This thesis explores the initial steps towards integrating high temperature scanning SQUID-on-tip (SOT) with quartz tuning fork atomic force microscopy (QTF-AFM). By combining these imaging...Show moreThis thesis explores the initial steps towards integrating high temperature scanning SQUID-on-tip (SOT) with quartz tuning fork atomic force microscopy (QTF-AFM). By combining these imaging techniques into one sensor, the local magnetic field variations and surface topography of a sample can be mapped simultaneously. This allows the SQUID to scan with ultra-sensitive flux sensitivity and nanometer spatial resolution, while its position on the surface remains identifiable. Specifically, this thesis addresses the low operating temperature of conventional SOT by developing a fabrication method that uses BSCCO, a high temperature superconductor, to create SQUIDs through gallium focused ion beam (FIB) milling. The electrical contacting procedure of BSCCO involves mechanical exfoliation and electron-beam lithography. The results yield contact resistances in the order of 100 ohm, which are sufficiently low to perform current transport experiments. The flakes are then structured into 1 micrometer SQUIDs. The Josephson junctions are created by introducing ion beam induced damage to the crystal lattice of BSCCO to suppress superconductivity. The transport measurements reveal no conclusive evidence of SQUID features. However, it is shown that milling sub-200 nm wide structures does not alter the electronic properties of BSCCO, indicating that this nanostructuring method can potentially be applied in fundamental research into high temperature superconductors. This thesis also focuses on depositing multiple SQUID electrodes along a QTF, while keeping the self-sensing and -actuating capabilities of the force sensor intact. The QTF is insulated with a 100 nm thick SiOx layer. It is then covered with a laser micro-machined hardmask, through which 50 nm of titanium is evaporated in the shape of SQUID electrodes. Through fabrication alterations, certain issues involving alignment and electrode interruption can be solved. However, the evaporation method inexplicably compromises the integrity of the insulating barrier, thereby forming electrical shorts. Overall, the findings indicate that while substantial progress has been made in developing fabrication methods for the different components, significant technical hurdles remain. These need to be addressed to realize the potential of BSCCO scanning SQUID-on-tip for atomic force microscopy.Show less
We successfully produce Josephson junctions by stacking NbSe2 flakes, where the twist angle and strain are responsible for a weak link. Field sweep measurements are taken to characterize these...Show moreWe successfully produce Josephson junctions by stacking NbSe2 flakes, where the twist angle and strain are responsible for a weak link. Field sweep measurements are taken to characterize these devices. These measurements highlight the importance of the direction of the applied magnetic field. Prompted by this, simulations of the critical current given an arbitrarily shaped interface area are presented and compared to measurements.Show less
In this project, a tight binding approach was used to investigate how strain can alter the electronic properties of and create a periodic nanopattern in single layer graphene. The motivation for...Show moreIn this project, a tight binding approach was used to investigate how strain can alter the electronic properties of and create a periodic nanopattern in single layer graphene. The motivation for this research is formed by the question how superconductivity can be introduced in graphene. It has been shown that under sufficiently large uniaxial strain along the zigzag direction, a bandgap can open, consistent with previous literature. Biaxial strain calculations led to behaviour previously known from superconducting twisted bilayer graphene, namely a decrease in the spacing between van Hove singularities. Finally, simulations were made on a layer of graphene periodically strained by a hexagonal lattice of nanospheres. It was found that for a Gaussian height profile of the graphene layer with height variations of 1 nm, a maximum effective strain of 0.13% can be expected. This corresponds to a decrease of the tight binding hopping elements of 0.22%. This idea was also carried out experimentally, by spincoating 20 nm diameter SiO2 nanoparticles on a 5 mm x 5 mm n-doped Si/285 nm SiO2 substrate and placing a graphene layer on top by means of a wet transfer. Atomic force microscopy images showed that monolayers were formed, albeit not with the desired hexagonal structure. The graphene transfer on to these spheres was however successful.Show less
Focused electron-beam induced deposition (FEBID) is a non-destructive, relatively fast and cheap method with applications in AFM, plasmonics and nanomagnetics amongst others. In this thesis, three...Show moreFocused electron-beam induced deposition (FEBID) is a non-destructive, relatively fast and cheap method with applications in AFM, plasmonics and nanomagnetics amongst others. In this thesis, three dimensional superconducting arches were fabricated using this technique with the help of a computer aided design (CAD) program. Magnetic field sweeps were performed at low temperatures (∼2-5 K) to characterize these arches. With a few improvements, more complex 3D structures can be fabricated, paving the way towards the numerous applications people now can only dream of.Show less
This work presents a study of two types of spin-triplet Josephson junctions that are characterised by superconducting quantum interference and Shapiro step measurements. The first type of system is...Show moreThis work presents a study of two types of spin-triplet Josephson junctions that are characterised by superconducting quantum interference and Shapiro step measurements. The first type of system is a cobalt/niobium disk where long-range triplets are generated by the spin texture of a ferromagnetic vortex. The second system is a mesoscopic ring of Sr2RuO4, which is expected to host intrinsic Josephson junctions by the presence of chiral domain walls. For the cobalt/niobium disk, two supercurrent channels have been found surprisingly close to the edges of the disk by quantum interferometry measurements while indications of 0-pi segments have been found by the observation of half-integer Shapiro steps. Two rings with different inner and outer radii of Sr2RuO4 are shown to behave as Josephson junctions. The configurations of these junctions depend on the ring dimensions and the temperature. These findings make a convincing case for the presence of chiral domains in Sr2RuO4.Show less
In recent experiments on triplet superconductivity in full film CrO2-based triplet spin valves a surprisingly large proximity effect was shown under an out-of-plane magnet field by Singh et al. We...Show moreIn recent experiments on triplet superconductivity in full film CrO2-based triplet spin valves a surprisingly large proximity effect was shown under an out-of-plane magnet field by Singh et al. We aim to investigated this further with improved control over the current paths and magnetic non-collinearity in a new configuration. This is done by using more confined structures rather than a full film and by applying an in-plane field. We find that more control is needed over the magnetic non-collinearity required for triplet generation in in-plane experiments. A further improved design based on shape anisotropy is proposed to achieve this. Out-of-plane field measurements find a proximity effect coinciding in order of magnitude with the previously published results.Show less
In BCS superconductors the critical temperature is dependent on the phonon dispersion relation. We want to influence - and hopefully increase - the critical temperature by varying the phonon...Show moreIn BCS superconductors the critical temperature is dependent on the phonon dispersion relation. We want to influence - and hopefully increase - the critical temperature by varying the phonon dispersion. This can be done by altering the lattice structure of a material on the nanoscale using nanofabrication. As a proof-of-principle project we simulate these phonon structures and see whether or not higher critical temperatures come out. In this project we tested the code for mistakes and physical inaccuracies. The code reproduced physical effects correctly, but artificial parameters which we need to put in because of numerics issues have some influence on the outcomes. However, we think this is not a big problem because in the end we calculate the ratio of critical temperatures between two systems. This means that all that is constant in the whole calculation will be divided out.Show less
Triplet superconductivity in lateral Josephson junctions has only been shown in the ferromagnet CrO2. We succeeded in creating a disk-shaped lateral cobalt Josephson junction with a trench width...Show moreTriplet superconductivity in lateral Josephson junctions has only been shown in the ferromagnet CrO2. We succeeded in creating a disk-shaped lateral cobalt Josephson junction with a trench width below 20 nm. The device shows promising signs of superconducting triplet correlations. For exchange fields up to 15 mT we see an increase in the critical current, possibly caused by an increase of the magnetic non-collinearity as the Ni generating layer aligns with the field. We also observe oscillations in the critical current. These oscillations have a period of approximately 4 mT and are believed to be the result of a flux quantisation effect in the superconducting cobalt disk.Show less
In superconductor-ferromagnet hybrids the opposite-spin singlet Cooper-pairs are transformed to equal-spin triplet correlations. A magnetic non-collinearity at the SF interface is essential for...Show moreIn superconductor-ferromagnet hybrids the opposite-spin singlet Cooper-pairs are transformed to equal-spin triplet correlations. A magnetic non-collinearity at the SF interface is essential for this conversion. This can be achieved in systems where two ferromagnets have a misaligned magnetization with respect to each other. The optimal system for triplet generation is then investigated with the help of micromagnetic simulations. Out of the various systems studied, a disk geometry consisting of nickel and cobalt layers proved to be the most promising. This system was fabricated and subsequently measured. Although the existence of triplet correlations are yet to be verified, a fully proximized junction was observed. In addition, an attempt is made to realise current induced magnetization switching by utilising 100% spin-polarized current from half-metallic CrO2. We expect the required current density to be significantly reduced, thus minimizing the energy dissipation, which is currently the greatest obstacle in fabricating fully functional spintronic devices.Show less
The working temperature is a limiting factor for improving the sensitivity of Magnetic Resonance Force Microscopy towards imaging of a single nuclear spin. In this report we take a step in lowering...Show moreThe working temperature is a limiting factor for improving the sensitivity of Magnetic Resonance Force Microscopy towards imaging of a single nuclear spin. In this report we take a step in lowering the working temperature by reducing the energy loss of the radio frequent source, using a superconducting NbTiN microwire. A cryogenic calorimeter with 100 nW resolution at 4 Kelvin is developed to investigate the power dissipation of the detection chip, giving new insights on the current design. The use of a NbTiN microwire enables to keep the mixing chamber of a dilution refrigerator at 10mK. The presence of flux vortices penetrating the NbTiN material seems to play an active role in the energy losses. Improvements are proposed on the design of the detection chip to reduce the power dissipation further for future experiments.Show less
