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In the present work, growth and structural characterization of CrB2/TiB2 superlattices on (0001) Al2O3 substrate is studied. The superlattices are grown using a direct current magnetron sputtering (DCMS) system with a base pressure of <9E-7 Torr. For structural characterization X-ray diffraction (XRD), X-ray reflectivity (XRR), transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), and scanning transmission electron microscopy (STEM) are used. Hardness is measured using nanoindentation technique. For growth of CrB2/TiB2 superlattices it is shown that the sputtering gas pressure of PAr= 4 mTorr, and substrate temperature of T= 600 °C are optimized parameters for growing well-structured superlattices with good interface quality. Superlattices with a layer thickness ratio of Γ= 0.43 (Γ= DTiB2/DCrB2+DTiB2) and a total thickness of 1 μm are deposited with different modulation periods Λ=1, 2, 6, 8, and 10 nm to see the layer-thickness affect on the quality of the structures. XRD and TEM results show that by increasing the modulation period, the quality of superlattices with smooth interfaces increases. The superlattices with modulation period Λ=8 nm is shown to be the best structure having coherent lattice and smooth interfaces up to ~20 periods. The STEM analysis shows that after about 20 periods, grains started to grow at slightly different orientations. A superlattice of TiB2/CrB2(having TiB2 as the first deposited layer) with modulation period Λ=8 nm shows an epitaxial growth of TiB2 on Al2O3 (0001) substrate, however, no big difference between the structure of TiB2/CrB2 and CrB2/TiB2 superlattices is seen. EDX maps and line profiles show that there is a diffusion of CrB2 into TiB2 layers which is a serious problem for obtaining sharp interfaces. STEM also shows that for a small modulation period of Λ=1 nm, there is a faint layered structure, whereas EDX, SAED and XRD indicates a homogenous textures Ti-Cr-B film in this sample. Finally, the hardness measurement shows a hardness value of 29-34 GPa for different modulation periods. The lowest hardness value is related to the sample with modulation period of Λ=1 nm with about 29 GPa, and the highest hardness is related to the sample with Λ=8 nm ith around 34 GPa.

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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.

<p>Some molecular complexes exhibiting bistability between two different spin states have been subject to vast investigations. Spin crossover and valence tautomerism interconversion molecular compounds are such examples showing this dynamic switching behavior and are a route toward designing molecular devices with a facile readout due to the change in the spin state that accompanies the change in conductance. Due to their extensive potential applications in industry and research, they are among the most interesting topics in spintronics. Spin state switching processes provide the foundation for applications in molecule-based devices. The main goal is to study the parameters that affect the intramolecular electron transfer between different spin states in spin crossover molecular thin films and the electron transfer between the metal center and redox-active ligands in valence tautomer thin films. </p> <p>Because substrate effects are important for any molecular-based device, there are increasing efforts to study the influence of the substrate on spin state transition. While some non-metallic substrates like graphite seem to be promising from experimental measurements, theoretical and experimental studies indicate that 2D semiconductor surfaces will have minimum interaction with spin crossover molecules.</p> <p>In this work, the functionality of two different spin crossover molecules sublimated on the ferroelectric Polyvinylidene Fluoride Hexafluoropropylene (PVDF-HFP) layer and 2D Ti3C2 MXene thin film is studied. We report the temperature-dependent spin state transition of spin crossover molecules, [Fe{H2B(pz)2}2 (bipy)], thin films, and valance tautomer [Co(sq)(cat)(3-tpp)2] thin films. Using a UV-Vis spectrometer and a specific lab-built sample holder we were able to perform the measurement at temperatures as low as 90 K and as high as 440 K. Temperature-dependent UV-Vis data show that the transition temperature from the high spin state to the low spin state and vice versa is well below the room temperature. However, for isothermal switching purposes, we designed and fabricated a bilayer device with PVDF-HFP thin films as the substrate to facilitate isothermal switching close to room temperature. The retention of voltage-controlled nonvolatile changes to the electronic structure in bilayers of PVDF-HFP/[Fe{H2B(pz)2}2(bipy)] and PVDF-HFP/[Co(sq)(cat)(3-tpp)2] strongly depends on the thickness of the PVDF-HFP layer. </p> <p>While the electric transport measurement revealed that doping the SCO layer with Ti3C2 MXene flakes can significantly improve the conductivity of the spin crossover molecular thin films, the temperature-dependent UV-Vis measurements represent that the adjacent MXene layer can cause a lock in the given spin state or a change in the transition temperature dramatically.</p> <p><br></p>

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.