This research aims to improve the existing capabilities of Scanning Tunneling Microscopy (STM) in a dry-dilution refrigerator, at the Oosterkamp group. The group currently has an STM setup that can...Show moreThis research aims to improve the existing capabilities of Scanning Tunneling Microscopy (STM) in a dry-dilution refrigerator, at the Oosterkamp group. The group currently has an STM setup that can resolve step edges in highly oriented pyrolytic graphite (HOPG) and is poised to achieve atomic resolution [1]. Currently the STM is mounted in the dry-dilution refrigerator, which allows for the study of samples at milli-Kelvin temperatures. The use of a dry-dilution refrigerator gives rise to some complications. Specifically we analyse the contributions of the compressor from the pulse tube cooling system. It was found that this compressor is the dominant source of mechanical noise in setup. We determined that the vibrations enter through both the compressor lines and via the ground through the outer frame. The compressors vibrations gave rise to 0.88 ̊ARMS noise. By placing the compressor on compressed gas springs and running the compressor lines through a trough filled with sandbags, we were able to reduce this noise to 0.13 ̊ARMS. This should no longer stand in the way of reaching atomic resolution STM.Show less
Scanning Tunneling Microscopy (STM) is a technique that allows its user to make scans of the surface topography of a sample. Via a capacitive approach, a tip is brought in close proximity to the...Show moreScanning Tunneling Microscopy (STM) is a technique that allows its user to make scans of the surface topography of a sample. Via a capacitive approach, a tip is brought in close proximity to the sample, by which eventually a tunneling current is measured that is used to obtain a scan. It can be interesting to do STM experiments at extremely low temperatures (< 4 K), which can be done by placing an STM into a dilution refrigerator. It is difficult to do so in a dry dilution refrigerator, because of the pulse tube system that brings lots of mechanical vibrations. This research shows the performance of the newly developed `PAN-motor' and `linear cryo-walkers', two new types of motors for STM-approaches at extremely low temperatures. The cryo-walkers in particular have allowed for a full approach at millikelvin temperatures due to its low heat dissipation, whereas this approach normally has to be done above 4 K. By realizing dry dilution refrigerator STM, more research at millikelvin temperatures would be made possible, due to these dry dilution refrigerators being closed systems, allowing for time-unlimited measurements. This advancement could allow for experiments with low T_c superconductors, or shot noise experiments with the new Multi-LC circuit, that has shown to enable the measurement of shot noise with a total measuring time of \tau \approx 98 h.Show less
Inelastic electron tunneling spectroscopy (IETS) is a tool used to research the vibrational modes in a tunneling junction. This report describes the setup of a cryogenic scanning tunneling...Show moreInelastic electron tunneling spectroscopy (IETS) is a tool used to research the vibrational modes in a tunneling junction. This report describes the setup of a cryogenic scanning tunneling microscope (STM). Potentially this STM can be used in the research to the edge modes of graphene edge junctions. Gold samples were used for the calibration of the z stage of the setup by using two different methods (histogram based/line cut based). Experiments at cryogenic temperatures seemed to have difficulties measuring topographic images at 77K. These difficulties are most likely caused by condensation of atmospheric gasses still present in the system. To solve this problem, a heater is required to prevent material condensing on the sample or tip. In order to implement IETS in the system it is proposed to used a combination of a heater and thermometer in order to keep the system clean.Show less
Current imaging is crucial to condensed matter physics, materials research and industry. State-of-the-art current imaging setups revolve around SQUID-on-tip (SOT) probes, that scan over a sample to...Show moreCurrent imaging is crucial to condensed matter physics, materials research and industry. State-of-the-art current imaging setups revolve around SQUID-on-tip (SOT) probes, that scan over a sample to locally measure magnetic fields and temperature. The resolution of such systems is presently limited by the lack of a robust method to control the probe-sample distance. In this thesis, we develop probes for hybrid microscopy that combine SOT with STM. We theoretically investigate interesting systems, and find that our approach would considerably improve on past magnetic investigations of vortex matter. We use focused-ion-beam milling to fabricate SOT probes on top of a commercial AFM-cantilever, and show these to be very sensitive to changes in applied magnetic field and temperature. We develop a novel readout scheme to simultaneously measure a magnetic and a tunneling signal. We present a proof-of-concept STMSOT probe that displays magnetic sensitivity inside a cryogenic STM setup, and use it as an STM probe to see the topography of a NbSe$_2$ crystal. Our approach will culminate in the development of a STMSOT setup in the near future.Show less
This project follows a design that aims at enabling multi-frequency readout of STM current noise in the MHz regime, with future prospects to amplify and record such signals with a SQUID amplifier....Show moreThis project follows a design that aims at enabling multi-frequency readout of STM current noise in the MHz regime, with future prospects to amplify and record such signals with a SQUID amplifier. The design consists of multiple LC resonators to allow multi-frequency readout. Design choices, freedoms and restrictions are noted and a method to built and test the model is given. Steps are taken to allow testing of a SQUID readout experiment inside a dry dilution cryostat and the dynamic range of the SQUID is calculated and compared to typical STM (noise) currents. Mainly the feasibility of the design is studied and further effort to quantitatively explore the design is suggested.Show less
To study the coupling mechanism in high-Tc superconductors we would like to observe them using STM while suppressing the superconductivity with high currents. As the superconductor under study we...Show moreTo study the coupling mechanism in high-Tc superconductors we would like to observe them using STM while suppressing the superconductivity with high currents. As the superconductor under study we choose Bi2S2C1C2O8+x because of easy exfoliatability and doping. Wanting to achieve the required high current densities we decide on lithographically contacting a flake and performing a cleaving in the STM. Several methods are attempted and successful cleaving outside of the STM is achieved. The procedure, however, cannot be reproduced reliably inside the chamber. The journey leading to the result does yield some promising insights to complete the final step.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
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
When cuprate compounds are sufficiently doped with extra holes, the Mott insulating phase gives way to the puzzling phenomenon of high-temperature superconductivity. Here, we use spectroscopic...Show moreWhen cuprate compounds are sufficiently doped with extra holes, the Mott insulating phase gives way to the puzzling phenomenon of high-temperature superconductivity. Here, we use spectroscopic-imaging scanning tunnelling microscopy (SI-STM) to probe two overdoped cuprate samples belonging to the family of BSCCO. The two samples have slightly different doping levels and critical temperatures TC of 3 K and 12 K. At this doping level, the band structure contains a saddle point close to the Fermi surface. As such, one expects to see a van Hove singularity (vHS) peak in the local density of states at every spatial position, i.e. in every STM dI/dV spectrum. Surprisingly, we find that the vHS peak is absent in part of the measured dI/dV spectra. Hence, to enable further investigation into the partial absence of the vHS peak, we developed a phenomenological model that is capable of fitting all the single dI/dV spectra. Using this model, we are able to spatially map the presence of the van Hove singularity and to correlate its energy to the width of the measured gap.Show less
In the search for substrates used in the investigation of catalytic nanoparticles we have grown alumina films on NiAl(110) past 5.35 Å, the maximum thickness that can be obtained with molecular...Show moreIn the search for substrates used in the investigation of catalytic nanoparticles we have grown alumina films on NiAl(110) past 5.35 Å, the maximum thickness that can be obtained with molecular oxygen. A NiAl(110) sample was first oxidized to that thickness with molecular oxygen, after which it was stepwise oxidized with NO2. The film thickness, crystal structure and chemical composition were measured using Low Energy Electron Diffraction (LEED), X-ray Photoelectron Spectroscopy (XPS) and Scanning Tunneling Microscopy (STM). We have confirmed that film grown with NO2 follows the theoretical model of Cabrera and Mott up to 6.9 Å. STM images show an increase in the number of islands and vacancies in the metal, indicating that this growth happened at the film-metal interface and not the film surface.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
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
This thesis presents the characterization of a broadband radio frequency power detector, they key element of the detector being the Herotek DZM020BB diode. The detector has been developed to...Show moreThis thesis presents the characterization of a broadband radio frequency power detector, they key element of the detector being the Herotek DZM020BB diode. The detector has been developed to measure shotnoise signal orig- inating from a UHV low temperature STM tunnel junction. The detector provides fast measurement times of 1 sec while providing enough resolu- tion 0.06μVrms to distinguish between different noise voltages. This detec- tor has been compared to the Zurch MFLI lock in amplifier which measures the shotnoise with measuring times in the order of 10 seconds. Systematic effects in the detector have been studied and possible solutions were sug- gested to eliminate these effects.Show less
In this work, the possibility to measure time- and spatially resolved spin fluctuations using Scanning Tunneling Microscopy is investigated. By using an impedance matching circuit as described in ...Show moreIn this work, the possibility to measure time- and spatially resolved spin fluctuations using Scanning Tunneling Microscopy is investigated. By using an impedance matching circuit as described in [1], the bandwidth of conventional STM can be increased opening up possibilities for new kinds of experiments. When combined with the technique of spin-polarized STM, it theoretically becomes possible to track spin states of individual atoms. Here, we present an overview of existing literature on this topic and propose several experiments to test this hypothesis. Finally, with a python simulation, we test the viability of EPR-STM measurements on a single atom and provide directions to expand upon this work.Show less
During this project we have tested and characterized a new cryogenic STM (Scanning Tunneling Microscope) with a uniquely designed coarse XY stage. We have used this STM the perform STM break...Show moreDuring this project we have tested and characterized a new cryogenic STM (Scanning Tunneling Microscope) with a uniquely designed coarse XY stage. We have used this STM the perform STM break junction experiments on both clean gold samples and samples covered by OPE-3 molecules. In the results for the clean gold samples we retrieve the characteristic and well known features of single atom gold wires. For the results of the samples covered by molecules we see faint traces of the molecules in the conductance histogram, although they are not as clear as compared to results in literature. Cryogenic experiments using this STM have proven to be difficult and only partially successful.Show less
In this thesis, we describe a new sample production method for the graphene nanogap junction of the AMC group. This production method features a new, standardised, sample layout with integrated...Show moreIn this thesis, we describe a new sample production method for the graphene nanogap junction of the AMC group. This production method features a new, standardised, sample layout with integrated metal contacts and a back gate. A large part of the sample production is outsourced commercially. This leads to mass production yielding more reliable samples. We have also introduced some new pieces of equipment that make the rest of the sample production both easier and more efficient. Additionally, we have proposed a design for a new sample holder. This will allow us to perform the experiment in cryogenic environments.Show less
Currently the shot noise signal from the STM is measured with the Zurich MFLI lock in amplifier, which has measuring times on the order of 10 seconds. In this research we will build a RF diode...Show moreCurrently the shot noise signal from the STM is measured with the Zurich MFLI lock in amplifier, which has measuring times on the order of 10 seconds. In this research we will build a RF diode detector. Starting out with the Herotek DZM020BB RF diode, we add additional components to increase its performance at measuring small signals. Then we compare its accuracy and measuring times with the Zurich MFLI. The detector had 1/f noise, which was eliminated with the ”tic toc” method. We find a final resolution of 0.06 μV for the detector with 1 s measuring time, which means that it performs better than the Zurich MFLI.Show less
In this bachelor thesis, we investigate cobalt on a Au(111) surface in a sulfiding H2S background by deposition and extended H2S exposure to gain insight in the interactions between Co, H2S and the...Show moreIn this bachelor thesis, we investigate cobalt on a Au(111) surface in a sulfiding H2S background by deposition and extended H2S exposure to gain insight in the interactions between Co, H2S and the Au(111) surface. In particular, we found two CoSx equilibrium structures. A hexagonal structure a which is about 2.1nm in size and is similar to an elsewhere reported Co3S4 structure. Structure g is more rectangular with a size of approximately 1.3nm x 0.8nm. Additionally we observed a structure x decorating the step edge. The experiments done to investigate the mobilization of gold due to H2S in presence of cobalt and the formation of H2S adsorption sites by Co replacing Au atoms, remain inconclusive. Due to the low intensities of the XPS spectra, the elemental compositions of observed CoSx structures could not be calculated and require further research.Show less
The goal of this thesis is to investigate the possibilities of building a GHz compatible circuit that will allow high frequency measurements with a Scanning Tunneling Microscope. In this frequency...Show moreThe goal of this thesis is to investigate the possibilities of building a GHz compatible circuit that will allow high frequency measurements with a Scanning Tunneling Microscope. In this frequency range, many interesting properties of materials could be accessed, as for example shot noise in the tunneling current. The main problem in these kinds of measurements is the mismatch between the very high impedance of the tunneling junction and the 50 Ω impedance of the measurement circuitry, which causes the high frequency signal to be reflected back. Here, two solutions to this problem - lumped and distributed impedance matching - are theoretically described and simulated in order to determine their advantages and disadvantages. Lastly, a distributed resonating circuit is built and measured, with the purpose of investigating potential difficulties in distributed circuits on a PCB.Show less
