This thesis investigates how the low-energy magnetic configuration of a mesoscopic chain can be tuned by geometrical modifications. The magnetic arrays made by single-domain stadium shaped elements positioned side-by-side were fabricated by patterning into a sputtered ferromagnetic thin film. The thickness of the thin film was determined by X-ray reflectivity measurements while Scanning Electron Microscopy and Atomic Force Microscopy were used to characterize the surface morphology of the nanostructures. Magnetic Force Microscopy was used to image the magnetic configuration of mesoscopic chains after applying a thermal annealing protocol and a field demagnetization protocol. By gradually modifying the geometrical arrangement of the half of mesospins, the magnetic chain is found to exhibit a transition from antiferromagnetic to dimer antiferromagnetic configuration after the thermal annealing treatment. After the field demagnetization protocol, both antiferromagnetic and dimer antiferromagnetic domains are formed. Micromagnetic simulations were performed to investigate how the interaction between the mesospins is affected by the geometrical modifications and a qualitative method was invented to examine the theoretical low-energy state of the magnetic chains. It is found that the low-energy magnetic configuration of the mesoscopic arrays is formed after the competition and collaboration of different interactions and is the one observed after the thermal annealing treatment.

One of the main features of topological phases is the presence of robust boundary states that are protected by a topological invariant. Famous examples of such states are the chiral edge states of a Chern insulator, the helical edge states of a two-dimensional Z2 insulator, and the Fermi arcs of Weyl semimetals. Despite their omnipresence, these topological boundary states can typically only be theoretically investigated through numerical studies due to the lack of analytical solutions for their wave functions. In the rare cases that wave-function solutions are available, they only exist for simple fine-tuned systems or for semi-infinite systems. Exact solutions are, however, common in the field of flat bands physics, where they lead to an understanding of the bulk bands rather than the boundary physics. It is well known that fully-periodic lattices with a frustrated geometry host localized modes that have a constant energy throughout the Brillouin zone. These localized modes appear due to a mechanism referred to as destructive interference, which leads to the disappearance of the wave-function amplitude on certain lattice sites. Making use of this mechanism, it is shown in this licentiate thesis that exact wave-function solutions can also be found on d-dimensional geometrically frustrated lattices that feature (d − 1)-dimensional boundaries. These exact solutions localize to the boundaries when the frustrated lattice hosts a topological phase and correspond to the robust, topological boundary states. This licentiate thesis revolves around the publication, which describes the method to finding these exact, analytical solutions for the topological boundary states on geometrically frustrated lattices, which was authored by the author of this licentiate thesis together with Maximilian Trescher and Emil J. Bergholtz and published in Physical Review B on August 30, 2017 with the title Anatomy of topological surface states: Exact solutions from destructive interference on frustrated lattices. An introduction is given on topological phases in condensed matter systems focussing on those models of which explicit examples are given in the paper: two-dimensional Chern insulators and three-dimensional Weyl semimetals. Moreover, by making use of the kagome lattice as an example the appearance of localized and semi-localized modes on geometrically frustrated lattices is elaborated upon. The chapters in this licentiate thesis thus endeavor to provide the reader with the proper background to comfortably read, understand, place into context and judge the relevance of the work in the accompanying publication. The licentiate thesis finishes with an outlook where it is discussed that the method presented in the paper can be generalized to an even larger class of lattices and can also be applied to find exact solutions for higher-order topological phases such as corner and hinge states.

Proteins often carry an intrinsic electric dipole moment, which can interact with external electric fields and cause protein motion. Previous research has found that the orientation of small proteins in gas phase can be controlled in a static electric field. This effect is hoped to benefit applications such as single-particle imaging, and possibly other techniques involving proteins in electric fields. With the purpose of improving our understanding and modeling of protein orientation, this project investigated the scarcely explored quantum mechanical aspects of the process, namely the polarizability. Ground-state electronic structure simulations of three small model proteins, ubiquitin, Trp-cage and lysozyme, under the influence of electric fields were performed in vacuum. The electric dipole moments of the proteins were extracted from simulations with an applied electric field of strength 1 V/nm for varying angles, with respect to a body fixed reference frame. A Python program was written to analyze and visualize the results. The results point to a connection between the polarizability and the structure of the proteins, as well as size. Next a 3D rigid rotor model was developed using Mathematica in order to study the orientation dynamics classically in a simplified and time efficient way, with the possibility of including the previous quantum results. A comparison between a simulation of ubiquitin with and without polarizability concluded that the polarizability seems to have a damping effect on the orientation dynamics, at least for the initial conditions tested in this study. Further research is necessary to validate the model and perform statistical analysis of many simulations with varying initial conditions. / Proteiner bär ofta på ett inneboende elektriskt dipolmoment, som vid interaktion med externa elektriska fält och orsakar rörelse hos proteinerna. Tidigare studier har funnit att orienteringen av små proteiner i gasfas kan kontrolleras i ett statiskt elektriskt fält. Den effekten kan förhoppningsvis vara en fördel i tillämpningar såsom single-particle imaging, och eventuellt andra tekniker som innefattar proteiner i elektriska fält. I syftet att förbättra vår förståelse och modellering av protein-orientering, har detta projekt undersökt de föga utforskade kvantmekaniska aspekterna av processen, nämligen polariserbarheten. Kvant-baserade simuleringar av grundtillståndet av tre små proteiner, ubiquitin, Trp-cage och lysozym, under påverkan av elektriska fält utfördes i vakuum. Proteinernas elektriska dipolmoment extraherades från simuleringar med ett elektriskt fält med styrkan 1 V/nm för olika vinklar, med avseende på ett kroppsfixerat koordinatsystem. Ett Python-program skrevs för att analysera och visualisera resultaten. Resultaten tyder på att polariserbarheten beror på strukturen och storleken av proteinerna. Därefter utformades en stel-rotor-modell med hjälp av Mathematica för att studera prienteringen klassiskt på ett förenklat och tidseffektivt sätt, med möjligheten att inkludera de tidigare kvantmekaniska resultaten. En jämförelse mellan en simulering av ubiquitin med och utan polariserbarhet konstaterade att polariserbarheten verkar ha en dämpande effekt på orienteringen, åtminstone för begynnelsevillkoren som testades i denna studie. Vidare forskning krävs för att styrka modellen och utföra statistisk analys av många simuleringar med varierande begynnelsevillkor.

The study of magnons is an essential part of combining quantum information science and spintronics, allowing for the investigation of quantum properties such as entanglement in solid-state devices. Magnons are commonly described using the theory of T. Holstein and H. Primakoff, associating the spin operators with bosonic creation and annihilation operators. The quantum mechanical properties inherent to this description are the commutation relations. These relations must be conserved under transformation of the basis. This requires the application of pseudo-unitary transformations U (p, q) when studying the magnon eigenspectrum for example. Depending on the system at hand, the groups U (1, 1) and U (2, 2) are of particular interest and will be the focus of this work. We present a general formalism that leads to a representation of pseudo-unitary matrices via their self-adjoint elements. We apply this representation in examples involving magnons in antiferromagnets to find an explicit picture in connection to material properties. Finally, we explore numerical methods for determining magnon energies and compare them to the analytical counterpart.

In this study the effect of hydrogenation on the optical properties in the wavelength range 400-1023 nm of an ultrathin iron-vanadium superlattice is investigated. Specifically, mea- surements of transmission are performed under different states of hydrogenation, along with measurements of absolute hydrogen concentration and hydrogen site occupancy. The trans- mission measurements are used to construct pressure-concentration isotherms. Isotherms and transmission data are in turn correlated to concentration and hydrogen occupancy.  The results show a wavelength dependent decrease in transmission with hydrogenation. The decrease is greatest around 550 nm, and the wavelength of maximum decrease shifts to higher wavelengths with increasing hydrogen pressure. The non-uniform decrease will make the use of transmission as a measurement of hydrogen concentration dependent on the wavelength of the probing light.  15N resonant NRA is used to perform direct, real-space measurement of absolute hydro- gen concentration. The achieved concentrations are 0.092, 0.38 0.40 H/V. Comparing the concentrations and corresponding transmissions to the location of the plateau region in the transmission based isotherms, it appears that the system is in a single phase at 0.38 and 0.40 H/V, and in a mixed phase at 0.092 H/V. Using a combination of resonant NRA and RBS, while exploiting crystal lattice ion channeling, indirect measurements of hydrogen site occupancy are performed. At all investigated concentrations the system does not display tetrahedral site occupancy, but it remains uncertain whether the occupancy is octahedral or some dislocated octahedral-tetrahedral intermediate.  The relation of hydrogen concentration and optical transmission is investigated via a linear regression analysis. The data points generally deviate by more than one standard deviation from the fitted lines, and lie outside of the error estimation. These deviations might indicate that a linear model is inappropriate, where one possible explanation could be that the mapping from transmission to concentration is dependent on the phase of the system. / Den här studien undersöker upptag av väte i en supertunn kristallstruktur bestående av omväxlande lager av vanadin och järn, samt vätets inverkan på de optiska egenskaperna i våglängdsområdet 400-1023 nm. Specifikt genomförs mätningar av genomsläpp av ljus, under olika nivåer av väteupptag. I samband med dessa mätningar genomförs också mätningar av absolut vätekoncentration och av väteatomernas position i kristallstrukturen. Mätningarna av ljusgenomsläpp används för att skapa isotermkuror över tryck och koncentration. Isotermkurvorna och genomsläppligheten av ljus korreleras till vätekoncentration och väteatomernas position i kristallstrukturen.  Resultaten visar en våglängdsberoende minskning av ljusgenomsläppligheten med en ökande mängd väte i kristallstrukturen. Minskningen är som störst omkring 550 nm, samtidigt som våglängden för störst minskning flyttas mot högre våglängder med högre koncentration av väte. Att minskningen i genomsläpplighet är beroende av våglängd innebär att ljusgenomsläpp som metod för att mäta vätekoncentration är beroende av den ljusvåglängd som används. Metoden 15N resonant NRA används för att genomföra direkta mätningar av absolut vätekoncentration. De uppmätta koncentrationerna är 0.092, 0.38 och 0.40 H/V. När dessa koncentrationsmätningar jämförs med genomsläpplighet och tillhörande isotermkurvor, så verkar det som att systemet befinner sig i en enskild fas vid koncentrationerna 0.38 och 0.40 H/V, och i en blandad fas vid koncentrationen 0.092 H/V. Indirekta mätningar av vätets position i kristallstrukturen genomförs baserat på en kombination av resonant 15N NRA och RBS, där det utnyttjas att projektiljonerna under vissa förutsättningar kan komma att styras in i kristallstrukturen (på engelska crystal lattice ion channeling). Vid de tre uppmätta koncentrationerna så visar systemet inga tecken på att väteatomerna finns på tetrahedrala positioner. Det är inte helt uppenbart om väteatomerna istället finns på oktahedrala positioner, eller om det handlar om förskjutna positioner som är mellanliggande till oktahedrala och tetrahedrala. Relationen mellan vätekoncentration och optisk genomsläpplighet analyseras med linjär regression. Datapunkterna avviker generellt med mer än en standardavvikelse från de anpassade linjerna, och ligger utanför feluppskattningen. De här avvikelserna kan indikera att en linjär modell inte är lämplig, och en möjlig förklaring kan vara att ljusgenomsläppligheten beror av den fas i vilken systemet befinner sig.

Understanding the low-temperature properties of strongly correlated materials requires accurate measurement of the physical properties of these systems. Specific heat and nuclear spin-lattice relaxation are two such properties that allow the investigation of the electronic behavior of the system.  In this thesis, nanocalorimetry is used to measure specific heat, but also as basis for new experimental approach, developed to disentangle the different contributions to specific heat at low temperatures. The technique, that we call Thermal Impedance Spectroscopy (TISP) allows independent measurement of the electronic and nuclear specific heat at low temperatures based on the frequency response of the calorimeter-sample assembly. The method also enables simultaneous measurements of the nuclear spin-lattice relaxation time (T1). The nuclear spin lattice relaxation, as 1/T1T, and electronic specific heat, as C/T, provide information about the same quantity, electronic density of states, in the system. By comparing these properties in strongly correlated systems, we can obtain insights of electronic interactions.  Metallic indium is studied using thermal impedance spectroscopy from 0.3 K to 7 K at 35 T. The magnetic field dependence of nuclear spin-lattice relaxation rate is measured. Indium is a simple metallic system and the expected behavior of the nuclear spin-lattice relaxation is similar to that of the electronic specific heat. The results of the measurement are matched with the expectation from a simple metallic system and Nuclear Magnetic Resonance (NMR) measurements. This demonstrates the effectiveness of the new technique.  The heavy-fermion superconductor CeCoIn5 is studied using thermal impedance spectroscopy and ac-calorimetry. This material is located near a quantum critical point (QCP) bordering antiferromagnetism, as evidenced by doping studies. The nature of its quantum criticality and unconventional superconductivity is still elusive. Contrasting specific heat and nuclear spin-lattice relaxation in this correlated system helps to reveal the character of its quantum criticality.  The quantum criticality in CeCoIn5 is also studied using X-ray Absorption Spectroscopy (XAS) across the superconducting transition and X-ray Magnetic Circular Dichroism (XMCD) at 0.1 K and 6 T. The element-specific probe zooming in on cerium in this material indicates two things, a mixed valence of Ce in the superconducting state and a very small magnetic moment, that implies resonance-bond like antiferromagnetic local ordering in the system.

The work in this thesis is basically divided into two related but separate investigations. The first part treats simple chemical reactions of adsorbate molecules on metallic surfaces, induced by means of a scanning tunneling probe (STM). The investigation serves as a parameter free extension to existing theories. The theoretical framework is based on a combination of density functional theory (DFT) and non-equilibrium Green's functions (NEGF). Tunneling electrons that pass the adsorbate molecule are assumed to heat up the molecule, and excite vibrations that directly correspond to the reaction coordinate. The theory is demonstrated for an OD molecule adsorbed on a bridge site on a Cu(110) surface, and critically compared to the corresponding experimental results. Both reaction rates and pathways are deduced, opening up the understanding of energy transfer between different configurational geometries, and suggests a deeper insight, and ultimately a higher control of the behaviour of adsorbate molecules on surfaces. The second part describes a method to calculate STM images in the low bias regime in order to overcome the limitations of localized orbital DFT in the weak coupling limit, i.e., for large vacuum gaps between a tip and the adsorbate molecule. The theory is based on Bardeen's approach to tunneling, where the orbitals computed by DFT are used together with the single-particle Green's function formalism, to accurately describe the orbitals far away from the surface/tip. In particular, the theory successfully reproduces the experimentally well-observed characteristic dip in the tunneling current for a carbon monoxide (CO) molecule adsorbed on a Cu(111) surface. Constant height/current STM images provide direct comparisons to experiments, and from the developed method further insights into elastic tunneling are gained.

In this master thesis, a 2D device simulator run on a hybrid classical-quantum computer was developed. The simulator was developed to treat statistical quantum effects such as quantum tunneling and quantum confinement in nanoscale transistors. The simulation scheme is based on a self-consistent solution of the coupled non-linear 2D SchrödingerPoisson equations. The Open Boundary Condition (OBC) of the Schrödinger equation, which allows for electrons to pass through the device between the leads (source and drain), are modeled with the QuantumTransmitting Boundary Method (QTBM). The differential equations are discretized with the finite-element method, using rectangular mesh elements. The self-consistent loop is a very time-consuming process, mainly due to the solution of the discretized OBC Schrödinger equation. To accelerate the solution time of the Schrödinger equation, a quantum assisted domain decomposition method is implemented. The domain decomposition method of choice is the Block Cyclic Reduction (BCR) method. The BCR method is at least 15 times faster (CPU time) than solving the whole linear system of equations with the Python solver numpy.linalg.solve, based on the LAPACK routine _gesv. In the project, we also propose an alternative approach of the BCR method called the "extra layer BCR" that shows an improved accuracy for certain types of solutions. In a quantum assisted version, the matrix inverse solver as a step in the BCR method was computed on the D-Wave quantum annealer chip ADVANTAGE_SYSTEM4.1 [4]. Two alternative methods to solve the matrix inverses on a quantum annealer were compared. One is called the "unit vector" approach, based on work by Rogers and Singleton [5], and the other is called the "whole matrix" approach which was developed in the thesis. Because of the limited amount of qubits available on the quantum annealer, the "unit vector" approach was more suitable for adaption in the BCR method. Comparing the quantum annealer to the Python inverse solver numpy.linalg.inv, also based on LAPACK, it was found that an accurate solution can be achieved, but the simulation time (CPU time) is at best 500 times slower than numpy.linalg.inv.

In today's data-driven society, data holds immense value and is sought after across various domains, including the realm of science. Materials science, in particular, relies heavily on data acquisition and analysis to further advancements in the field. In this study, data for an alloy database is generated through high-throughput calculations, serving as a valuable resource for investigating the effects of vacancy concentration on the structural and mechanical properties of (TM)0.5Al0.5N1-x random alloys. The alloys, comprised of Ta or Nb (TM), exhibit promising potential for diverse applications such as cutting tool, electrical and optical devices, etc. To accurately represent the average behavior of real random alloys, special quasirandom structures (SQS) are utilized, and the short-range order (SRO) parameters are analyzed using the ATAT code. A two-level computational approach with different convergence criteria is used to investigate the influence of vacancy concentration on alloys. This approach utilizes the high throughput toolkit (httk) software package in conjunction with VASP calculations. The calculations are performed on supercomputer resources provided by the National Academic Infrastructure for Supercomputing in Sweden (NAISS). The elastic constants of the alloys are calculated using the httk software, providing insights into their mechanical properties. The findings highlight the substantial influence of vacancy concentration on the structural and mechanical behavior of (TM)0.5Al0.5N1-x random alloys.

Nanophotonic and plasmonic research have seen many breakthroughs lately which has created a demand for automated design algorithms to optimize optical elements at the nanometer scale. This work focuses on plasmonic nanostructures that are small metal particles interacting with electromagnetic radiation on length scales typically less than the wavelength. Plasmonic effects from nanostructures are used for enhancing and manipulating electromagnetic fields at the nanometer scale which have seen many applications in sensing requiring an ultra-high sensitivity and a small resolution. This work is about how deep learning methods can be used for designing plasmonic gold nanostructures. In particular, we investigate how convolutional neural networks can be used to predict the optical properties of nanostructures and how conditional generative adversarial networks (cGAN) can be used for designing structures with desired optical properties. The data in this work consist of images with differently shaped nanostructures and the corresponding spectral data for the scattering cross section, the absorption cross section, the polarization rotation and the polarization ellipticity. Utilizing the convolutional EfficientNet architectures, we train a forward model to predict the spectral data of anisotropically shaped nanostructures with images of the structure shape as input. We achieve a mean squared error of 2.5 × 10−4, 2.5 ×10−4, 6.0 ×10−4, and 4.9 ×10−4 respectively for each variable which agrees with the literature for similar problems. For the inverse design models, we show that label projection can be used to improve the results of two common GAN architectures in combination with a novel label embedding network. We use the Wasserstein GAN method with gradient penalty for training the models to generate images of nanostructure shapes conditioned on spectral data. The best model achieves a pixelwise mean absolute error of 4.9×10−3 and an estimated spectral mean absolute error of 8.4×10−3 between original and generated images when trained on a dataset containing cylindrical dimer structures. Furthermore, we have shown that the pixelwise mean absolute error is reduced when more conditional input variables are added to the model and that the model can learn different nanostructure shapes when trained on a large dataset containing different anisotropic gold nanostructure shapes. The best pixelwise mean absolute error found is 1.1×10−2 and the estimated spectral mean absolute error is 1.7 × 10−2 on the full dataset using all available input data.