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This thesis describes a series of experiments and a theoretical study in order to understand and control the behavior of electrons in many-body systems. In particular, the experiments concentrate on competition between the - antagonistic - electronic states of superconductivity and ferromagnetism, and are strongly motivated by the recent proposal of a new type of superconductivity which appears to defy the Pauli principle, if defined in the usual sense. As the central ingredient of our experiments, we use high-quality thin films of half-metallic Chromiumdioxide (or CrO2), best known for its large scale application in the magnetic recording industry, with an experimentally verified close to 100% spin-polarization. We start the experiments with a magnetotransport study of the magnetic properties of these films, and find a Planar Hall effect which demonstrates that the films, which are ordinarily supposed to have a single easy axis in the plane of the film, instead possess a biaxial magnetic anisotropy, which is analyzed in detail. We then contact the CrO2 films to a conventional (s-wave singlet) superconductor, using microfabrication techniques, and show that superconducting correlations originating in the superconductor, enter the CrO2 and persist over distances incompatible with spin singlet and ballistic transport, thereby providing strong evidence for the presence of s-wave triplet correlations. Moreover, from an application point of view, we show that the superconductor-half-metal-superconductor devices we have used to measure this effect are superconducting transistors that can be switched "on" and "off" with the magnetic field. The theoretical part focusses on the influence of voltages, rather than currents or temperature, on the collapse of the superconducting state in a short mesoscopic superconducting wire between normal metal contacts. It is shown that the collapse of the superconducting state is driven by the applied voltage, and not by the resulting current

The electron transport though ferromagnetic metal-superconducting hybrid devices is considered in the non-equilibrium Green's function formalism in the quasiclassical approximation. Attention if focused on the limit in which the exchange splitting in the ferromagnet is much larger than the superconducting energy gap. Transport properties are then governed by an interplay between spin-accumulation close to the interface and Andreev reflection at the interface. We find that the resistance can either be enhanced or lowered in comparison to the normal case and can have a non-monotonic temperature and voltage dependence. In the non-linear voltage regime electron heating effects may govern the transport properties, leading to qualitative different behaviour than in the absence of heating effects. Recent experimental results on the effect of the superconductor on the conductance of the ferromagnet can be understood by our results for the energy-dependent interface resistance together with effects of spin- accumulation without invoking long range pairing correlations in the ferromagnet

In this Master thesis I report results on a route to find Majorana fermions in indium antimonide nanowires in contact with a superconductor. Theoretically Majorana fermions appear in one-dimensional nanowires with strong spin-orbit coupling, in proximity with a superconductor and an external magnetic field applied parallel to the nanowire. The nanowires are deposited by a deterministic method, in this way the external magnetic field is perfect aligned with the nanowires up to a few degrees. Results we observed are a possible magnetic field tunable pi-junction, measurements of an induced gap in the nanowire and a robust zero-bias peak that persist in both gate and magnetic field scans. This zero-bias peak can be split and recombine with varying the applied magnetic field and the local gate potential.

This thesis describes a series of experiments aimed at understanding the low-temperature electrical transport properties of semiconductor nanowires. The semiconductor nanowires (1-100 nm in diameter) are grown from nanoscale gold particles via a chemical process called vapor-liquid-solid (VLS) growth. The huge versatility of this material system (e.g. in size and materials) results in a wide range of potential applications in (opto-)electronics. During the last few years many important proofs of concept have already been provided like lasers, field-effect transistors, light emitting diodes, and biochemical sensors. Simultaneously, the versatility of semiconductor nanowires creates new opportunities for the study of quantum transport phenomena. The quantum mechanical properties of semiconductor nanowires become visible at low temperatures (below a few Kelvin) and can be very different from room-temperature transport properties. For instance, the confinement of electrons in a small nanowire segment results in a discrete electronic energy spectrum forming a quantum dot, or artificial atom.

Spectroscopic observations of a large number of galaxies in the far-infrared are critical in the study of galaxy evolution. These observations are difficult at the moment, because of the lack of good far-infrared (FIR) detector technology. Microwave Kinetic Inductance Detectors (MKIDs) are showing promising results to fill this void. A common MKID design is the antenna-coupled coplanar waveguide (CPW) resonator patterned in a superconducting film. These resonators are extremely sensitive to changes in the Cooper pair density. FIR photons are energetic enough to break Cooper pairs. This means a superconducting resonator can be used as a pixel for a FIR camera. The inherent multiplexing advantage of MKIDs means they can easily be fabricated in arrays of up to 5000 pixels. By putting such an array in the SAFARI instrument on the cooled SPICA telescope a blind spectroscopic survey that can detect Milky Way sized galaxies out to redshift z = 2 will be possible. However, to achieve background limited performance on a cooled telescope like SPICA the sensivity of MKIDs needs to be improved by at least an order of magnitude. This project aims to investigate if this can be achieved by reducing the width of the CPW to much below what has typically been made using optical lithography (> 1 μm). CPW resonators with a central line width as narrow as 300 nm were made in NbTiN using electron beam lithography and reactive ion etching. Using this fabrication method NbTiN resonators with a central line width varying between 0.3 μm and 3 μm were made. These resonators were used to do a systematic study of the width dependence of the kinetic inductance fraction, quality factor, responsivity, read-out power, noise and NEP. It is shown that the width dependence of all these properties does not change down to 300 nm. Encouraged by these results two prototype submicron pixel arrays were made in aluminum. These showed sensitivities comparable to resonators with a few micron wide CPW. The main reason for this lack of sensitivity improvement is the lower maximum read-out power of submicron wide aluminum resonators. Based on these results power handling seems a key issue that will determine if submicron resonators are a viable route to improve the MKIDs sensitivity. These results of this project indicate that aluminum has a different width dependence of its power handling compared to NbTiN. Therefore, a systematic study of submicron resonators in aluminum, which focuses on the power handling, is required.

As a response to the need for lower carbon transmissions for means of transportation, the aerospace industry is looking into more electric aircraft. One concept of such an aircraft is a hybrid helicopter that uses highly efficient diesel engines and electrical propulsion motors. The feasibility of this concept is greatly determined by the feasibility of its main propulsion motor. The very challenging specifications of such a motor means that new technologies have to be used. One of these possible technologies is superconductivity. In this thesis a first technical feasibility study on a direct drive superconducting propulsion motor for a hybrid helicopter concept is performed. Superconductors are special materials that have virtually no electrical resistance when cooled down below a certain critical temperature. This property makes it possible to build high field and low weight coils for electrical machines. A helicopter’s electrical propulsion motor requires a very high torque density. Conventional electrical machines like permanent magnet designs are too heavy for this application. Superconducting motors may achieve the required torque densities, because of their high performance at low weight. In this thesis, first a literature study of superconductivity is presented. After that the machine is designed in three parts: The armature design, the superconducting field windings design and the cryostat design. After that all the results are combined and the complete machine parameters, like the power density, are defined. Finally a conclusion is given on the feasibility of the superconducting propulsion motor and recommendations are given for future work.

As a result of progress in nanotechnology, smaller and smaller electronic circuits can be made. The stage of electrically contacting even a single molecule has now been reached. This stimulates both fundamental and applied research alike. Molecular electronics is hence a booming new field that draws a lot of attention. In this research project we have studied fundamental electrical transport properties of single molecules at low temperatures. In collaboration with chemists, a special kind of molecules has been synthesized for this purpose: molecular magnets. These molecules individually behave as tiny magnets. In this thesis, we describe the effect of the magnetic properties on the conductance of the molecule. Quantum mechanical effects play an important role in this respect. Furthermore, we looked at the conductance of a novel material system: graphene an atomic layer of graphite. Graphene is a semi-metal, in which electrons behave as relativistic, massless particles. By coupling graphene to superconducting electrodes, we were able to induce a supercurrent in graphene. The supercurrent in graphene can be tuned by a gate-electrode and hence the device behaves as a superconducting transistor. Our measurements provide new insights in the properties of this exotic material.

One of the greatest challenges in astronomy is observing star and planetary formation, redshifted distant galaxies and molecular spectral ‘fingerprints’ in the far-infrared spectrum of light, using highly sensitive and large cameras. In this thesis we investigate superconducting resonators for photon detection. In superconductors the electrons are paired. The incoming light then breaks these pairs into unpaired electrons, so-called quasiparticles, influencing the superconductor’s inductance. Consequently, the resonance frequency shifts. These resonators are extremely sensitive, since they are operated at temperatures where less than a billionth of the electrons are unpaired. By giving each resonator (or pixel) a slightly different length, like the pipes in an organ, many can be read out simultaneously. This allows for the construction of large cameras. These cameras work best when pairing is slow – the quasiparticles eventually recombine and the signal is lost – and the noise is low. In this thesis we focus on two main topics: the quasiparticle recombination process as well as the frequency noise of these resonators. At low temperatures we find relaxation times as long as milliseconds for Al and several tens of microseconds for Ta. The relaxation times clearly saturate at low temperatures in both materials, indicating an additional recombination channel in the superconducting films. The low temperature relaxation is made faster by the implantation of magnetic as well as nonmagnetic atoms, indicating that it arises from an enhancement of disorder. In addition, we show that the frequency noise mainly arises at interfaces, whereas deviations in the temperature dependence of the resonance frequency arise from dipole defects in the volume of dielectrics. Finally, we significantly decrease the noise by widening the geometry of the resonator waveguide.

Ferromagnetism and superconductivity have long been thought to be mutually exclusive. Recently however it was found that the compounds UGe2, URhGe and UIr belong to a class of materials in which ferromagnetisme and superconductivity appear simultaneously. One characteristic property of these compounds is the existence of strong correlations between the magnetic moments of the uranium ions and the conduction electrons. These correlations lead to unusual magnetic properties at low temperatures. By applying external pressure the magnetic correlations can be varied. The fact that superconductivity in these materials is found only for those pressures, at which the magnetic correlations are strongest, indicates that the effective attracting force between the conduction electrons responsible for superconductivity has a magnetic origin. In this research the magnetic correlations of the ferromagnetic superconductors are investigated in order to better understand the unusual coexistence of ferromagnetism and superconductivity. Besides the dilatometry, specific heat, magnetization, and three-dimensional neutron depolarization techniques, the muon spin relaxation (muSR) technique is frequently used in the study of de magnetic properties of the ferromagnetic superconductors. The muon experiments indicate that unusual excitations exist in these materials which are possibly responsible for the superconductivity.

We report the characterization of layered 2H-type CuxTaS2 for 0≤x≤0.12. The charge density wave (CDW), at 70 K for TaS2, is destabilized with Cu doping. The sub-1 K superconducting transition in undoped 2H-TaS2 jumps quickly to 2.5 K at low x, increases to 4.5 K at the optimal composition Cu0.04TaS2, and then decreases at higher x. The electronic contribution to the specific heat, first increasing and then decreasing as a function of Cu content, is 12 mJ mol−1 K−2 at Cu0.04TaS2. Electron-diffraction studies show that the CDW remains present at the optimal superconducting composition but with both a changed q vector and decreased coherence length. We present an electronic phase diagram for the system.

High critical current-density (10 to 420 kA/cm2) superconductor-insulator-superconductor tunnel junctions with aluminum nitride barriers have been realized using a remote nitrogen plasma from an inductively coupled plasma source operated in a pressure range of 10−3–10−1 mbar. We find a much better reproducibility and control compared to previous work. From the current-voltage characteristics and cross-sectional transmission electron microscopy images it is inferred that, compared to the commonly used AlOx barriers, the polycrystalline AlN barriers are much more uniform in transmissivity, leading to a better quality at high critical current densities.

The coupling efficiency of a Nb superconducting transmission line has been measured using a Fourier transform spectrometer for different magnetic fields. It is found that the coupling decreases with increasing magnetic field when the frequency is close to the gap of the Nb superconductor. This is attributed to the changes of the surface impedance of the proximity-coupled superconductor/normal-metal bilayers in the transmission line.