Examensarbeten för masterexamen
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- PostCarbon Nanotube Networks as Thermally Conducting Layers(2022)Flexible and thermally conductive materials with microfabricated structures are important in a number of different research fields. Different approaches for integration of such materials into functioning devices have been implemented in a plethora of ways. Carbon nanotube networks have been the subject of many studies due to their remarkable physical properties, including high thermal conductivity, high electron mobility, high Young’s modulus and their flexibility, but challenges still remain. One hurdle to overcome is the lack of efficient bonds between nanotubes in meshes. In this project, the viability of a nickel/carbon nanotube network have been investigated in the context of a potential thermal spreading hybrid material. Carbon nanotubes of with different lengths were grown on silicon substrates, dispersed in acetone and mixed into solutions containing Nickel-oxide particles. The blends were deposited onto new Silicon substrates where they formed networks. The Nickel particles stuck to strands and bundles of nanotubes, forming bridges between them. Thermal treatment of the networks were performed at different time scales in order to study the effects of annealing on the networks. The characteristics of the Ni/CNT networks were finally investigated using scanning electron microscopy and Raman spectroscopy in order to study potential changes within them. An increase of the D-peak/G-peak intensity ratio corresponding to longer thermal treatment of the substrates were concluded to be a plausible indicator of increased bonding between the Ni-particles and CNTs. In addition, a simulation was made of a CNT-CNT electron tunneling junction. This was done in order to provide the theoretical backround for the challenges regarding CNT meshes. The lack of chemical bonds between tubes were calculated to increase the resistance of a square CNT thin film by approximately 150%.
- PostLa2-xSrxCuO4 thin films and nanostructures to study local ordering phenomena in a striped superconductor(2022)Since their discovery in 1986, cuprates high critical temperature superconductors (HTS) represent one of the most fascinating class of materials in condensed matter physics. Understanding the underlying mechanism behind high-Tc superconductivity is a real challenge, which could results in the possibility to design in the future a room-temperature superconductor, a technological holy grail allowing an energy-efficiency revolution, and the large-scale realization of applications such as magnetically levitated trains and quantum computers. The strong electron-electron correlations in HTS lead to the formation of exotic charge and spin orders such as charge density waves (CDW) and spin density waves (SDW), that are respectively charge and spin density periodic spatial modulations. In La-based cuprates, as La2-xSrxCuO4 (LSCO), CDW and SDW are characterized, in a well-defined portion of the phase diagram, by a well-defined relation of periodicity, forming the so-called stripe order. The understanding of these local orders is crucial, since they have been recently reported to be responsible for the superconducting and normal state of HTS. Their nature can be effectively investigated in thin films, where the strain induced by the substrate, and the confinement of the HTS at the nanoscale, have been proven to be two powerful knobs to manipulate these orders and understand their mutual interaction. The fabrication of HTS nanostructures is a very challenging task, and up to now relevant results were obtained mainly for YBa2Cu3O7-δ. We optimized the growth of 20 nm thick optimally doped LSCO thin films on LaSrAlO4 (001) substrates by Pulsed Laser Deposition. The films are smooth, as confirmed by atomic force microscopy and reflection high-energy electron diffraction, and highly crystalline, as confirmed by X-ray diffraction. Our best films show a Tc ∼ 39 K, comparable to the bulk value. Finally, we realize LSCO nanowires down to 50 nm width. We measure their Jc values and study the Jc(T). To prove the high degree of homogeneity of our nanowires, we compare the value of J0 c , obtained by fitting the Jc(T) with the Bardeen expression, to the Ginzburg-Landau theoretical limit for the depairing current Jv, due to vortex motion. Our results pave the way for the study of LSCO ground state at the nanoscale.
- PostTensile-strained micromechanical resonators made from crystalline InGaP with low mechanical dissipation and high optical reflectivity(2022)The interest in micro- and nanomechanical resonators has grown rapidly during the last decade due to their broad applicability within metrology and fundamental science. They have, for instance, been brought to the quantum regime and have also been demonstrated to be incredibly precise detectors of small masses, forces, or displacements. A micromechanical resonator’s motion can be detected via optical means, where the resonator’s reflectivity enhances the coupling between it and the light —– resulting in a more efficient read-out. High-reflectivity micromechanical resonators can be achieved by alternating their in-plane dielectric constants with structures known as photonic crystals. On the other hand, a precise read-out of the resonator’s displacement requires low mechanical dissipation for the measurement signal to exceed the thermal noise floor. Dissipation can be minimized by carefully selecting the appropriate material, design, and operating environment for the resonator. Furthermore, mechanical dissipation can even be diluted by introducing tensile strain to the material, which acts as additional storage of energy. The quality factor, which is the ratio between the total energy stored in the system and energy lost during one cycle, is commonly used to quantify mechanical dissipation. This thesis uses highly tensile strained crystalline InGaP to realize micromechanical resonators with low mechanical dissipation. Crystalline materials are promising candidates for highly sensitive micromechanical resonators due to their potentially low intrinsic dissipation, high intrinsic strain, and yield strength. The first part of this thesis investigates the first two of these properties for InGaP by comparing fabricated doubly-clamped strings with analytical models. It was inferred that the stress depends on the crystal direction and varies between 200-500 MPa. Further, the InGaP used in this thesis shows an intrinsic quality factor between 5700±1000 up to 7900 ± 1700. The second part of this thesis focuses on optimizing the geometry of trampoline-shaped micromechanical resonators to enhance their mechanical quality factor and their optical reflectivity. An improved optical reflectivity was observed for fabricated devices patterned with a photonic crystal. FEM-based simulations were made to find dimensions of the trampoline that considerably reduce mechanical clamping loss to the surrounding supporting structure. Implementing these designs to trampoline-shaped micromechanical resonators showed a quality factor of 7 · 10^6 at low temperatures, which was not limited by clamping loss. It was instead demonstrated that the quality factor was limited by gas damping. The mechanical and optical properties of micromechanical resonators fabricated from crystalline InGaP demonstrated in this thesis have shown promising results and provide all requirements for the resonators to be used in optomechanical systems in the near future.
- PostImpact of Equilibration on the Heat Conductance and Noise of non-Abelian fractional Quantum Hall Edges(2022)Performing resistance measurements in a cold 2D electron gas allows to observe the quantum Hall effect. It comes along with a quantized transverse and simultaneously vanishing longitudinal resistance as well as transport along the edge in chiral channels. Some of the discovered fractional quantum Hall states are predicted to host non-Abelian quasi particles that obey exotic exchange statistics with potential use for quantum computation. An essential step towards the manipulation of these particles is to uncover the edge structure of the underlying state and thus verifying the usability of their non-Abelian properties. Recently, a novel method to distinguish between potential candidates for the fractional quantum Hall edge at filling 5/2 has been established using a combination of heat transport and noise arguments. In this thesis, the role of equilibration between counter-propagating edge modes on the heat conductance and the generation of noise at the 5/2 edge is investigated theoretically. This includes an analysis of potential structures describing the 5/2 edge within a common transport scheme and a comparison to experimental results. It is furthermore shown that the heat conductance of the most promising candidate is expected to be quantized to different values of the quantum of heat κ0 = π2kB2 /(3h) depending on the degree of thermal equilibration between the involved modes. Performing experiments with controlled thermal equilibration are therefore predicted to uncover even more details of the underlying structure.
- PostTowards flux-tunable superconducting coplanar waveguide resonators for inductive coupling to levitated superconducting particles(2022)Superconducting magnetic levitation is a promising technique to study potential limits of quantum mechanics for mesoscopic objects due to the levitated object being extremely isolated from the environment. Using optomechanics techniques, the centre-of-mass motion of a levitated particle can be controlled and cooled down to its motional ground-state thereby bringing it into the quantum regime. This would enable macroscopic quantum experiments as well as ultrasensitive force and acceleration sensing. In order to realize this, the motion of the particle’s centre-of-mass would be coupled to a flux-tunable superconducting resonator, which would allow control of the particle motion through the state of the resonator. This thesis investigates the microwave properties of superconducting coplanar waveguide (CPW) resonators. At first, non-flux tunable CPW resonators were fabricated from Aluminum and Niobium and measured in a cryostat at mK temperatures. The best performing CPW resonators achieved unloaded quality factors of ~10^5 and ~10^6 at 10^6 average number of intra-cavity photons for Aluminum and Niobium, respectively. The quality factors of these resonators were found to be one order of magnitude lower for Al, but higher for Nb, when compared to the state-of-the-art. Subsequently, flux-tunable resonators were fabricated by embedding a SQUID into Aluminum-based CPW resonators. The flux tunability of these resonators was studied and was found to be much lower than expected. The reason for the low tunability was due to the fabrication process of the Josephson junctions of the embedded SQUID, in which the junction width was too wide causing an excess amount of aluminum to have deposited onto the substrate. Based on these identified issues, suggestions have been proposed to improve upon the fabrication of flux-tunable CPW resonators, such that in the future the desired frequency tuning through a change in magnetic flux can be observed.