This licentiate thesis concerns topics in non-interacting and interacting quantum physics in one dimension. We present the notions of Wannier functions and tight-binding models. Quantum walks are discussed, quantum mechanical analogues to random walks. We demonstrate the ideas of Bloch oscillation and super-Bloch oscillation - revivals of quantum states for particles in a periodic lattice subject to a constant force. Next, the Rabi model of light-matter interaction is derived. The concept of quantum phase transitions is presented for the Dicke model of superradiance. The idea of adiabatic elimination is used to highlight the connectedness of the Dicke model. Finally, we present a one-dimensional interacting system of resonators and artificial atoms that could be built as a superconducting circuit. Using adiabatic elimination as well as matrix product states, we find the phase diagram of this model.

To reach the error threshold required to successfully perform error-correcting algorithms in quantum computers, geometric quantum gates have been considered because of their natural resilience against noise. Non-cyclic geometric gates have been proposed to reduce the run time of conventional geometric gates, to further guard against decoherence. However, while these proposed gates remove the dynamical phase from the computational basis, they do not in general remove it from the eigenstates of the time evolution operator. For a non-cyclic gate to genuinely be considered geometric the dynamical phase should be removed from both the computational basis and the eigenstates. Here, a scheme for finding genuine non-cyclic geometric gates is proposed. The gates are designed to evolve the computational basis along non-cyclic paths, consisting of two geodesic segments, chosen such that the dynamical phase is removed from the eigenstates. The gates found with this scheme did not have shorter runtimes than cyclic gates, but it was possible to implement any gate with this scheme. The findings are important for the understanding of how general quantum computations can be implemented with geometric gates.

High entropy alloys (HEAs) are made of at least five principal elements in near-equimolar proportions. The vast number of possible alloys and unconventional combinations of properties are the main benefits of HEAs. Ti, Nb, Cr, Al and N were chosen in order to create a hard, corrosion resistant coating with good thermal stability. TiNbCrAl multicomponent nitride thin films with Ti content between 0 to 14.4 at.% were deposited using multi-magnetron reactive high power impulse magnetron sputtering (R-HiPIMS) to investigate the feasibility of this method and to study how the Ti content affects the properties of the film. The samples deposited using reactive direct current magnetron sputtering (R-DCMS) were used as benchmarks. The settings required for near-equimolar composition were fixed, with Ti magnetron power as the only variable. Substrate was grounded and not intentionally heated. The composition of HiPIMS samples was more stable while the DCMS samples had significant fluctuations in Al and N content when varying the Ti target power, and were understoichiometric in nitrogen, (T iCrN bAl)1N1−δ, due to low degree of ionization of N. All crystalline samples had NaCl-type fcc structure. Crystalline DCMS samples were (111) textured, while the higher ionization characteristic for HiPIMS resulted in samples with competitive growth between two growth directions. The energetic particle bombardment caused the columnar structure of the film to be denser and less jagged, while DCMS samples containing Ti were significantly more porous. Denser, harder and stiffer films with significantly higher compressive stress were produced with HiPIMS. The hardness and stiffness were almost linearly dependent on Ti content, with density slightly decreasing as the Ti content increased. Higher Ti content increased the rate of corrosion of the films.

Fiber optic sensors offer the ability to measure different types of physical quantities in more harsh environments, such as temperature, pressure and deformations. Some of these demanding environments include chemicals that affect the sensitivity of the sensor, and therefore its resili-ence deteriorates. This work focuses on using experimental techniques to find a method that protects the optical fiber in these chemically demand-ing environments, by coating the fiber with a polymer layer which has the task of protecting it in such environments. A challenge that comes with coating the fiber optic sensor with a polymer layer is that the ability to obtain information becomes more difficult as, its sensitivity deterio-rates. In this project, a type of polymer called polyimide will be tested, using different concentrations and number of layers coated on the optical sensor to investigate the extent that these factors affect the sensor´s ability to cope in chemically demanding environments and also how the sensi-tivity is affected. Thus, the coating method used was soap film coating (SFC). A spectrometer was used to examine the sensitivity of the sensor (using total internal reflection (TIR) and surface plasmon resonance (SPR)). The examination of the resistance of the optical fiber was meas-ured by immersing the polymer-coated sensor in a corrosive liquid for various time intervals and then examining its protective ability. The re-sults obtained through this work demonstrate that polyimide as a coating material provides a protective effect by improving the resistance. The sen-sitivity was most affected when the concentration of the polyimide layer increased from 1-layer to 2-layer polyimide at high concentrations. Re-sistance also increased as the concentration increased, however, 1-layer and 2-layer protection did not have a major impact. The results of this project can be used to further test different types of polymers, for example PVDF. Even more tests with the same attitude and conditions should be carried out to ensure the conclusions and results, and to estimate the measurement uncertainties in the work.

Quantum conductance is a phenomenon associated with nanowires / quantum point contacts where the current through a wire is quantized. Experiments have shown that this phenomenon can be manifested at room temperature using macroscopic wires. This project is aimed to recreate these experiments with emphasis on simplicity. By briefly contacting gold wires and measuring the current using an oscilloscope, current quantization can occasionally be seen as the contact breaks.

Syftet med denna avhandling är att bidra till forskningen av befintliga TMD:er (på Engelska transition metal dichalcogenide-TMD) som visar laddningstäthetsvågor och supraledning vid låga temperaturer (som 2H-TaS2). 2H-TaS2 är också känt för att visa supraledning vid 2K. Dessutom kommer en betydande del av denna avhandling att ägnas åt analysen av den elektronstrukturen nära Ferminivån av Cu-interkalerad TiSe2 och speciellt dess laddningstäthets-beteende vid temperaturer under 200K. Medan de teoretiska modellerna överlåts till teoretikerna, är följande sidor tillägnad att ge ett kvalitativt perspektiv på materialen. Avhandlingen är uppdelad i fyra huvudavsnitt: grundläggande begrepp, experiment-ella tekniker, tidigare rön och dataanalys. Det första avsnittet syftar till att introducera de viktigaste relevanta begreppen för att spåra de många möjliga fenomen som händer i bulk-TMD, speciellt Fe-dopade TaS2 i 2H-fasen och Cu-interkalerade TiSe2 i 1T fas. Elektronisk dispersion i fasta ämnen kommer att diskuteras på ett inledande och fenomenologiskt sätt utan rigorösa härledningar och ska hjälpa läsaren att förstå kapitlen därefter. / Motivation: The family of transition metal dichalcogenides (TMDs) has captured the fascination of researchers worldwide due to their remarkable properties and vast potential for various applications. These 2D materials exhibit a wide range of electronic, optical, and mechanical characteristics, making them incredibly versatile. From semiconductors to superconductors, TMDs offer a rich playground for exploration in condensed matter physics and materials science. Their unique properties are paving the way for breakthroughs in electronics, optoelectronics, energy storage, and beyond. As we delve deeper into the world of TMDs, we uncover new opportunities to revolutionize technology and enhance our understanding of the fundamental principles governing the behavior of matter. Joining the journey of discovery within the TMD family promises exciting challenges and the potential to contribute to the forefront of scientific and technological advancement.  The aim of this thesis is to add to the canon of existing TMDs that display charge density waves and superconductivity at low temperatures (like 2H-TaS2). 2H-TaS2 is also known to display superconductivity at 2K. Additionally, a substantial part of this thesis will be dedicated to the analysis of the electronic structure near the Fermi level of Cu-intercalated TiSe2 and especially its charge-density behaviour at temperatures below 200K. While the theoretical models are left to the theoreticians, the following pages are dedicated to giving a qualitative perspective on the materials. Thesis Outline: The primary goal of this thesis is to provide an introduction to both widely utilized and cutting-edge experimental setups employed by physicists worldwide. This will enable the acquisition of practical experience, facilitating the mastery of best practices and analysis techniques within the realm of experimental condensed matter physics. The thesis is split into four main sections: fundamental concepts, experimental techniques, previous findings and data analysis. The first section is occupied to introduce the main relevant concepts to trace the many possible phenomena happening in bulk TMDs, specifically Fe-doped TaS2 in the 2H phase and Cu-intercalated TiSe2 in the 1T phase. Subjects such as electronic dispersion in solids will be discussed in a rather introductory and phenomenological manner without rigorous derivations and shall aid the reader in understanding the chapters thereafter.

The intention with this thesis is to investigate a dimer in a spin chain. Inorder to do that, a model from Z2-gauge theory is taken as the theoretical motivation to construct a discrete lattice with Ising spin properties. A dimer is then allowed to exist indirectly in the empty space between sites. We choose to tackle the problem through a quantum mechanical approach in 1+1-dimensions, distancing ourselves from the original description in quantum field theory. The exposition begins by reviewing the spatial construction of the entire chain as well as its components, and ends with a discussion of time development where the main concern is dispersion in addition to reflection against a static charge.

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Humanity has great energy demands, and must simultaneously combat climate change by curbing anthropogenic greenhouse gas emissions. Perovskite solar cells (PSC) provide a low-carbon energy source, at lower production costs than traditional silicon-based solar cells. PSC's suffer some issues with long-term stability. This report presents a measurement aimed at better understanding interfacial dynamics of PSC's, using X-ray Photo-electron Spectroscopy (XPS). By collecting data at the synchrotron BESSY II, material compositions at different depths in the cell were successfully measured. An unexplained shift in binding energy was observed for configurations with an external light source on and off.

With the immense increase in the computational power available for the material science community in recent years, a range of new discoveries were made possible. Accurate investigations of large scale atomic systems, however, still come with an extremely high computational demand. While the recent development of Graphics Processing Unit (GPU) accelerated supercomputing might offer a solution to some extent, most well known electronic structure codes have yet to be fully ported to utilize this new power. With a soaring demand for new and better materials from both science and industry, a more efficient approach for the investigation of material properties needs to be implemented. The use of Machine Learning (ML) to obtain Interatomic Potentials (IP) which far outperform the classical potentials has increased greatly in recent years. With successful implementation of ML methods utilizing neural networks or Gaussian basis functions, the accuracy of ab-initio methods can be achieved at the demand of simulations with empirical potentials. Most ML approaches, however, require high accuracy data sets to be trained sufficiently. If no such data is available for the system of interest, the immense cost of creating a viable data set from scratch can quickly negate the benefit of using ML. In this diploma project, the elastic and thermodynamic properties of the Ti0.5Al0.5N random alloy at elevated temperature are therefore investigated using an Active Learning (AL) approach with the Machine Learning Interatomic Potentials (MLIP) package. The obtained material properties are found to be in good agreement with results from computationally demanding ab-initio studies of Ti0.5Al0.5N, at a mere fraction of the demand. The AL approach requires no high accuracy data sets or previous knowledge about the system, as the model is initially trained on low accuracy data which is removed from the training set (TS) at a later stage. This allows for an iterative process of improving and expanding the data set used to train the IP, without the need for large amounts of data.