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
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
Bi(2)Sr(2)Cu(1)O(6+x) is a high-temperature superconductor exhibiting strange metal behaviour. A strange metal shows linear resistivity over a long range of temperatures. The strange metal...Show moreBi(2)Sr(2)Cu(1)O(6+x) is a high-temperature superconductor exhibiting strange metal behaviour. A strange metal shows linear resistivity over a long range of temperatures. The strange metal behaviour can possibly be explained by the Anti-de Sitter (AdS)/Conformal Field Theory (CFT) correspondence. In order to investigate the correspondence, a method for reliably measuring the strange metal phase is required. Because measurements on macroscopic crystals deviate from the expected linear resistivity due to C-axis contribution, microscopic Bi(2)Sr(2)Cu(1)O(6+x) flakes had to be used for the measurements. Therefore macroscopic crystals were exfoliated and the resulting flakes were contacted with electron beam lithography. Observing the strange metal regime of Bi(2)Sr(2)Cu(1)O(6+x) under the superconducting dome furthermore requires high current densities and high magnetic fields. These prerequisites for breaking the superconducting phase were obtained by structuring the contacted flakes using a Focused Ion Beam. After successfully contacting the flakes, linear resistivity was actually observed. Calculations on a Hall-bar and a constriction then yielded values for the resistivity of Bi(2)Sr(2)Cu(1)O(6+x) in agreement with literature. By contacting flakes showing the strange metal behaviour the first step for research into the AdS/CFT correspondence has been taken.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
Ultra-thin films with different thicknesses of SrRuO3 were grown epitaxially on SrTiO3. The anomalous Hall effect (AHE) with additional peaks was observed in Hall resistivity measurements as a...Show moreUltra-thin films with different thicknesses of SrRuO3 were grown epitaxially on SrTiO3. The anomalous Hall effect (AHE) with additional peaks was observed in Hall resistivity measurements as a function of field and temperature in a 5 unit cell SrRuO3 film without capping layer. The additional peak phase matches literature, whereas the actual resistivity size is found to be lower. The peaks could be explained by either a topological Hall effect (THE) caused by the presence of a skyrmion lattice or two phases of the AHE corresponding to different interfaces. If the additional effect is a THE, this study confirms the presence of a robust skyrmion phase in ultra-thin SrRuO3 on SrTiO3 without capping layer, while gating experiments indicate that the skyrmion size could be tuned by an electric field.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
We describe a method to increase the critical temperature of BCS superconductors. The method is based on altering the electronic properties of a thin film of a superconductor by periodically...Show moreWe describe a method to increase the critical temperature of BCS superconductors. The method is based on altering the electronic properties of a thin film of a superconductor by periodically fabricating holes in the crystal lattice. We use a MATLAB simulation to demonstrate that certain patterns enhance the coupling between electrons and phonons, which increases the transition temperature. In this project we attempted to improve the simulation such that it executes faster and is compatible with hexagonal structures.Show less
Prior research has demonstrated by theory and simulation that the creation of holes in two-dimensional superconductors by periodic nanopaterning can increase the critical temperature. To test this...Show morePrior research has demonstrated by theory and simulation that the creation of holes in two-dimensional superconductors by periodic nanopaterning can increase the critical temperature. To test this theory it must be implemented experimentally and the resulting critical temperature must be measured. Preceding the modification of these superconductors, the cryogenic resistance-temperature measurement must be tested and optimized. This thesis discusses the steps, and the problems in them, of the measurement process and provides possible explanations and solutions.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 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
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
In this thesis we discuss the results of different resonant circuits to measure shot noise in an STM. We found two circuits where a change in the shot noise can be detected relatively easily. One...Show moreIn this thesis we discuss the results of different resonant circuits to measure shot noise in an STM. We found two circuits where a change in the shot noise can be detected relatively easily. One of the circuits has a relatively large bandwidth and the other has a relatively large total signal. Furthermore we benchmarked an RF diode detector (envelope detector) to improve the measurement speed for shot noise measurements. We conclude that we need an additional amplifier to amplify the RF output signal of the resonant circuit $2\cdot 10^6$ to $3\cdot 10^6$ times to use the RF diode detector.Show less
Motional control of mechanical resonators is crucial for their applications. In particular, cooling the mechanical mode to overcome the thermal noise has been greatly explored, and has recently...Show moreMotional control of mechanical resonators is crucial for their applications. In particular, cooling the mechanical mode to overcome the thermal noise has been greatly explored, and has recently been pushed into the quantum regime. In this thesis, we study an almost-forgotten cooling technique: resistive cooling with an artificial cold resistor (ACR) which is physically at room temperature. We perform a proof-of-principle demonstration to cool a mechanical mode of a quartz crystal with a “cold” resistor. The “cold” resistor is realised either by a normal resistor cooled by liquid nitrogen or by an ACR made of a special circuitry. We show that the ACR can cool the mode in the same way as a real cold resistor, and the cooling mechanism can be qualitatively understood in the basic thermodynamic picture. We also discuss the feasibility of applying such resistive cooling to an optomechanical system, with a nested trampoline resonator.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
Scanning Tunnelling Microscopy (STM) is a well established and widely used technique in the world of surface physics, capable of measuring atomic resolution topographs within seconds. There are...Show moreScanning Tunnelling Microscopy (STM) is a well established and widely used technique in the world of surface physics, capable of measuring atomic resolution topographs within seconds. There are however still improvements we can make. Where spatial resolution is almost perfect, the temporal resolution of STM is quite terrible, limiting the measurement of rapid fluctuations in the tunnelling current. This withholds STM from for example measuring shotnoise and single atom spin relaxation. We try to solve this issue by designing a small cryogenic amplifier and implementing it close to the tip of a STM setup, increasing its bandwidth around 2.8MHz. We discus simulations as well as test results from our amplifier. Finally, we give an outlook on how to improve this design in order to measure shotnoise.Show less
Physicists like to find out how things work on the smallest level. When doing experiments in which we want to achieve atomic resolution, there are many factors that can influence the experiments,...Show morePhysicists like to find out how things work on the smallest level. When doing experiments in which we want to achieve atomic resolution, there are many factors that can influence the experiments, for example: thermal fluctuation and external vibrations. The latter is what we focused on during this project. At the Leiden Institute of Physics we have built a new low vibration laboratory in order to perform experiments that can achieve atomic resolution. In this report we will describe and compare the vibrations in both the old and new low vibration laboratory. We have found that the vibrations in the new laboratory are much less and that this new lab is comparable with some of the most successful scanning tunneling microscopy laboratories in the world.Show less
