1,721,305 research outputs found
description of transverse transport due to impurity scattering in transition-metals
Full text linkThis thesis attempts to shed light on various spin-orbit driven transport phenomenain materials, as a crucial for the further development of the field of spintronics. Inparticular, we address the skew-scattering mechanism in dilute alloys, which gives rise to the anomalous and spin Hall effect, as well as spin-relaxation processes. We create the tools to access these quantities from calculations in the framework of the full-potential all-electron Korringa-Kohn-Rostoker Green-function method, by (a) developing and implementing a new tetrahedron method for the calculation of complicated, multi-sheeted Fermi surfaces even of complex transition-metal compounds, and (b) developing an efficiently parallelized and thus highly scalable computer program (up to thousands of processors) for the precise calculation of scattering properties. In a first application of the new tetrahedron method, we calculate the Elliott-Yafet spin-mixing parameter on the Fermi surfaces of 5 and 6 metals, and discover a yet unexplored dependence on the electron's spin-polarization direction. As we show, this anisotropy can reach gigantic values in uniaxial hcp crystals due to the emergenceof large spin-ip hot-areas or hot-loops on the Fermi surface, supported by the low symmetry of the hcp crystal. A simple model is able to reveal an interesting interplay between the orbital character of the states at special points, lines or areas in the Brillouin zone and the matrix-elements of the spin-flip part of the spin-orbit coupling operator. We further calculate the skew-scattering contribution to the anomalous Hall effect(AHE) in dilute alloys based on a ferromagnetic host for the first time. A systematic study of 3 impurities in bcc Fe, as well as the non-magnetic hosts Pd, Pt and Au, allows us to identify trends across the periodic table. In all our calculations, we also observe a strong correlation between the spin Hall effect and anomalous Hall effect in these materials, which is of interest for the creation and detection of strongly spin-polarized currents. A Fermi-surface analysis of the contributions to the AHE reveals a non-trivial, peaked behavior at small hot-spots around spin-orbit lifted degeneracies. We then proceed to the more complicated 1-ordered alloy FePt and address different kinds of disorder. We showcase the power of our method by treating the very complicated compounds FeMnSi and MnSiGe, based on the non-Fermi liquid manganese silicide (MnSi). Finally, we also calculate the pure spin Hall effect for 4/5 and 5/6 impurities in fcc Ir and hcp Re hosts. For the latter, we discover a strong dependence on the electron's spin-polarization direction
Intrinsic and extrinsic spin-orbit torques from first principles
No full textThis thesis attempts to shed light on the microscopic mechanisms underlying the current-induced magnetic torques in ferromagnetic heterostructures. We have developed first principles methods aiming at the accurate and efficient calculation of the so-called spin-orbit torques (SOTs) in magnetic thin films. The emphasis of this work is on the impurity-driven extrinsic SOTs. The main part of this thesis is dedicated to the development of a formalism for the calculation of the SOTs within the Korringa-Kohn-Rostoker (KKR) method. The impurity-induced transitions rates are obtained from first principles and their effecton transport properties is treated within the Boltzmann formalism. The developed formalism provides a mean to compute the SOTs beyond the conventional constant relaxation time approximation. We first apply our formalism to the investigation of FePt/Pt and Co/Cu bilayers in the presence of defects and impurities. Our results hint at a crucial dependence of the torque on the type of disorder present in the films, which we explain by a complex interplay of several competing Fermi surface contributions to the SOT. Astonishingly, specific defect distributions or doping elements lead respectively to an increase or a sign change of the torque, which can not be explained on the basis of simple models. We also compute the intrinsic SOT induced by electrical and thermal currents within the full potential linearized augmented plane-wave method. Motivated by recent experimental works, we then investigate the microscopic origin of the SOT in a AgBi-terminated Ag film grown on ferromagnetic Fe(110). We find that the torque in that system can not be explained solely by the spin-orbit coupling in the AgBi alloy, and instead results from the spin-orbit coupling in all regions of the film. Finally, we predict a large SOT in Fe/Ge bilayers and suggest that semiconductor substrates might be a promising alternative to heavy metals for the development of SOT-based magnetic random access memories. We show the strong dependence of the SOT on the stacking direction, thereby providing important guidelines for future experimental works. We also compute the sublattice-resolved SOTs in an antiferromagnetic Fe/Ge thin film and find a large anisotropy of the torkance tensor
Elektronenstrukturtheorie von magnetoresistiven Effekten in atomaren Kontakten induziert durch Spin-Bahn-Kopplung und Spin-Nichtkollinearität
Full text linkIn this thesis magnetoresistive effects caused by spin-orbit coupling (SOC) and spin non-collinearity are studied on the atomic scale based on electronic structure theory. The full-potential linearized augmented plane wave method is applied, which relies on density functional theory (DFT). The electronic structure is projected on Wannier functions from which a tight-binding (TB) like Hamiltonian is constructed. This Hamiltonian is used in a Green's function formalism to obtain the transmission function within the Landauer approach.
The anisotropic magnetoresistance (AMR) of symmetric Ni monowire junctions terminated by Co, Rh, and Ir apex atoms is investigated as a function of the distance between the apex atoms. A non-trivial distance dependence is found, which is due to the interplay of the magnetization-direction-dependent SOC-induced orbital mixing and the decay of the transition-matrix element across the gap between the apex atoms, which both depend on the orbital symmetry. These findings allow to explain scanning tunneling microscopy (STM) experiments of the distance dependence of the AMR in Co and Ir adatoms on W(110).
Furthermore, single-molecule junctions consisting of metal-benzene complexes contacted by Ni and Co monowires are studied. The hybridization of the molecular orbitals with the orbitals of the adjacent metal atom results in an orbital-symmetry-filtered transmission function, which leads to a giant molecular AMR.
Going beyond the monowire geometry, the AMR is investigated in Pt break junctions, in which Pt is expected to become magnetic. Pt trimers contacted by bulk-like bcc-(001) electrodes are studied finding an AMR of up to 20% in agreement with recent experimental results.
Finally, it is shown based on TB and DFT that spin mixing caused by non-collinear spin structures leads to a tunneling non-collinear magnetoresistance, which has been discovered in STM experiments probing magnetic skyrmions in PdFe/Ir(111) with non-magnetic tips.In dieser Arbeit werden magnetoresistive Effekte durch Spin-Bahn-Kopplung (SBK) und nicht-kollineare (NK) Spinstrukturen auf der atomaren Skala mittels der Elektronenstrukturtheorie untersucht. Die “Full potential linearized augmented plane wave” Methode, die auf der Dichtefunktionaltheorie (DFT) beruht, wird angewendet. Die elektronische Struktur wird auf Wannier-Funktionen projiziert, mit denen eine “Tight-Binding” (TB) ähnliche Hamiltonmatrix konstruiert wird. Dieser Hamilitonian wird in einem Greensche-Funktionen-Formalismus verwendet, um die Transmissionsfunktion innerhalb der Landauer-Methode zu gewinnen.
Der anisotrope magnetoresistive Effekt (AMR) von symmetrischen, einatomigen Ni-Ketten mit Co-, Rh- und Ir-Endatomen wird als Funktion des Abstandes zwischen den Endatomen untersucht.
Die Abstandsabhängigkeit entsteht durch das Zusammenspiel des magnetisierungsrichtungsabhängigen, SBK-induzierten Mischens der Orbitale und dem Abfallen der Übergangsmatrixelemente zwischen den Endatomen entsteht, die beide von der Orbitalsymmetrie abhängen. Diese Erkenntnisse erlauben es Rastertunnelmikroskopie (RTM) Experimente des abstandsabhängigen AMR von Co und Ir Adatomen auf W(110) zu erklären.
Weiterhin werden Einzelmolekülkontakte bestehend aus Metall-Benzol-Komplexen, die von einatomigen Ni- und Co-Ketten kontaktiert werden, studiert. Die resultierende, orbitalsymmetriegefilterte Transmissionsfunktion führt zu einem gigantischen molekularen AMR.
Anschließend wird der AMR in Pt-Bruchkontakten, in denen Pt magnetisch werden soll, untersucht. Von bcc-(001)-Elektroden kontaktierte Pt-Trimere werden studiert und ein AMR von bis zu 20% in Übereinstimmung mit jüngsten, experimentellen Daten wird gefunden.
Schließlich wird basierend auf TB und DFT gezeigt, dass das Mischen der Spinkanäle durch NK Spinstrukturen zum NK magnetischen Tunnelwiderstand führt. Dieser wurde in RTM-Experimenten, in denen Skyrmionen in PdFe/Ir(111) mit nicht-magnetischen Spitzen sondiert wurden, entdeckt
Ballistischer Transport in eindimensionalen, magnetischen Nanokontakten: Ein first-principles Wannierfunktion Ansatz
Full text linkThe discovery of the giant magnetoresistance (GMR) effect by Grünberg and Fert initiated the field of spintronics, which holds high promise for future devices of high speed and low power consumption by not only utilizing the charge of the electron in an electronic device but also its spin degree of freedom. With the ongoing miniaturization to nano-scale structures, their size becomes eventually smaller than the mean free path of an electron. In this regime an electron is 'ballistically' transmitted through a nanocontact without changing its momentum or its energy due to inelastic scattering processes. The emerging effects, e.g. the quantization of conductance and spin-dependent transport phenomena, are of quantum mechanical nature. In this thesis, a novel theoretical approach is introduced to calculate quantum transport for such nano-scale spintronic systems. In accordance to the basic geometry of state-of-the-art experiments, based e.g. on the spin-polarized scanning tunneling microscope (SP-STM), those one-dimensional nanojunctions are treated as a scattering region connected to two macroscopic electrodes by leads. The electronic and magnetic properties of such a nanojunction are described by density functional theory (DFT) within the high-precision full-potential linearized augmented plane-wave (FLAPW) method, as implemented in the FLEUR code. The quantum transport calculations are performed within the Landauer formalism based on non-equilibrium Green's functions (NEGF) in the linear response regime for small bias voltages. Since the NEGF method relies on a localized basis set, Wannier functions (WFs) are used to map the electronic structure of the FLAPW calculations onto a tight-binding like Hamiltonian. They also provide a reliable framework to save a considerable amount of computational effort within the newly employed "locking-technique". The first important aspect studied with the novel transport code is the influence of spin-orbit coupling (SOC) on quantum transport. Due to SOC the magnetization direction can strongly influence the conductance, resulting in e.g. the ballistic anisotropic magnetoresistance (BAMR). SOC also allows electrons to switch between both spin-channels and, thus, can be responsible for spin-flip scattering processes. These effects are investigated for a magnetic Co impurity in a nonmagnetic Pt monowire and for a nonmagnetic Pt impurity in a ferromagnetic Co monowire. It is e.g. shown, that due to the presence of an impurity the BAMR which occurs for a perfect wire changes to the conventional anisotropic magnetoresistance (AMR). The second important aspect studied is the effect of complex non-collinear spin-structures on ballistic conductance, which is investigated in a nanojunction consisting of two semi-infinite ferromagnetic Co monowires with magnetic Mn apex atoms, brought from the tunneling- to the contact-regime. A stable non-collinear solution occurs in the contact regime due to competing exchange interactions, which results in distinctive fingerprints in the conductance or magnetoresistance.Die Entdeckung des Riesenmagnetwiderstandes (engl. GMR) durch Grünberg und Fert begründete das Feld der Spintronik, das die Entwicklung von Bauteilen mit hohen Schaltraten bei niedrigem Energieverbrauch verspricht, da nicht nur die Ladung eines Elektrons, sondern zusätzlich auch dessen Spin-Freiheitsgrad berücksichtigt wird. Durch die fortwährende Miniaturisierung zu nanoskaligen Strukturen kann die Größe solcher Bauteile kleiner als die mittlere freie Weglänge eines Elektrons werden. In diesem Bereich passieren die Elektronen ein Bauteil "ballistisch", das heißt, ohne Änderung ihres Impulses oder ihrer Energie durch unelastische Streuprozesse. Die hierbei auftretenden Effekte, wie z.B. die Quantisierung des Leitwerts oder spinabhängige Transportphänomene, sind quantenmechanischem Ursprungs. In dieser Arbeit wird ein neuartiger, theoretischer Ansatz vorgestellt, um den Quantentransport durch solche nanoskaligen, spintronischen Systeme zu berechnen. In Übereinstimmung mit der grundlegenden Geometrie moderner Experimente, wie z.B. dem spinpolarisierten Rastertunnelmikroskop (SP-RTM, engl. SP-STM), werden eindimensionale Nanokontakte duch eine Streuregion beschrieben, die durch Zuleitungen mit zwei makroskopischen Elektroden verbunden ist. Die elektronischen und magnetischen Eigenschaften solch eines Nanokontakts werden anhand von Dichtefunktionaltheorie (DFT) mit der sehr präzisen Full-Potential Linearized Augmented Plane-Wave (engl. FLAPW) Methode in der Implementierung des FLEUR-Codes bestimmt. Der Quantentransport wird im Landauer-Formalismus mit Nichtgleichgewichtsgreensfunktionen (engl. NEGF) im Bereich der linearen Reaktion für kleine Biasspannungen berechnet. Da die NEGF Methode auf einem lokalisierten Basissatz beruht, werden Wannierfunktionen (engl. WFs) benutzt, um die elektronische Struktur der FLAPW-Rechnungen auf einen Tight-Binding-artigen Hamiltonoperator abzubilden. Wannierfunktionen bieten zudem zuverlässige Rahmenbedingungen, um den benötigten Rechenaufwand anhand der neuentwickelten "Locking-Technique" zu reduzieren. Der erste wichtige Aspekt, der mit der neuen Transportmethode untersucht wird, ist der Einfluss der Spin-Bahn Kopplung (engl. SOC) auf den Leitwert. Durch die Spin-Bahn Kopplung kann die Magnetisierungsrichtung den Leitwert stark beeinflussen, was z.B. im ballistischen anisotropen Magnetwiderstand (engl. BAMR) resultiert. Die Spin-Bahn Kopplung ermöglicht es Elektronen zudem, zwischen den Spin-Kanälen zu wechseln und kann daher Spin-Flip Streuprozesse hervorrufen. Diese Effekte werden anhand einer magnetischen Co Störstelle in einem nichtmagnetischen Pt Monodraht und einer nichtmagnetischen Pt Störstelle in einem ferromagnetischen Co Monodraht analysiert. Es wird z.B. gezeigt, dass der ballistische anisotrope Magnetwiderstand, der bei perfekten Drähten auftritt, durch eine Störstelle in den konventionellen anisotropen Magnetwiderstand (engl. AMR) übergeht. Der zweite wichtige Aspekt, der Einflusses komplexer, nichtkollinearer Spinstrukturen auf den ballistischen Leitwert, wird in einem Nanokontakt untersucht, der aus zwei halbunendlichen, magnetischen Co Monodrähten mit magnetischen Mn Spitzenatomen besteht, die vom Tunnel- in den Kontaktbereich gebracht werden. Im Kontaktbereich kann durch konkurrierende Austauschwechselwirkungen ein stabiler, nichtkollinearer Zustand entstehen, der kennzeichnende Leitwert- und Magnetwiderstandsignaturen besitzt
Machine learning models for chiral transport in magnetic systems
No full textAchievements in the field of solid state physics have shaped our way of life profoundly over the last century, propelling mankind into an era of wearable electronics, worldwide access to electronically stored information, and computing power previously unheard of. Mobile phones, which can be found in almost everyone's pocket today, perfectly illustrate the pivotal conflict between simultaneous miniaturization of devices and increases in computing power and storage density, that has been at the heart of solid state research since the fabrication of the world's first transistor. In addition, there is much interest in computing architectures beyond the standard, semiconductor-based computers. Now, materials with non-collinear magnetization textures, such as domain walls or skyrmions, present themselves as prime candidates in the quest for miniature electronics and unconventional computing platforms alike, due to their small size, their stability, and their intrinsic, non-linear characteristics, enabling complex arithmetic operations. However, a clear description of charge, heat, or spin transport in complex magnetization textures is still sought after. Therefore, a systematic way of conquering the complexity in canted magnets or chiral textures is a key objective in the field of spintronics. In this thesis, explicit tight-binding calculations of the anomalous Hall effect on a two-dimensional, magnetic honeycomb lattice, are exploited in a threefold manner in order to introduce the vector chirality of a magnetic texture as a powerful order parameter. First, the chiral Hall effect in canted magnets is established on equal footing with the anomalous Hall effect of collinear ferromagnets and antiferromagnets, by identifying the chiral Hall effect as the contribution to the anomalous Hall effect linear in vector chirality. Second, by classifying different parts of the Hall signal with respect to the vector chirality of the magnetic configuration and the underlying crystal symmetry, the numerical data reproduces the functional form and directional dependence obtained from an expansion of the anomalous Hall effect in gradients of the magnetization. Lastly, numerical data is utilized for training a linear model of the anomalous Hall effect, encompassing effects up to arbitrary order in the magnetization, which is constructed from the symmetric invariants of the lattice symmetry. Through explicitly training non-chiral and chiral models, this constructive method demonstrates the fingerprint of chiral magnetic textures in electric transport properties
Topological Matter - Topological Insulators, Skyrmions and Majoranas
No full textCondensed matter physics is currently undergoing a revolution through the introduction of concepts arising from topology that are used to characterize physical states, fields and properties from a completely different perspective. With the introduction of topology, the perspective is changed from describing complex systems in terms of local order parameters to a characterization by global quantities, which are measured nonlocally and which endow the systems with a global stability to perturbations. Prominent examples are topological insulators, skyrmions and Majorana fermions. Since topology translates into quantization, and topological order to entanglement, this ongoing revolution has impact on fields like mathematics, materials science, nanoelectronics and quantum information resulting in new device concepts enabling computations without dissipation of energy or enabling the possibility of realizing platforms for topological quantum computation, and ultimately reaching out into applications. Thus, these new exciting scientific developments and their applications are closely related to the grand challenges in information and communication technology and energy saving. Topology is the branch of mathematics that deals with properties of spaces that are invariant under smooth deformations. It provides newly appreciated mathematical tools in condensed matter physics that are currently revolutionizing the field of quantum matter and materials. Topology dictates that if two different Hamiltonians can be smoothly deformed into each other they give rise to many common physical properties and their states are homotopy invariant. Thus, topological invariance, which is often protected by discrete symmetries, provides some robustness that translates into the quantization of properties; such a robust quantization motivates the search and discovery of new topological matter. So far, the mainstream of modern topological condensed matter physics relies on two profoundly different scenarios: the emergence of the complex topology either in real space, as manifested e.g. in non-trivial magnetic structures or in momentum space, finding its realization in such materials as topological and Chern insulators. The latter renowned class of solids attracted considerable attention in recent years owing to its fascinating properties of spin-momentum locking, emergence of topologically protected surface/edge states governed by Dirac physics, as well as the quantization of Hall conductance and the discovery of the quantum spin Hall effect. Historically, the discovery of topological insulators gave rise to the discovery of a whole plethora of topologically non-trivial materials such asWeyl semimetals or topological superconductors, relevant in the context of the realization of Majorana fermions and topological quantum computation. [...
Investigating the role of Chaos and characteristic time scales in Reservoir Computing
No full textDynamical systems suited for Reservoir Computing (RC) should be able to both retain information for sufficiently long times and exhibit a rich representation of the input driving. However, selecting and tuning system parameters as well as choosing a sufficient input encoding has yet to be standardized as a procedure. This work attempts to make progress in this regard by focusing on the input and dynamical timescales in RC systems. Two qualitatively different models are studied: An adaptation of the Fermi-Pasta-Ulam-Tsingou model made suitable for Reservoir Computing and sparsely connected networks of spiking excitatory/inhibitory neurons. By comparing input injection frequencies to system relaxation timescales, and measuring its effects on the degree of chaos in the dynamical system, a relationship between timescales and the performance on a short term memory and parity-check tasks is established. We find that both systems rely on a close matching of their relaxation timescales with the input frequency in order to memorize and make precise use of the most recent information in the input. This was consistent across both models, implying greater generalizability. Furthermore, we find that a high degree of chaos deprecates memory in the Fermi-Pasta-Ulam-Tsingou model, while at the same time enhancing performance in the parity-check task, suggesting the edge of chaos to be an optimal tradeoff. The networks of spiking neurons show similar performance on the performance tasks, suggesting that nonlinear computations happen on a much faster timescale
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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