Scanning SQUID-on-tip (SOT) microscopy offers topographic, magnetic and thermal imaging at high sensitivities. This project focused on the development of a SOT from a self-sensing, self-actuating...Show moreScanning SQUID-on-tip (SOT) microscopy offers topographic, magnetic and thermal imaging at high sensitivities. This project focused on the development of a SOT from a self-sensing, self-actuating tuning fork AFM probe. Patterning the superconducting contacts to the SQUID was identified as the main challenge. The non-planar geometry of the probe discourages continuous film growth and prohibits the use of lithography to pattern the film. The superconducting element of the SOT must be electrically isolated from the adjacent tuning fork actuation circuit. Off-axis sputtering of 60nm NbTi was found to minimize short circuits and result in continuous superconducting films. The steps necessary to pattern the NbTi film were identified; Off-axis sputtering at a slight incline with respect to the deposition substrate and a better-fitting micromachined hard mask will enable the fabrication of a SOT atop a tuning fork AFM probe.Show less
Efficient single-photon sources based on semiconductor quantum dots typically rely on resonant excitation schemes with a high degree of control. In particular, having access to continuous-wave (CW)...Show moreEfficient single-photon sources based on semiconductor quantum dots typically rely on resonant excitation schemes with a high degree of control. In particular, having access to continuous-wave (CW) and pulsed excitation without changing the center frequency is highly desirable. CW excitation is useful for alignment and characterization, while pulsed excitation is essential for on-demand single-photon production. We present a technique based on ultra-fast electro-optic modulation to directly synthesize optical pulses from a narrow linewidth CW laser. With custom-built ultra-fast electronics, we demonstrate tunable pulse lengths down to 50 ps. Pulses longer than 100 ps achieve a typical extinction ratio of 300, and the 50 ps pulses still show an extinction ratio of 150. We then use these pulses to excite a single InAs quantum dot in a micropillar cavity and show the generation of true single photons. This technique allows for full control over the experiment in the temporal-spectral domain, and is significantly simpler compared to using conventional Ti:Sa mode-locked laser oscillators in combination with grating-based pulse shaping.Show less
The application of denoising machine learning to STM data has several advantages, such as improving data quality, aiding visual interpretation of data, and speeding up measurement time. With...Show moreThe application of denoising machine learning to STM data has several advantages, such as improving data quality, aiding visual interpretation of data, and speeding up measurement time. With experimental data, the absence of a ground truth poses a problem for traditional supervised learning techniques. In this work, state-of-the art self-supervised machine learning techniques are applied to reduce noise in quasiparticle interference data of overdoped cuprates, using only the noisy measurements. The machine learning methods are shown to outperform traditional denoising methods. Further ideas to improve and generalize the denoising of quasiparticle interference data are proposed.Show less
The main goal of this project is to study the conductivity of pentacene thin films in the herringbone standing-up (HSU) phase as a function of layer count and to investigate the role of film...Show moreThe main goal of this project is to study the conductivity of pentacene thin films in the herringbone standing-up (HSU) phase as a function of layer count and to investigate the role of film defects in the charge transport through these pentacene layers. To grow these films, atomically flat and spatially uniform substrates are required, for which hexagonal boron-nitride (hBN) is used. Several techniques to clean these substrates are tested and multiple growth experiments are performed on our samples. We also develop a very general approach, which we have named the Laplace conductivity algorithm, to extract local conductivities on a sample from experimental potential maps. This algorithm is tested using simulations, with very promising results being achieved.Show less
Pentacene, a semiconducting organic molecule that is able to grow in crystalline structures, is a versatile material for many semiconductor applications. In this thesis, the transmissivity and...Show morePentacene, a semiconducting organic molecule that is able to grow in crystalline structures, is a versatile material for many semiconductor applications. In this thesis, the transmissivity and reflectivity of low energy electrons (0-10 eV) through standing-up phase pentacene in the transversal direction is studied. From these values, the total, inelastic and elastic mean free path (MFP) can be obtained from which the latter contains information about the unoccupied bands present in the pentacene. Using a graphene-hBN substrate on a TEM-grid, pentacene is epitaxially grown into 2 or 3 layer thick dendrites. Low energy electron microscopy (LEEM) and low energy transmissive electron microscopy (eV-TEM) are then used to find the transmissivity and reflectivity. First the substrate is studied, then, using a mathematical model to disentangle the MFP of pentacene from the substrate it is found that standing-up phase pentacene contains an unoccupied band around 3.2 eV. Furthermore, it is found that the disordered layer of non-crystalline pentacene molecules have a large influence on the measured reflectivity and transmissivity of the substrate.Show less
Twisted bilayer graphene at a magic angle might be used to increase our understanding of high temperature superconductivity. Studying 2D-materials with an Atomic Force Microscope (AFM) yields...Show moreTwisted bilayer graphene at a magic angle might be used to increase our understanding of high temperature superconductivity. Studying 2D-materials with an Atomic Force Microscope (AFM) yields information on its surface properties, such as its topography or its conductivity. In this thesis, AFM work has been done on different samples, with the final goal of identifying moiré patterns. AFM tapping mode shows clear roughness decline of a silicon/silicon oxide wafer after annealing 500 degrees for 24 hours. AC mode on layered graphene placed on an identical substrate was able to identify different atomic layers clearly. Sputtered gold, graphite and evaporated gold have been analyzed with current sensing AFM. Finally, I/V-curves have been acquired with AFM contact mode on sputtered gold, corresponding to theoretical expectations. All initial work has been done to measure I/V curves and start conductive AFM measurements on twisted bilayer graphene, in hopes of identifying moiré patterns in the current map.Show less
We conduct a feasibility study of nitrogen-vacancy (NV) ensemble magnetometry of the two-dimensional ferromagnet Cr2Ge2Te6 (CGT). The studied sample consists of thin flakes of exfoliated CGT...Show moreWe conduct a feasibility study of nitrogen-vacancy (NV) ensemble magnetometry of the two-dimensional ferromagnet Cr2Ge2Te6 (CGT). The studied sample consists of thin flakes of exfoliated CGT stamped onto a diamond with shallow NV centers. First, we simulate the NV center response to the magnetic stray fields produced by a monolayer of CGT and conclude a good signal to noise ratio should be attainable in a shot noise limited picture. Subsequent room temperature photo luminescence experiments reveal two key challenges: optical dimming underneath the samples and inherent low contrast of the NV electron spin resonance (ESR) spectrum. We can explain the optical dimming by a near-surface nanophotonic effect. The low observed contrast can partially be accounted for by the ionization of the shallow NV centers to the neutral charge (NV0). Also, it is found that contrast can be largely regained by rigorous cleaning of the diamond surface.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
The medical technique of Magnetic Resonance Imaging (MRI) is barely available in developing countries because of its high cost and the strong requirements on infrastructure. To address this problem...Show moreThe medical technique of Magnetic Resonance Imaging (MRI) is barely available in developing countries because of its high cost and the strong requirements on infrastructure. To address this problem, we are developing a permanent magnet-based head scanner that is affordable (<50,000 EUR) and portable. Here, we report on the first observations of magnetic resonance in our custom magnet array with a field strength of 59 mT. Using custom made volume coils, we observe using Hahn echo (Spin echo) and CPMG pulse sequences. We discuss the step towards 2D imaging using rotating spatially encoding magnetic fields (rSEMs) and show simulations that indicate this is feasible in our setup. Finally, we discuss the technical challenges that still have to be overcome to turn this prototype into a diagnostic device for those in need.Show less
We measured the homogeneous linewidth $\Gamma_h$ of ytterbium atoms inside a whispering gallery mode ring resonator. Using two-pulse photon echoes, we looked at the temperature dependence of $...Show moreWe measured the homogeneous linewidth $\Gamma_h$ of ytterbium atoms inside a whispering gallery mode ring resonator. Using two-pulse photon echoes, we looked at the temperature dependence of $\Gamma_h$ for an applied external magnetic field of $B=0$ and $B=0.3$ T. We found a clear narrowing of the linewidth at each temperature. Furthermore, we found saturation of $\Gamma_h$ for $T<30$ mK for both $B=0$ and $B=0.3$ T. At $T=10$ mK and $B=0.3$ T, we have observed a linewidth of 22 kHz, which is very near the lifetime limit of 1 kHz. Our results for the homogeneous linewidth of ions in glasses show that current theories have to be reconsidered. In addition, our results for $T_2$ ensure that rare earth element-doped cavities are a candidate for AFC quantum memory.Show less
