No description available.

Complexity, a fundamental concept in physics, encompasses phenomena spanning atomic to cosmic scales. The natural emergence of complexity can be explained by self-organized criticality. In this work, two complexity measures in magnetic systems are explored. The multiscale structural complexity (MSC) and spin temperature both capture complexity but are fundamentally different in nature and hence behave differently when subject to various temperature profiles. The MSC is extended to incorporate time correlations and compared to the time-averaged static MSC for examining spin glasses and bcc Fe at different temperatures. The spin glass transition temperature is determined with an accuracy of 1 K using the time-extended MSC, outperforming similar estimates based on the heat capacity in terms of accuracy, computational cost, and efficiency. Future work includes the optimization of coarse-graining scales in spin glasses, the investigation of transient magnetization dynamics, and the influence and loss of information of averaging magnetic unit cells before computing complexities.

Graphene, the first experimentally isolated atomically thin crystal has displayed numerous superlative properties for quantum and spin-based electronics, as evidenced by research results of more than a decade. The scalable form of graphene, produced by the chemical vapor deposition (CVD) method has been increasingly attracting scientific and technological interest, as outstanding properties are combined with large scalability and high quality. The high-performance devices based on large-scale polycrystalline graphene growth capabilities with efficient charge and spin transport make it prospective for practical implementation into future spintronic and quantum integrated circuits. While CVD graphene presents unlimited prospects for exploring spin currents, there exist challenges along the way in terms of scalability of efficient performance, and reliability. Deformations, wrinkles, and structural (electronic) modifications caused at the interfaces with contacts remain key concerns for device performance. In particular, oxide-based interfaces with graphene are central to both graphenes electronic and spintronic devices. For high-performance scalable devices, it is of crucial significance to understand the details of these interfaces and how devices of CVD graphene with polycrystallinity respond to high current limits. In this thesis, we discuss a systematic study of the effect of e-beam evaporated ultra-thin titanium oxide (TiOx) and aluminum oxide (AlOx) on graphene; which are conventionally used as tunnel barriers in spintronic and nanoelectronics devices. Characteristic topographic features of both metal oxides on the graphene surface were revealed by atomic force microscopy. To estimate the impact of these oxides on graphene, electrical measurements were performed on graphene spin devices with and without metal oxides on the same devices. These measurements show significant p-type doping for both metal oxides, with sustained sheet conductance (σ0) and mobility (μ) values. Strikingly, Raman spectroscopy and X-ray photoelectron spectroscopy show the emergence of significant sp3 carbon for AlOx on graphene, in sharp contrast to TiOx. Our results and observations, together with theoretical calculations provide new insights into how sp3 carbon for AlOx can lead to new memristive mechanisms and explicate enhanced spin relaxation into graphene with AlOx devices, which was widely attributed to the presence of interface pinholes. Here we also investigate how CVD graphene-based devices respond to high current stress to understand their stability and robustness. Despite the grainy and wrinkled structure, we observed the highest till-date current density of 5.2 × 108 A/cm2, remarkably higher than previously reported values for multilayer graphene and graphene nanoribbons. The recorded reversible regime (~108 A/cm2) for device operation allows reliable spin transport measurements with an observable spin signal up at such high current density. Furthermore, our investigation also encompasses cyclical current-voltage electrical measurement, to unveil the stability of graphene/ultra-thin oxide interfaces in graphene devices. Overall, these results present significance for CVD graphene device engineering for nanoelectronics and spintronics.

This report presents the work done on an optical setup combining Raman and BLS spectroscopy and discusses the achieved improvements of the system. After some general optimization and minor redesigning, a new alternative beam path was implemented. This new beam path allows to freely adjust all three volume Bragg grating filters in the setup, which are used to suppress the elastic line and which feature an exceptionally narrow filter window. This enables measurement of BLS signal closer to the elastic line, with a frequency separation of only 30 GHz from it achieved during the project. For measuring below this threshold, additional suppression is needed. As a first technique to achieve this, the Fabry-Pérot interferometer provided by the BLS spectrometer was partially put to use. At the point of this report, this has not been completed successfully and the setup also suffers from other adverse effects in the form of signal fluctuations and spurious measurements close to the elastic line. Nonetheless, the setup has showed promising intermediate results and can be expected to perform well once these challenges have been overcome.

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To study the effect of carbon interstitials in austenitic steels on plastic deformation mechanisms is the main goal of the present thesis. Using first-principlesmethods, the generalized stacking fault energy (GSFE) of C-alloyed γ-Fe is firstcalculated. The GSFE curve includes several prominent stacking fault energiesthat are fundamental for, e.g, predicting critical twinning stress and twinnability. The C effect was previously investigated in γ-Fe assuming nonmagnetic(NM) state. However, paramagnetic (PM) state with local magnetic momentson each site and total magnetization equal to zero is a more appropriate description for austenites. The Exact Muffin-Tin Orbitals (EMTO) method is capableof modelling the PM state together with the Coherent Potential Approximation (CPA). We also compare the NM GSFEs of C-alloyed γ-Fe obtained fromEMTO and Vienna Ab initio Simulation Package (VASP) to evaluate the performance of EMTO on handling the C-interstitial structure. The EMTO resultsare verified to fit reasonably well with VASP results so the GSFE calculationfor the C-alloyed γ-Fe is further extended to the PM state.The influence of C interstitials on the GSFE for PM γ-Fe is significantly different from what is predicted for NM γ-Fe. Though the GSFE is increased byC addition for both NM and PM γ-Fe, the C-driven change on the GSFE ascompared to pure γ-Fe at the PM state deviates from that at the NM state:paramagnetism significantly weakens the C impact on the intrinsic stacking faultenergy while strengthens it on the unstable stacking fault energy as comparedto the hypothetical NM case. The different behaviours uncovered for the intrinsic and unstable stacking fault energies due to the presence of local magneticmoments is illustrated by the magnetic structures of the Fe-C alloys as a function of volume, which mainly emerged from the suppression effect of C on themagnetic moments of its adjacent Fe neighbours.Using the generalized stacking fault as an approximation for the partial dislocation core, we investigate the minimum energy path (MEP) for C diffusionin the dislocation core (i.e., for various displacement vectors ) for NM γ-Feusing VASP. In contrast to the common assumption of stationary interstitialatoms during the passage of fast-moving dislocations, a pair of partial dislocations moves C atoms forward on the slip plane by one full Burgers vector. Thisdissociated dislocation-mediated transport mechanism for C is a strain inducedprocess, which is present even when the normal thermally activated diffusion isinoperative. Moreover, at the stacking fault ribbon and especially near the partial dislocation core, the in-plane diffusion energy barriers for C are significantlyreduced compared to that in bulk, opening a fast diffusion pathway for C. Themagnetic effect is also indirectly considered for the in-plane C diffusion energybarrier by calculating the MEP in high-spin ferromagnetic (HS-FM) Fe and ferromagnetic (FM) Ni. It is concluded that the presence of magnetic couplingdoes not change this trend. Therefore, contrary to the previously suggestedmechanism based on the reorientation of Mn-C short range order, our resultsreveal that the fast pipe diffusion of C at the dissociated dislocations is primarily responsible for the dynamic strain aging (DSA) in Fe-Mn-C steels and themechanism for DSA-mediated formation of deformation twinning is proposed to understand the strain rate dependence of deformation twinning in the presenceof DSA.

Superconductors are important to study due to their many applications. Superconductivityarises when electron pairs, so-called Cooper pairs, form in the material.To understand the mechanism behind the formation of the Cooper pairs, the pairingsymmetry, described by the order parameter, must be found. One method to experimentallydetermine the order parameter is through quasi-particle interference.Quasi-particle interference (QPI) is a real space perturbation due to a defect andcan be experimentally imaged using scanning tunneling microscopy (STM), wherean atomic sized tip is scanned over a surface at a bias measuring the tunneling currentthrough the sample surface. QPI can also be theoretically calculated using atight-binding model describing the electronic structure and then using an interpolationbetween unit cells using wave functions to carry intra-unit cellular information,so-called Continuum QPI (CQPI). This report studies the influence of different superconductingorder parameters in a one-band cuprate-like material on theoreticallycalculated phase-referenced Fourier transformed CQPI (PRFT-CQPI). The resultsshow that this method gives qualitatively distinct results depending on the ordersymmetry, suggesting that it can be used to experimentally determine the superconductinggap symmetry. The results are also compared with experiment and withpreviously published CQPI calculations.

No description available.

This report presents the significant advancements made in the UppASD software package, a tool for atomic spin dynamics simulations, through the integration of GPU acceleration. Originally developed at Uppsala University, UppASD primarily utilized CPU-based computations. This project aimed to enhance its computational efficiency and capability by extending and rigorously evaluating its GPU-based implementation. The focus was on integrating a variety of magnetic interactions, including tensorial coupling, anisotropy models, and the Dzyaloshinskii-Moriya interaction, into the GPU framework. We also assessed the impact of GPU acceleration on the calculation of key observables. The methodology involved a comprehensive evaluation of the existing code structure, meticulous debugging, and performance benchmarking. This report details the development process, highlights the challenges encountered, and provides insights into future directions for the UppASD project. / <p>Held internally sharing the progress made.</p>

Performance and energy consumption of large-scale computing infrastructures could be improved significantly by adoption of superconducting electronics. Rapid single flux quantum (RSFQ) circuits with logic based on the vortex state in Type II superconductors were first demonstrated several decades ago and as of now, research focusing on developing modern superconducting electronic components operating on the level of a single Abrikosov vortex is ongoing, where RSFQ serves as a convenient reference point. The main focus of this project was one crucial aspect for such components, namely reliable manipulation of vortex dynamics under transport current. A set of realistic and relevant micron scale geometries of superconducting thin films with a circular vortex trap were simulated by obtaining numerical solutions of time-dependent Ginzburg-Landau equations in two spatial dimensions. Within the scope of this work, successful manipulation of vortex dynamics means being able to perform each of the following actions on-demand: introduce one vortex into the device, pin it to the trap, remove it from the device. Current and time-dependent behaviour of vortices in different device geometries was studied and an important role of seemingly small changes in geometric parameters was established. Certain geometric configurations were discovered to be inherently more favourable for deterministic control of vortex dynamics, while others were identified as inherently unfavourable. Practically conceivable methods to separate between the two were introduced. Repeated sequences of "write" (trapping a vortex) and "erase" (removing a vortex) operations with square waves of transport current tailored to a particular device geometry were simulated as a demonstration of successful vortex manipulation. Findings of this thesis are expected to improve the success rate of physically conducted experiments within the area of superconducting vortex-based electronics.