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Een enkel elektron draagt een elementaire lading -e. Het manipuleren van elektronen op basis van hun lading evenals de componenten die dit verwezenlijken, staan bekend onder de naam elektronica. Dit soort componenten zijn sterk geïntegreerd in de samenleving. Een voorbeeld is de transistor, die toelaat om de hoeveelheid lading die door de transistor stroomt, actief te beïnvloeden door middel van een elektrisch veld. Het elektron heeft echter nog een tweede intrinsieke eigenschap naast zijn lading, namelijk de spin. Wanneer er een as gedefinieerd is, bijvoorbeeld door het aanleggen van een magnetisch veld, dan kan deze spin twee waarden aannemen, namelijk 'op' en 'neer' wat betekent dat de spin zich respectievelijk parallel of antiparallel met de as oriënteert. Een nieuwe technologie die naast de lading van het elektron ook diens spin aanwendt voor bijvoorbeeld informatieverwerking, is in ontwikkeling en wordt 'spintronica' genoemd. Het theoretisch en experimenteel onderzoek dat in het afgelopen decennium is verricht naar spintronica, voorspelt dat deze technologie op meerdere fronten de conventionele elektronica significant kan verbeteren. In deze thesis wordt het transport van elektronspins in zogenaamde non-local spin valves (letterlijk in het Nederlands: niet lokale spin kleppen) bestudeerd. Dit is een component van enkele tientallen nanometer groot die ideaal is voor het bestuderen van spin dynamica. Deze component laat toe om spin transport te induceren en te detecteren, waarbij het spin transport gemanipuleerd kan worden door middel van elektrische en magnetische velden. De spin valve bestaat uit een ferromagnetisch en een niet-magnetisch materiaal. De ferromagnetische delen staan in voor het injecteren en het detecteren van spins, terwijl het niet-magnetisch deel de regio is waar het spin transport plaatsvindt. Deze thesis beschrijft het eerste onderzoek dat is gebeurd naar non-local spin valves (NLSV) aan de KU Leuven. De twee belangrijke resultaten van deze thesis zijn: (1) het ontwikkelen van een volledige metallische NLSV en (2) het ontwikkelen van een procedure die toelaat om fenomenen gerelateerd aan spin transport te meten in NLSV. Resultaat (1) is bereikt door voor het eerst een schaduw opdamptechniek te implementeren in het Ion and Molecular Beam Lab (IMBL) aan de KU Leuven. De metallische NLSV bestond uit kobalt ferromagneten en een niet-magnetische nanodraad van aluminium. Het spin valve effect is gedetecteerd in deze structuren, en deze succesvolle waarneming van spin transport bevestigde het slagen van de fabricatie. Een tweede type van NLSV die is onderzocht bestaat uit grafeen als niet-magnetisch materiaal en kobalt ferromagneten. Grafeen is een tweedimensionaal materiaal dat bestaat uit een rooster van koolstofatomen. Het heeft eigenschappen die het zeer geschikt maakt voor spin transport. De elektronische eigenschappen van grafeen zijn onderzocht, en ook het spin valve effect en spin precessie zijn geobserveerd in de grafeen NLSV. Uit metingen van deze twee effecten kunnen belangrijke parameters voor spin transport, zoals de spin diffusielengte en de spin levensduur, worden bepaald. Met het observeren van deze effecten in NLSV is voldaan aan doelstelling (2). Beide typen spin valves kunnen als startpunt dienen voor verder onderzoek naar spin transport (binnen de KU Leuven). Een uitzicht naar experimenten die verder bouwen op het werk beschreven in deze thesis is daarom ook beschreven.

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The discovery of superconductivity over 110 years ago has impacted both fundamental and applied research. Its unique properties of dissipationless current flow and perfect diamagnetism could not be explained by classical mechanics, instead requiring a quantum mechanical description. This was ultimately achieved by Bardeen, Cooper, and Schrieffer by developing the BCS theory. The theory explains that electrons are subjected to an attractive interaction, which results them to be in bound states called Cooper pairs. Being a boson, the Cooper pair form a condensate and populate the superconducting ground state in the superconductor. When two superconducting materials are connected by an insulator or a weak link, a Josephson junction is formed. This junction is central to the practical applications of modern day superconducting devices, due to the Josephson Effect. The Josephson Effect is manifested by two different mechanisms. (i) The DC Josephson effect is the Cooper pair tunnelling through the junction from one side of the superconducting material to the other, giving rise to dissipationless supercurrent, which depends linearly on the gauge invariant phase difference between the wavefunctions of the superconducting materials. (ii) The AC Josephson effect is the oscillation of the supercurrent when the junction is under a DC bias. Consequently, the Josephson junction can be thought of as a DC-to-AC converter, functioning as a radiation source. Conversely, the Josephson junction can be thought of as an AC-to-DC converter by displaying Shapiro steps in the V(I) curve when it is under AC bias, functioning as a radiation detector. This is also known as the inverse AC Josephson Effect. In the context of the Third Quantum Revolution, the Josephson junction has great potential to be implemented for qubit technology. Qubit manipulation and readout typically operate in the frequency range between 4-8 GHz. As a result, connections using expensive Radio Frequency (RF) cables from room temperature to cryogenic temperature are required. Moreover, the presence of large components inside the cryostat takes up space sacrifices cooling power. The Josephson junction can be a solution to these problems as an on-chip emitter and detector of radiation. A single junction has a low impedance, which prevents an easy integration with electronics. Fortunately, using a 2D array of Josephson junctions provides a better control on the impedance of the resulting device; additionally, it provides an increased power and sharper line-width, features that scale proportionally to the number of junctions in the array. This thesis studies the 2D Superconductor/Normal/Superconductor (SNS) Josephson junction array. The array consists of Molybdenum Germanium (MoGe) superconducting islands deposited on a Gold on Titanium Ti/Au thin film. The work in the thesis built on previous research done in the group on radiating superconducting arrays and aimed to (i) increase the output power and decrease line-width of the emitted radiation, (ii) investigate the reproducibility of emission by measuring two Josephson junction arrays in one sample, and (iii) perform an on-chip radiation and detection test on the same sample. These different goals were explored. Additionally, numerical simulations simulating the Josephson junction array under different biases and magnetic field were developed and conducted.

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A superconductor is an example of a so-called macroscopic quantum phenomenon where effects that normally only occur on the smallest of scales are observable on a macroscopic scale. It gives rise to the superconductors most famous property: for sufficiently low temperatures, they can conduct current without dissipation. In the case of superconductors, this is because all electrons are in exactly the same, shared state. A characteristic property that describes this common electron state is the macroscopic phase, a purely quantum mechanical property. In 1962, Brian D. Josephson considered what would happen when one places two superconductors next to each other, but separated by a thin, insulating weak link, creating a superconductor- insulator- superconductor (SIS) Josephson junction. Josephson showed that, without external stimuli, a supercurrent could run between the two superconductors whose value depends sinusoidally on the difference in phase between the superconductor macrostates; this effect was named the junctions currentphase relationship. The investigated experimental method in this thesis can potentially reveal the exact relation between the supercurrent and the phase difference, which can differ from the sine dependence when replacing the insulator with other link materials. These unconventional materials (for example a one-atom thick sheet of carbon atoms) can result in novel functionalities and applications of Josephson junctions. The experimental technique measures the current-voltage (IV) characteristics of Josephson junction with a metallic weak link (SNS-junctions) when irradiating the structures with radio-frequency radiation. The IV characteristic will show distinct steps of constant voltage that acts as a fingerprint for the exact relation between the supercurrent on the phase difference. Instead of using a single junction, arrays containing thousands of SNS junctions are used in order to increase the sensitivity. The design of the array is of fundamental importance in order to discern the response corresponding to a single junction while remaining insensitive to spurious effects related to e.g. external magnetic fields and inter-junction coupling. It was found that when designing an array with a high anisotropy (reducing the interjunction coupling), it serves as an excellent platform to perform the above described experimental method. It is clearly demonstrated that for the SNS-based array, we observe the same dependence of the supercurrent on the phase difference as expected from theory. Therefore, it is clear that anisotropic Josephson junction arrays can serve as an interesting playground to investigate the exact nature of current-phase relationships.

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Over the years, processing power of computers, smartphones and other electrical devices has increased dramatically, to the point where a modern Wi-Fi router has more processing power than the computers used during the Apollo 11 mission, which was responsible for humans stepping on the moon for the first time. This rapid technological development is, in part, due to the increasing number-per-chip and reduction in size of electrical switches regulating the flow of current (transistors). With the size of transistors steadily approaching the size of atoms, beyond which no further miniaturisation is possible, alternative ways of improvement in information processing techniques have to be explored. One such alternative is offered by the field of spintronics. Where in conventional electronics, the electrical charge of electrons is used to transmit information, in spintronics another property of electrons, their spin, is used. Just as copper is used in electrical wiring to conduct currents of electrons, a single layer of pencil lead (which does not, in fact, contain any lead), an atom-thin sheet of carbon atoms arranged in a honeycomb structure, can be used as a conductor in spintronics. Clusters, pieces of material more than 10 000 times smaller than the width of a hair, can be used to change graphene's properties. For instance, adding clusters made of magnetic material may induce magnetic properties in the non-magnetic graphene sheet. As magnetism and spin are connected, in fact the spin of electrons is one of the origins of magnetism, the behaviour of the electrons may be influenced by the presence of the cluster. This thesis' focus was on measuring the impact of clusters made up of three gold atoms on graphene, using electrical and magnetic based techniques. The comparison of measurements on graphene without clusters and with clusters did indicate changes in the electrical properties of graphene (the addition of more charge carriers, e.g., electrons, and the reduction of the mobility, a measure of how fast electrons can travel in a material), which have been observed before. They did not, however, indicate that magnetic properties were induced by the clusters. The low number of clusters deposited on the graphene may be one reason for this. Another is that perhaps gold does not induce (strong enough) magnetic properties to measure. Clusters made from materials exhibiting more profound magnetic properties, such as iron or nickel may produce a measurable response in the graphene and could be studied in further experiments.