The classical Landau-Lifshitz-Gilbert (LLG) equations are of crucialimportance in micro-magnetism, but a true quantum-mechanical descriptionwas not found until 2013. However, very few realistic quantumsystems have been modeled using it. This project describes the quantum LLG dynamics of a dimer system,accounting for the Heisenberg exchange and Dzyaloshinskii-Moriyainteractions, as well as local dephasing as an open system effect.Equations of motion are derived using an appropriate Hamiltonian, Wieser’s non-linear master equation and a two-qubit parametrization,then solved numerically. The non-locality and entanglenment of thesystem were then investigated using the CHSH inequality and concurrence. The solutions for the dimer system show oscillations in the Blochvector components aligned with the external magnetic field, and inthe anti-ferromagnetic case, both CHSH inequality violation and entanglementwere initially found, but underwent ”sudden death” anddisentanglement as the evolution continued, due to dephasing. Analysisof the kT-Bz parameter space reveals combinations which produceentanglement without violation of the Clauser, Horne, Shimony, Holt (CHSH) inequality, and regions of Bz where increasing kT increasesentanglement. This set of solutions to Wieser’s quantum LLG equation suggests thatthe disentangling effect of dephasing and other open-system effects willbe obstacles for future practical efforts in quantum communication.

ELNES-spectra of the L3 and L2-edge of bcc-iron are simulated using a Bloch wave method for the calculation of the DDSCS within a 1st-order Born approximation of the inelastic electron scattering process in order to study the influence of non-dipole terms on the EMCD-signal. The necessary electronic structure information is obtained from a DFT-calculation using WIEN2k.Two different different ways of extracting the EMCD-signal are considered: the pEMCD-signal, which is extracted from the pure imaginary part of the MDFF, and the eEMCD-signal, extracted via an EMCD difference method.A non-negligible contribution of the 13 cross term to the eEMCD-signal is found. Furthermore it is shown that the double difference method and the single left-right difference method cancel out the contributions of the 01-term to the eEMCD-signal. The pEMCD-signal is found to be influenced by non-dipole terms only for large scattering angles. Conclusive quantitative results on the influence of non-dipole contributions to the eEMCD-signal remain to be found, however, since it is uncovered that the eEMCD-signal is strongly disturbed by the choice of the post-edge normalization range due to the inaccurate description of the post-edge region in the present simulation. Furthermore a not anticipated "apparent anisotropy" of the real part of the MDFF is found, whose cause is presently unknown.As a byproduct of these investigations deeper insight is gained on a reason, why the double difference method is superior to the other extraction methods. It practically eliminates the effect of non-dipole terms 01 and 12.Lastly two effects are encountered that might pave the way to a deeper understanding of why the L2-edge is experimentally often observed to be weakened or suppressed relative to the L3-edge in comparison with simulations.

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.

No description available.

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.