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
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
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
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
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
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
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
