A common problem in Magnetic Resonance Force Microscopy (MRFM) is the spin-induced damping of the cantilever, which drastically limits the sensitivity to spin signals. In order to solve this...Show moreA common problem in Magnetic Resonance Force Microscopy (MRFM) is the spin-induced damping of the cantilever, which drastically limits the sensitivity to spin signals. In order to solve this problem, we have developed improvements to a Persistent Current Switch (PCS) that make it less dissipative and capable of creating a stronger magnetic field at the sample. On top of this, the low noise level that our detection setup requires is conserved. The improvements are based on the use of a low-temperature magnetic core material called MetGlas [1]. We have measured the full B-H curve of the MetGlas and verified that it decreases the current required to switch a Niobium wire to the resistive state by a factor of 30. Furthermore, we have used this data to calculate the performance of a transformer made using this material, and we have calculated the expected extremely low noise level that this circuit will cause in our SQUID.Show less
In this thesis, we have worked on devices for two oxide systems, with which spin-polarized currents could in future be controlled. Firstly, we have worked on the optimization of growth parameters...Show moreIn this thesis, we have worked on devices for two oxide systems, with which spin-polarized currents could in future be controlled. Firstly, we have worked on the optimization of growth parameters for the depositions of La0.7Sr0.3MnO3 thin-films in off-axis sputtering. We have characterized the grown films using Atomic Force Microscopy, X-Ray Diffraction and resistance measurements. La0.7Sr0.3MnO3 is a ferromagnetic oxide, which we believe could be used in conjunction with the oxide superconductor Sr2RuO4 to induce polarized supercurrents. Further, we illustrate a lithography procedure which allows for the patterning of LSMO films into Hall bar structures. Next, we have designed a side-gated Hall bar pattern for LaAlO3/EuTiO3/SrTiO3 devices. This system has been shown to give rise to a spin-polarized two dimensional electron gas [1]. Our structure is designed to allow for local control of this spin-polarization.Show less
Motivated by the recent discovery of superconductivity in Magic Angle Twisted Bilayer Graphene, we discuss preparatory work for STM measurements on micrometer-sized graphene flakes. We develop and...Show moreMotivated by the recent discovery of superconductivity in Magic Angle Twisted Bilayer Graphene, we discuss preparatory work for STM measurements on micrometer-sized graphene flakes. We develop and fabricate a dummy sample for testing capacitive navigation in any STM setup with an XY-stage. Furthermore, we calculate the effect of a biased STM tip on the charge distribution in a graphene bilayer. We also estimate the chemical potential shift in a gated graphene sample as a result of the probe tip. All results unambiguously suggest that the STM tip will strongly influence the electrical behavior of the graphene system.Show less
DNA methylation is the phenomenon where a methyl group attaches to the nucleotide cytosine or adenine. We study CpG methylation and specifically its role in nucleosome positioning. Methylation...Show moreDNA methylation is the phenomenon where a methyl group attaches to the nucleotide cytosine or adenine. We study CpG methylation and specifically its role in nucleosome positioning. Methylation changes the mechanical properties of the CpG step and therefore the mechanical affinity of DNA to form nucleosomes. We use a rigid base pair model to emulate DNA where the mechanical and geometric properties of a base pair step depend on which base pairs it is made of. A 147 base pair long piece of DNA is forced into the shape known for wrapped DNA inside a nucleosome by attaching it to the nucleosome core proteins at 28 positions. Monte Carlo simulations then allow us to measure various quantities like the average energy of a given sequence or the occurrence rate of certain base pair triplets. The latter is used to inform a trinucleotide Markov model that generates free energy landscapes. From these landscapes we calculate free energy it takes to place a nucleosome at any position close to transcription start site of genes. On average human genes attract nucleosomes around their transcription start sites. We find in this thesis that methylation turns these nucleosome attractive regions into nucleosome expelling regions. We also study the role of entropy in nucleosome positioning by comparing energy landscapes to free energy landscapes. These comparisons show that at higher temperatures entropy becomes non-negligible. We propose a model that could generate energy landscapes at much lower calculational cost, a tool that can be useful in studying the role of entropy in nucleosome positioning on a genome wide scale.Show less
In this thesis we investigate conductivity changes due to magnetite in agarose gels mimicking grey brain matter. We use conventional MRI sequences to acquire B + 1 phase maps. Using the homogeneous...Show moreIn this thesis we investigate conductivity changes due to magnetite in agarose gels mimicking grey brain matter. We use conventional MRI sequences to acquire B + 1 phase maps. Using the homogeneous Helmholtz equation and the B + 1 phase-only approximation, we reconstruct conductivity maps. The current sensitivity of the reconstructions is too low to detect conductivity changes due to magnetite nanoparticles in the concentration found in the brain of Alzheimer’s disease patients. Nevertheless, we have promising indications that we have been able to observe a change in the standard deviation of the conductivity due to the presence of magnetite.Show less
In this study, we employed several methods to characterize iron-oxide nanoparticles using SQUID magnetometry and MRI. With SQUID magnetometry, we measured the Isothermal Remanent Magnetization of C...Show moreIn this study, we employed several methods to characterize iron-oxide nanoparticles using SQUID magnetometry and MRI. With SQUID magnetometry, we measured the Isothermal Remanent Magnetization of C. Elegans and two human brain samples. We obtained the iron concentration from the fit. We were able to detect changes in iron concentration due to mutations in C. Elegans. For the MRI measurements, we used Quantitative Susceptibility Mapping and an Off-Resonance Saturation method for brain phantoms. These phantoms consist of different concentrations of magnetite or ferritin dissolved in an agarose gel and mimics the human brain. With QSM we observed a comparable slope of the susceptibility/µg iron/ml. For the ORS method, a good agreement is found between the obtained iron concentration and the pre-determined iron concentration in the sample.Show less
Gold nanorods are used in various sensing applications. Through observation of their Surface Plasmon Resonance (SPR) at optical wavelengths, they offer a bridge to length scales below the...Show moreGold nanorods are used in various sensing applications. Through observation of their Surface Plasmon Resonance (SPR) at optical wavelengths, they offer a bridge to length scales below the diffraction limit. In a confocal Interference Scattering Microscopy (iSCAT) setup fluctuations of the SPR may be detected fast enough to draw conclusions about diffusion in the vicinity of the nanorod. We characterize such a setup and find its optimal working point.Show less
Spin waves are collective excitations of spins in magnetic materials, that can be used to transport and manipulate spin information in spintronic devices. In this work, we use the magnetic field...Show moreSpin waves are collective excitations of spins in magnetic materials, that can be used to transport and manipulate spin information in spintronic devices. In this work, we use the magnetic field generated by a radio frequency (RF) current in a microstrip to excite surface-confined Damon-Eshbach spin waves in an Yttrium Iron Garnet (YIG) thin film. First, we measure the propagating spin waves by recording the currents they generate via induction in a second microstrip. By sweeping the RF frequency, we map out the spin wave spectrum excited in the YIG. Next, we harness the spin associated to a nitrogen-vacancy lattice defect in diamond (NV center) to locally probe the magnetic stray fields generated by the spin waves. By monitoring the NV center’s electron spin resonance (ESR) contrast and Rabi frequency we register the spectrum of the spin waves in a unique way. Moreover, we directly observe the filter function decay of the spin wave’s magnetic field and characterise resonant spin wave excitation in a microstrip cavity. Finally, we use a single NV center embedded in an AFM scanning probe to directly image the wavefronts of spin waves in YIG. This was never done before and paves the way for magnon imaging at the nanoscale.Show less
We performed numerical simulations to verify a model of spontaneous collapse of the wavefunction by an infinitesimally small, non-unitary noise field. This noise field breaks time reversal and...Show moreWe performed numerical simulations to verify a model of spontaneous collapse of the wavefunction by an infinitesimally small, non-unitary noise field. This noise field breaks time reversal and translation symmetry of the Hamiltonian and depends only linearly on the wavefunction itself. We found that the probabilty of collapse from a uniform wavefunction goes to infinity in the limit of continuous space and depends on B^{2}m. Here B is the strength of the noise and m the mass of the system. Furthermore, we investigated the stability of a collapsed state and found that this depends on the shape of the noise distribution: an asymmetrical, positive distribution seems to enhance the stabilty. We finally performed analytical calculations to further understand this dependency and the role of normalisation of the wavefunction in our model.Show less
The spectroscopic techniques of ARPES and spectroscopic STM play a key role in strongly correlated electron research due to their ability to resolve k-space. However, due to their different...Show moreThe spectroscopic techniques of ARPES and spectroscopic STM play a key role in strongly correlated electron research due to their ability to resolve k-space. However, due to their different approaches in obtaining k-space information, the two techniques do not necessarily agree on the observed bandstructure. Here, in an attempt to clear the fog between the two, we present FT-STM results on the rhodate Sr2RhO4 focusing on the comparison between our data and previous ARPES studies on the same sample. We deduce the low-energy bandstructure through the modulations of the LDOS caused by impurity scattering. The Fermi surface area and self-energy are then calculated. We find a flattened dispersion compared to the ARPES result which is shown to be in line with previous FT-STM studies on other correlated electron materials.Show less
Curved crystals provide the possibility to study different vicinal surfaces in a single crystal. In this thesis, we analyzed a curved Pt (111) single crystal with kinked steps under UHV conditions...Show moreCurved crystals provide the possibility to study different vicinal surfaces in a single crystal. In this thesis, we analyzed a curved Pt (111) single crystal with kinked steps under UHV conditions using STM images to characterize the sample. We present and compare terrace width histograms of different images taken on the crystal.Show less
In this thesis research is presented into multiple unexplained phase transitions of the two antiferromagnets FePS3 and Fe1-xS (pyrrhotite). X-ray diffraction and resistivity measurements of...Show moreIn this thesis research is presented into multiple unexplained phase transitions of the two antiferromagnets FePS3 and Fe1-xS (pyrrhotite). X-ray diffraction and resistivity measurements of pressurised FePS3 show that this material undergoes at least two crystallographic phase transitions, strongly related to the appearance of an exotic metallic state when pressures over approximately 10GPa are applied. Magnetisation, specific heat and resistivity measurements of Pyrrhotite highlight for the first time the dual nature of the Besnus transition. Compelling evidence is presented that excludes the proposition of extrinsic magnetic coupling between differently ordered vacancy superstructures as the underlying mechanism. Instead the presence of local magnetic domain formation is highlighted.Show less
In this thesis we are interested in growing LaAlO3 films onto SrTiO3 substrates with an off-axis geometry radio frequency magnetron sputtering technique in order to study the properties of the...Show moreIn this thesis we are interested in growing LaAlO3 films onto SrTiO3 substrates with an off-axis geometry radio frequency magnetron sputtering technique in order to study the properties of the Q2DES found at the interface between this two band insulators. Hall bars with an Al2O3 hard mask were patterned onto TiO2-terminated SrTiO3 substrates. The selected lithographic process was electron beam lithography due to its high resolution. With respect to the growing of the Al2O3 hard mask two different approaches were used.Show less
This thesis comprises two research projects focused on investigating surface plasmon (SP) propagation on metal-dielectric interfaces. The characterization of a newly fabricated device, designed for...Show moreThis thesis comprises two research projects focused on investigating surface plasmon (SP) propagation on metal-dielectric interfaces. The characterization of a newly fabricated device, designed for further research, is also presented. This thesis first showcases a new form of SP lasing, which utilizes two metal hole arrays as cavity mirrors. Analysis of the lasing mode provides a direct view of SP propagation and allows for determining the SP group velocity on any metal-dielectric interface. Second, high-quality measurements of the dispersion characteristics of SP-mediated emission from an actively pumped elliptical-hole array are presented. These measurements resulted in the observation of an intriguing SP dispersion relation ω(k) and provide an insight into the effect of symmetry-breaking of SP scattering in metal hole arrays. Looking into the future, this thesis also describes the production process as well as subsequent characterization of a newly fabricated device. The device is found to perform worse than expected, leading to several interesting insights into the production process.Show less
An overview of Quantum Field Theory dualities is given, highlighting the tools physicists have been using to derive them and the importance of symmetries in searching for such dualities. Most...Show moreAn overview of Quantum Field Theory dualities is given, highlighting the tools physicists have been using to derive them and the importance of symmetries in searching for such dualities. Most duality derivations take place in 2+1d where one may use flux attachment to realise dualities between fermionic and bosonic theories. The phase transition method for finding dualities is then discussed in 2+1d and 3+1d, and a novel derivation of the Montonen-Olive duality is given using this method.Show less
In this work, we measure the internal magnetic field of a 100nm iron-doped palladium film by means of transversal field muon spin spectroscopy. The internal field and field inhomogeneity were...Show moreIn this work, we measure the internal magnetic field of a 100nm iron-doped palladium film by means of transversal field muon spin spectroscopy. The internal field and field inhomogeneity were measured in the bulk of the film from 3.5K to 100K and compared to measurements in a pure Pd film in order to investigate the effect of iron-doping. Furthermore, we compared these temperature sweeps to existing μSR measurements on bulk PdFe [1], in order to verify the quality of the film. Finally, we measured the internal field and field inhomogeneity at 3.5K as a function of muon implantation energy, in order to learn more about the spatial variation of the internal magnetic field. Ultimately, by doing this characterization, we hope that PdFe can serve as a testbed for other local magnetic field probe techniques.Show less
Majorana quasiparticles can arise at the ends of one dimensional superconducting quantum wires. As a true one-dimensional experimental realization of such a quantum wire, a carbon nanotube can be...Show moreMajorana quasiparticles can arise at the ends of one dimensional superconducting quantum wires. As a true one-dimensional experimental realization of such a quantum wire, a carbon nanotube can be employed. The one-dimensional carbon nanotube can inherit the superconducting properties by bringing it in close contact to a superconductor, such as van der Waals superconductor niobium diselenide. In this work, the design and fabrication of devices are discussed that contain a carbon nanotube connected to a few atomic layer thick niobium diselenide. Bi- or trilayers of niobium diselenide crystal flakes are obtained using mechanical exfoliation techniques in inert conditions. A stamping resolution of about 1 mm is obtained in positioning the flakes with respect to each other which is shown by Atomic Force Microscopy. Initial characterizations have shown an improvement of the conductance of more than two orders of magnitude. The stability diagrams, however, exhibit a complex behaviour which we were unable to explain in terms of a proximitized carbon nanotube. We have shown that it is possible to fabricate the complete hybrid device that is predicted to host Majorana Fermions in its non-trivial phase. This work was conducted at Regensburg University in the group of Professor Christoph Strunk.Show less
