie la nature du magnétisme dans une autre famille de IBSC, les germanides de fer. J'explore différents ordres magnétiques possibles avec des simulations DFT et leur concurrence (ce qui peut en principe favoriser la supraconductivité) dans plusieurs composés où différents substitutions sont appliquées au composé parent YFe2Ge2. J'étudie également l'effet de la pression chimique sur ce composé. / Materials where the electrons responsible for the low-energy properties experience strong correlations are today very investigated in search of emerging new phases with surprising and/or useful properties. Iron-based superconductors (IBSC) are now considered in this class of compounds. Using the many-body techniques necessary for the theoretical treatment of these correlations (slave-spin mean field theory - SSMFT- and dynamical mean field theory - DMFT- in conjunction with density functional theory, DFT), in this thesis I address several properties of IBSC.First I analyze the very hole-doped compounds in the IBSC family, that show experimentally some behaviors typical of the so-called “heavy fermions”, compounds typically of rare earth or actinides, where extremely correlated electrons coexist with others less correlated. In particular I focus on the specific heat and the thermoelectric power and show how these properties can be understood in the recently developed paradigm of “Hund’s metals”. Indeed the intra-atomic exchange (the “Hund’s coupling”) is responsible for these materials of transition metal elements showing heavy-fermionic physics. I show also that typical heavy-fermionic features of the excitation spectrum, known as Van Hove singularities are well captured by our modelization within DFT+SSMFT. I then use DMFT in a model in order to study the direct impact of the Van Hove singularities on the strength of correlations.In a second part I show how FeSe, the presently most studied IBSC, is also in a Hund’s metal phase, but it is brought to the frontier of this phase by pressure. This frontier is connected to an enhancement of the electronic compressibility which correlates positively then with the enhancement of superconductivity found in experiments. I perform an analogous study on the record holder for the highest critical superconducting temperature, the monolayer FeSe where I also find an enhanced compressibility. This supports the recent proposal that the frontier of a Hund's metal favors high-temperature superconductivity.Finally I study the nature of magnetism in another family of IBSC, the iron-germanides. I explore different possible magnetic orders with DFT simulations and study their competition (which can in principle favor superconductivity) in several compounds where different chemical substitutions are applied to the parent compound YFe2Ge2. I also study the effect of chemical pressure on this compound.

This work is about the properties of several low dimensional, small systems of interacting particles. We demonstrate that interaction between particles in the low dimensional small systems can lead to many unexpected effects. We considered electrons in a Luttinger liquid, in a superconducting state, and atoms in a magneto-optical trap. Using bosonization techniques we calculated the thermopower of a Luttinger liquid wire with an impurity. We predicted the appearance of a phase dependent force and resonant phase dependent magnetization in the nanoscopic superconductor - normal metal superconductor (or superconductor - two dimensional electron gas - superconductor) junction. We also considered plasma oscillations inside thin superconducting tubes and rings and predicted that the velocities of the plasmons in these systems are periodic functions of the magnetic flux. By considering neutral atoms in a harmonic trap we discovered that strongly repelling atoms do not form Bose-Einstein condensate at zero temperature but tend to occupy different orbitals with small mutual overlap, forming crystallite structures similar to Wigner molecules of electrons inside a quantum dot.