Within the diverse order of beetle species that produce color through structural coloration, some beetles produce light with a strong left-handed circular polarization caused by a chiral structure...Show moreWithin the diverse order of beetle species that produce color through structural coloration, some beetles produce light with a strong left-handed circular polarization caused by a chiral structure in their outer shells. From an evolutionary perspective, there should be no substan- tial benefit to left-handed over rather-handed polarization. This raises the question why no beetles have been found showing right-handed polarization. Large-scale beetle surveys are required to investigate this question further. A systematic beetle classification scheme based on polarization would aid in such a survey and highlight species with interesting optical prop- erties for further investigation. To this end we constructed a setup capable of measuring the Stokes parameters of a beetle specimen for angles of incidence and observation. We tested this setup on the beetle species protaetia speciosa jousselini and verified that the setup produces accurate results. We are however sceptical this method will result in identifying beetle-specific Stokes parameters, as their values vary with respect to both location on the beetle cuticle as well as observational angles. Furthermore we provide quantitative evidence for earlier re- ported inversion of the polarization handedness in beetle species jousselini at large angles of observation. We detect such inversion at angles of observation of at least 70◦.Show less
This work investigates hexagonal Boron Nitride (hBN), being a promising source of single photons, with a home-built confocal microscope. Using colloidal quantum dots as a reference, the setup was...Show moreThis work investigates hexagonal Boron Nitride (hBN), being a promising source of single photons, with a home-built confocal microscope. Using colloidal quantum dots as a reference, the setup was optimized and characterized. All in all it showed decent efficiency and resolution, close to the limit of the current objective. The hBN sample, provided by a scientific group in Denmark, was investigated in two stages. First, primary investigation revealed that there are two types of flakes present on the sample; we call them flat and fluffy flakes. They were observed using pump reflection, fluorescence and darkfield imaging. All measurements present clear distinction between the flakes including difference in form, structure, fluorescent and reflective properties. Fluffy flakes show larger absolute strength of fluorescence signal and pump reflection. Bleaching measurements indicate greater stability of flat flakes compared to fluffy flakes. In the second stage, special defects on the flat flakes were studied by means of spectral and lifetime measurements. These defects were found using position-resolved spectral scans. Some of the defects show spectra with two-peak structure and prominent spectral lines, resembling those reported in literature. Spectra of fluffy flakes present uniform distribution of bulk fluorescence. This research is a stepping stone to a better understanding of single-photon fluorescent defects in 2D materials.Show less
In order to curb the growing energy demand and environmental impact of data centers, breakthrough technologies bringing increased energy efficiency are required. Superconducting computing promises...Show moreIn order to curb the growing energy demand and environmental impact of data centers, breakthrough technologies bringing increased energy efficiency are required. Superconducting computing promises massive energy savings through near-dissipationless operation. So far, multiple designs for superconducting memory are proposed, but none meets all requirements for an implementable device. To this end, we propose a novel design of superconducting memory (SCM) device, using two stable magnetic spin textures in a single-layer elliptical SFS Josephson junction. We observe a change in critical current between the two states by a factor of five at remanence. Using a novel technique to quantify the effects of stray fields using micromagnetic simulations, we conclude that the shift in critical current is caused by a large contrast in stray field strength between the two magnetic states. Furthermore, we verify that the switching process is deterministic and unambiguous during read-out. Future pathways to scale up this memory device are proposed.Show less
Magnetic Resonance Force Microscopy (MRFM) is a sensitive method to investigate spin systems, which uses a flexible cantilever as mechanical amplifier of the forces on its magnetic tip. However,...Show moreMagnetic Resonance Force Microscopy (MRFM) is a sensitive method to investigate spin systems, which uses a flexible cantilever as mechanical amplifier of the forces on its magnetic tip. However, MRFM is generally limited in its application at milliKelvin temperatures because existing devices rely on laser interferometry to detect cantilever deflection, which heats the cantilever, leaving many condensed matter systems out of reach for MRFM. Furthermore, lower temperatures correspond to lower cantilever force noise, so samples with more diluted spins could be investigated. SQUID-detected MRFM, using the flux induced by a moving cantilever tip, does allow for operation at milliKelvin temperatures. Yet, SQUID-detecting setups have still been limited in sample accessibility because the detection loop is printed on the sample. This thesis reports on the construction of a SQUID-detected MRFM device that employs a single probe head design to overcome the issue. The design choices and assembly methods for this device, called the easyMRFM, are discussed, as well as models to predict the sensitivity. It was found that the coupling is large enough to do optimisations in liquid helium dipstick experiments, although the thermal cantilever motion signal will only barely rise above the flux noise level. Lastly, a room-temperature magnetometry setup for cantilever chips is discussed that has proven useful in characterising cantilevers before mounting them in more permanent setups.Show less
Copper oxide superconductors (cuprates) are perhaps among the best known strongly correlated materials. Upon chemical (hole) doping of the antiferromagnetic parent compound, a variety of different...Show moreCopper oxide superconductors (cuprates) are perhaps among the best known strongly correlated materials. Upon chemical (hole) doping of the antiferromagnetic parent compound, a variety of different electronic phases emerges, including a mysterious pseudogap phase and unconventional superconductivity. Alternatively, forcing a transport current can also induce phase transitions [1–4]. In this thesis, we outline the first steps towards the local characterization of these current-induced electronic properties using low-temperature Scanning Tunneling Microscopy (STM). After studying the viability of these experiments, we define different regions in the doping-current phase diagram suitable for the local characterization of the various phases. The large current densities and accurate doping control needed to reach these regions require the usage of thin samples that were proven hard to manufacture. However, we present the first steps towards a simple fabrication method that allows cleaving thin exfoliated flakes after being stamped on a Si/SiO2 chip with pre-patterned contacts. After studying and optimizing the cleaving processes of Bi2Sr2CaCu2O8+x flakes onto Gold and Si/SiO2 we conclude that the presented method is not ideal, and suggest more elaborate methods that include conventional lithography approaches.Show less
Mapping the local electronic structure on a nm scale has been vital to the understanding of cuprates. Experiments that probe the local DOS with STS have shown the superconducting gap to be...Show moreMapping the local electronic structure on a nm scale has been vital to the understanding of cuprates. Experiments that probe the local DOS with STS have shown the superconducting gap to be inhomogeneous in these compounds [1]. More recently, ARPES experiments on cuprates have shown the gap to fill rather than close, but it is yet unclear what this process looks like locally [2]. Other STS results have shown that the DOS in overdoped Bi-2201 also contains a pronounced peak attributed to the Van Hove singularity [3]. Such a momentum space origin of the peak remains contested, however, as this peak is reported as similarly inhomogeneous in real space. Presented in this thesis is a phenomenological analysis of STS spectra of Bi2Sr2CuO6+δ (Bi-2201) in the far overdoped regime (OD12K and OD3K). Two different fitting models demonstrate that this supposed VHS is responsible for electron-hole asymmetry of the coherence peaks of the superconducting gap by having the peak energy be close to gap size Δ. It it also shown that the gap filling hypothesis holds up with the addition of a phenomenological peak, although the best model for how the gap closes stays undecided on. Aside from fitting, peak detection analysis supports the same conclusions about the VHS. A more general dimensionality reduction approach is applied to investigate relations between features in the local DOS, utilising Principle Component Analysis PCA) and Uniform Manifold Approximation and Projection (UMAP). However no conspicuous clusters were found in the STS data. This thesis paves the way for more complete models of the still poorly-understood LDOS of cuprates and how it varies in real space. Furthermore, the affirmation of a filling gap adds to the evidence pointing to high-temperature superconductivity being limited by phase coherence. [1] Ø. Fischer, M. Kugler, I. Maggio-aprile, C. Berthod, and C. Renner, Scanning tunneling spectroscopy of high-temperature superconductors, 79 (2007). [2] T. J. Reber, S. Parham, N. C. Plumb, Y. Cao, H. Li, Z. Sun, Q. Wang, H. Iwasawa, M. Arita, J. S. Wen, Z. J. Xu, G. D. Gu, Y. Yoshida, H. Eisaki, G. B. Arnold, and D. S. Dessau, Pairing, pair-breaking, and their roles in setting the Tc of cuprate high temperature superconductors, (2015). [3] A. Piriou, N. Jenkins, C. Berthod, I. Maggio-Aprile, and Fischer, First direct observation of the Van Hove singularity in the tunnelling spectra of cuprates, Nature Communications 2, 221 (2011).Show less
Superconducting QUantum Interference Devices (SQUIDs) play a central role in numerous applications, ranging from cancer treatment to magnetic imaging of nanoparticles. Conventional fabrication...Show moreSuperconducting QUantum Interference Devices (SQUIDs) play a central role in numerous applications, ranging from cancer treatment to magnetic imaging of nanoparticles. Conventional fabrication methods revolve around a multi-step lithography process that encompasses etching, heating, chemical cleansing and coating, thereby limiting these techniques in practice. In this thesis we present a non-destructive alternative approach that does not entail pre- or post processing, namely, a direct-write printed SQUID patterned under 20 minutes using Focus Electron Beam Induced Deposition (FEBID). The Josephson behaviour of these devices is confirmed by performing out-of-plane magnetic field sweeps and measuring the corresponding oscillations in critical current. Our endeavours pave the way for printing sophisticated quantum systems and three dimensional superconducting sensors.Show less
Twisted bilayer graphene (TBG) provides a range of novel electrical, mechanical and optical properties, some yet to be discovered. Large differences between different samples suggest that the...Show moreTwisted bilayer graphene (TBG) provides a range of novel electrical, mechanical and optical properties, some yet to be discovered. Large differences between different samples suggest that the discovered phenomena are highly sensitive to small changes in the device quality. In-plane atomic relaxation of TBG at small twist angles provides a starting point for investigating displacement fields in the underlying atomic lattice of bilayer devices. A spatial lock-in algorithm is used to detect and measure small atomic displacements in both simulated graphene lattices and scanning tunnelling microscope (STM) topographs of reconstructed TBG devices. The algorithm is found to retrieve displacement fields at the atomic scale with high accuracy. Results for physical lattices show promising potential for use on high-quality STM scans with atomic resolution.Show less
Authentication of a communication channel usually requires that the parties meet physically; but there is one solution if it is enough to confirm the geographical location of a party: quantum...Show moreAuthentication of a communication channel usually requires that the parties meet physically; but there is one solution if it is enough to confirm the geographical location of a party: quantum position verification (QPV). This is based on quantum mechanics, the no-cloning theorem, and special relativity, the invariance of the speed of light. We shown an extension of a QPV protocol where quantum information is communicated via the polarization state of single photons including the effects of photon loss and polarization noise, and explore it by numerical simulations. Moreover, we have designed and implemented the first steps of a QPV demonstration using optical fibers. We have been able to calibrate the setup for horizontal and vertical polarization states where a visibility of approximately 0.85 has been measured.Show less
In this thesis, the potential of Chemical Exchange Saturation Transfer to image brain metabolites is presented. The Bloch-McConnell equations are simulated and the optimal parameters are found to...Show moreIn this thesis, the potential of Chemical Exchange Saturation Transfer to image brain metabolites is presented. The Bloch-McConnell equations are simulated and the optimal parameters are found to be 3.5uT, tsat = 1s for glutamate and 3uT, tsat = 1.5s for creatine. Furthermore, multiple quantification methods, including MTR asymmetry, Lorentzian fits and spinlock fits are evaluated for quantifying CEST signal from glutamate and creatine. The quantification methods are tested on the Bloch-McConnell simulations, 2-pool phantoms, 3-pool phantoms and in vivo.Show less
Well isolated mechanical systems have the potential to be developed into systems for magnetic, accelerometric and gravitational sensing, as well as to investigate the limits of quantum theory. This...Show moreWell isolated mechanical systems have the potential to be developed into systems for magnetic, accelerometric and gravitational sensing, as well as to investigate the limits of quantum theory. This holds especially for mechanical resonators which consist of levitated nano- and microparticles, since an advantage of this type of system is the lack of clamping losses, potentially resulting in an extremely low energy dissipation. Here, a mechanical resonator is presented, where a magnetic microparticle is levitated in a cylindrical trap of a type I superconductor. SQUID detection has been used to measure the vibrational modes of the particle. The damping factors of the resonator have been analytically calculated, resulting in an expected quality factor Q of 10^11. The coupling and energy of the six translational and rotational rigid body modes of the particle have been simulated, based on analytical approximations. Experimentally, a resonance is detected with a damping time of 47 seconds and a Q of 2.2*10^4. These are promising first results, since this difference in damping and Q factor can be explained as the Earth's magnetic field was trapped inside the experiment. With these complications resolved, an extremely sensitive micromechanical resonator can be developed. This opens a new road in the investigation of the boundary between the quantum and classical regime and gravitational research.Show less
Localization of light and cavity modes play an important role in optics. In photonic crystals, localization occurs at defects to the periodicity of the structure. Recently, twist in a coreless...Show moreLocalization of light and cavity modes play an important role in optics. In photonic crystals, localization occurs at defects to the periodicity of the structure. Recently, twist in a coreless photonic crystal fibre was shown to confine an optical mode. In this project, the analogous system of a rotating two-dimensional photonic crystal has been investigated. Building upon previous work by Götte, Barnett and Padgett, a wave equation for a rotating photonic crystal is derived. Although no explicit solutions have been found yet, several solving procedures are proposed. In addition, the effect of elastic deformations resulting from rotation-induced centrifugal force is investigated theoretically. We report the emergence of localized modes in a circular Bragg deformed by rotation. The wavelength of these modes is found to be proportional to the magnitude of the deformations. The localized modes reported here present a first step towards highly tunable cavity devices.Show less
In this thesis we will demonstrates how to construct a counting mechanical metamate- rial that is based on bi-stable buckling beams by creating a material that can count to ten. By coupling...Show moreIn this thesis we will demonstrates how to construct a counting mechanical metamate- rial that is based on bi-stable buckling beams by creating a material that can count to ten. By coupling bistable buckling beams in a suitable geometry we can design mate- rials that when cyclically compressed between two critical strains. We show that when two adjacent buckled beams touch, there is a characteristic length scale - the inversion length - that determines which of two beams snaps through when compressed to the snapping strain. We experimentally and analytically work out the characteristic inver- sion distance length scale that determines the outcome of the battle of the buckling beams and show how to modify beam geometries to design counting behaviour for many beam materials.Show less
Magnetic resonance force microscopy (MRFM) is a scanning probe technique capable of producing three-dimensional images with nanometer-scale spatial resolution. MRFM relies on the mechanical...Show moreMagnetic resonance force microscopy (MRFM) is a scanning probe technique capable of producing three-dimensional images with nanometer-scale spatial resolution. MRFM relies on the mechanical detection of a weak and oscillating magnetic force between a tip magnet attached to a high compliance cantilever and magnetic moments. Measuring a single electron spin (abbreviated as single-spin) would open the way towards a macroscopic spin-cantilever superposition and three-dimensional images of molecular complexes, e.g. protein structure, with angstrom precision. Although single-spin detection has already been accomplished at 1.6 K, we aim to repeat this feat at millikelvin temperatures to achieve an improved force sensitivity and reduced thermal noise. In this thesis we report on the requirements a setup has to satisfy to enable the detection of an individual spin at millikelvin temperatures. These conditions are drastically more stringent compared to the prerequisites of single-spin detection at a temperature of several kelvin. Moreover, it turned out that our setup does not meet the criteria so we studied several technical enhancements that bring single-spin detection within reach, such as a sample with a lower spin density, nanometer-scale probe magnets and nanometer-sized cantilevers. Provided that these improvements are implemented successfully, detection of an individual spin at millikelvin temperatures appears to be feasible. Furthermore, we present several test experiments with a novel piezoelectric based vibration isolation device. This damping apparatus was designed to actively reduce the level of environmental vibrations near the sample stage, which is required to be ultra-low to achieve a sufficiently large superposition to measure a visible interference.Show less
The study of stepped surfaces is fundamental to the fields of catalysis, nanostructure and chemical surface bonding. In this thesis, we characterize the range of surface structures present in a...Show moreThe study of stepped surfaces is fundamental to the fields of catalysis, nanostructure and chemical surface bonding. In this thesis, we characterize the range of surface structures present in a curved platinum crystal, miscut such that the curvature is perpendicular to the [112] direction. Surface preparations include argon sputtering and annealing cycles. Auger electron spectroscopy has been used to confirm the chemical composition at the surface and low energy electron diffraction has been used to determine the surface structure. Scanning tunneling microscopy was used to image the surface and study its terrace width distribution and step density. Vacancy islands were studied to determine the chirality of the surface. A short to long edge ratio of 0.65 +/- 0.06 is found in vacancy islands. Quantifying defect sites shows a surplus of reactive sites on the surface where step density is lower than 0.005 /nm.Show less
In this thesis, we address our progress to send high currents and generating high magnetic fields at milliKelvin temperatures for the use in MRFM measurements. Multiple ways for sending a current...Show moreIn this thesis, we address our progress to send high currents and generating high magnetic fields at milliKelvin temperatures for the use in MRFM measurements. Multiple ways for sending a current while at 20mK inside a dilution refrigerator are described. The use of a heatsink and an option for splitting the current over multifillament connections are analyzed and tested. We find starting resistances in our spotwelds contradicting with earlier measurements of 3pΩ conducted in our group. Next, the design of a transformer in the form of a cone complement is showed and preliminary tests are presented. Furthermore, the inductance is calculated from a sweep over a frequency range from 500Hz to 20kHz. Our measurements show high potential for an experiment to generate 500mT at 20mK. This experiment is described and in addition, a possible use for B1-fields of this cone complement coil is briefly discussed.Show less
The work presented in this thesis is part of a larger project towards the goal of reading DNA via the tunneling current in a graphene-graphene nanogap. This work focuses on preparing the transition...Show moreThe work presented in this thesis is part of a larger project towards the goal of reading DNA via the tunneling current in a graphene-graphene nanogap. This work focuses on preparing the transition of a graphene twisted edge junction from ambient conditions to liquid environments. Improvements to the fabrication of the electrodes are made by means of silanizing the Si/SiO2 substrate. The use of OTS seems promising, but more measurements are required for valid conclusions. The current contributions of the electrodes in Milli-Q water are investigated component-wise by cyclic voltammetry. Thin pour reprorubber epoxy is demonstrated to be sufficient in insulating the contact and wiring. A double-layer approach to insulating the graphene with PMMA is investigated, but the proposed method does not yield satisfactory results. In order to continue towards sufficiently insulated graphene electrodes, it is suggested to move on to different sample preparation methods or different insulating materials.Show less
The DNA in eukaryotic organisms is largely stored in a compact wrap around histone proteins to form nucleosomes. The mechanics of the DNA play a major role in the biological processes for which the...Show moreThe DNA in eukaryotic organisms is largely stored in a compact wrap around histone proteins to form nucleosomes. The mechanics of the DNA play a major role in the biological processes for which the DNA is used. In this thesis we will computationally show that we can study the mechanics of the DNA with a simplified computational model. By parametrically constraining the DNA around a superhelical curve we can calculate the mechanical energy of the DNA by only the sequence of the DNA and the positions along the curve. We will demonstrate how, with Monte Carlo methods we can effectively estimate the sequence and position statistics of nucleosomal DNA.Show less
Various experiments have indicated that electron transfer through nonmagnetic chiral hydrocarbon molecules is spin dependent. This property is called the chiral-induced spin selectivity (CISS)...Show moreVarious experiments have indicated that electron transfer through nonmagnetic chiral hydrocarbon molecules is spin dependent. This property is called the chiral-induced spin selectivity (CISS) effect and its presence cannot be explained with current theoretical models. To detect the CISS effect experimentally, chiral molecules have to be isolated. Therefore, transport through a single chiral molecule must be measured with STM. A fabrication procedure for sharp and stable nickel STM tips has been developed using electrochemical etching. The produced tips are very sharp and have an average opening angle of 18.9° and an apex size below 200 nm. The magnetic polarisation of the tip can be reversed in situ by a magnetic field, induced by a coil around the tip, which has been demonstrated. The quality and the magnetisation of the tips have been tested on HOPG, gold and nickel with STM. Clear large-scale images were obtained on all materials. Atomic resolution was obtained on HOPG. With these challenges resolved, STM measurements using a nickel tip with both magnetic polarisations can be performed in the study of single chiral molecules as spin filter and the investigation of the CISS effect.Show less
In this thesis I explore the possibility of excitation with a single photon source for biomicroscopy purposes. In biomicroscopy the fluorescent properties of molecules are used to label proteins or...Show moreIn this thesis I explore the possibility of excitation with a single photon source for biomicroscopy purposes. In biomicroscopy the fluorescent properties of molecules are used to label proteins or molecules of interest. A key limitation is the effect of photobleaching: after some time all fluorescent molecules are irreversibly converted to a dark state. Photobleaching processes involve highly reactive excited states of molecules and formation of radical oxygen. Excitation with single photons represses multiphoton processes and it is therefore expected that this will reduce the photobleaching rate. So far no experiments have been done with bright cavity based single photon sources. If the cavity based single photon source at our disposal is integrated in the microscopy setup, this could be a first test system for single photon microscopy.Show less
