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This dissertation deals with the dynamics of interacting quantum and classical spin models and the question of whether and to which degree the dynamics of these models agree with each other. For this purpose, XXZ models are studied on different lattice geometries of finite size, ranging from one-dimensional chains and quasi-one-dimensional ladders to two-dimensional square lattices. Particular attention is paid to the high-temperature analysis of the temporal behavior of autocorrelation functions for both the local density of magnetization (spin) and energy, which are closely related to transport properties of the considered models. Due to the conservation of total energy and total magnetization, the dynamics of such densities are expected to exhibit hydrodynamic behavior for long times, which manifests itself in a power-law tail of the autocorrelation function in time. From a quantum mechanical point of view, the calculation of these autocorrelation functions requires solving the linear Schrödinger equation, while classically Hamilton’s equations of motion need to be solved. An efficient numerical pure-state approach based on the concept of typicality enables circumventing the costly numerical method of exact diagonalization and to treat quantum autocorrelation functions with up to N = 36 lattice sites in total. While, in full generality, a quantitative agreement between quantum and classical dy- namics can not be expected, contrarily, based on large-scale numerical results, it is demonstrated that the dynamics of the quantum S = 1/2 and classical spins coincide, not only qualitatively, but even quantitatively, to a remarkably high level of accuracy for all considered lattice geometries. The agreement particularly is found to be best in the case of nonintegrable quantum models (quasi-one-dimensional and two-dimensional lattice), but still satisfactory in the case of integrable chains, at least if transport properties are not dominated by the extensive number of conservation laws. Additionally, in the context of disordered spin chains, such an agreement of the dynamics is found to hold even in the presence of small values of disorder, while at strong disorder the agreement is pronounced most for larger spin quantum numbers. Finally, it is shown that a putative many-body localization transition within the one- dimensional spin chain is shifted to stronger values of disorder with increasing spin quantum number. It is concluded that classical or semiclassical simulations might provide a meaningful strategy to investigate the quantum dynamics of strongly interacting quantum spin models, even if the spin quantum number is small and far from the classical limit.

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Thesis advisor: Michael J. Graf / In this work I present a characterization of spin dynamics in LiY1-xHoxF4 over a wide region of frequency - temperature - magnetic field - concentration phase space to probe the onset of magnetic correlations. Specifically, measurements were made of the T = 1.8 K magnetic field and frequency dependence of AC susceptibility, and temperature and field dependence of the longitudinal field positive muon spin relaxation (μSR) for LiY1-xHoxF4 with x = 0.0017, 0.0085, 0.0408, and 0.0855. To determine the concentration range over which the spin dynamics are determined primarily by the Ho3+-μ interaction rather than by the F-μ interaction I characterize the dynamics associated with the formation of the (F-μ-F)−complex by comparing data with Monte Carlo simulations. Numerical simulations of the susceptibility for the x = 0.0017 and 0.0085 are fit to my data and show that Ho-Ho cross-relaxation processes become important at higher concentration, signaling the crossover from single-ion to correlated behavior. The muon spin depolarization is simulated using the parameters extracted from the susceptibility, and the simulations agree well with data for these two samples. It is found that the susceptibility and μSR data for samples with x = 0.0408 and 0.0855 cannot be described within a single-ion picture, possibly due to the onset of collective phenomena. An unusual peak is also discovered in the magnetic field dependence of the muon relaxation rate in the temperature interval 10 – 20 K, and ascribed to a modification of the Ho3+ fluctuation rate due to a field induced shift of the energy splitting between the ground and first excited doublet crystal field states relative to a peak in the phonon density of states centered near 63 cm-1. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Despite of enormous progress in experimental nanophysics theoretical studies of low-dimensional electron systems still remains a challenging task. Indeed, most of the structures are strongly correlated, so that an effective perturbative treatment is impossible due to the lack of a small parameter. The problem can be partly solved within the dynamical mean-field theory (DMFT) paradigm, nevertheless the correlations in physically relevant high-temperature superconductors are of purely non-local nature. The recently developed dual fermion approximation, combining field-theoretical diagram technique and numerical methods, allows for explicit account of spatial correlations. The approximation was shown to be of fastest convergence compared with standard DMFT extensions, and along with renormalization group is used here to study Fermi condensation on a triangular lattice near van Hove singularities. The still debated phenomenon of Fermi condensation is believed to be a precursor to strongly correlated low-temperature instability and is found in this thesis to be robust even at high temperature, making its experimental verification feasible. Unlike homogeneous ferromagnetic ordering a variety of non-collinear ground state configurations emerge as a result of competition among exchange, anisotropy, and dipole-dipole interaction. These particle-like states, e.g. magnetic soliton, skyrmion, domain wall, form a spatially localized clot of magnetic energy. Consistent study of spin, which essentially is a quantum mechanical entity, led to the emergence of spintronics (spin-based electronics) and magnonics (photonics with spin waves), in the meanwhile topologically protected magnetic solitons and skyrmions might potentially be applied for data processing and information storage in next generation of electronic technology (rapidly advancing solitonics and skyrmionics). An ability to easily create, address, and manipulate such structures is among the prerequisite forming a basis of "-onics" technology. It is shown here that spins on a kagome lattice, interacting via Heisenberg exchange and Dzyaloshinskii-Moriya coupling, allow the formation of topologically protected edge states through which a skyrmion can propagate. Not only can chemical methods be used to design novel functionality, but also geometric structuring. It is demonstrated that for graphene sandwiched between two insulating media external circularly-polarized light serves as an effective magnetic field. The direct practical implication permits to control light polarization and induce spin-waves propagating on the surface of e.g. a topological insulator. The newly discovered Dirac materials, graphene and three-dimensional topological insulators, are not easy to handle. In fact, the quasiparticle band function is gapless preventing them from being used in integrated circuits, nevertheless the problem is shown here to be partially relaxed by placing a vacancy on top of it.

The existence of bulk magnetism in matter can be attributed to the magnetic properties of the sub-atomic particles that constitute the former. The fact that the origin of these microscopic magnetic moments cannot be related to the existence of microscopic currents became apparent when this assumption predicted completely featureless bulk magnetic properties in contradiction to the observation of various bulk magnetic properties [1]. This microscopic magnetic moment, independent of other motions, hints at the existence of a hitherto unknown degree of freedom that a particle can possess. This property has come to be known as the "spin" of the particle. The atomic nucleus is comprised of the protons and the neutrons which possess a spin each. The composite object- the atomic nucleus is therefore a tiny magnet itself. In the presence of an external bias like a magnetic field, the nucleus therefore evolves like a magnetic moment and attains a characteristic frequency in its evolution called the Larmor frequency given by, (formula) where η is the magnetogyric ratio of the particle and B is the applied magnetic field. The existence of a natural frequency presents the possibility of a resonance behaviour in the response of the system when probed with a driving field. This is the basic principle of magnetic resonance, which in the context of the atomic nucleus, was discovered independently by Purcell [2] and Bloch [3]. From its conception, the technique and the associated understanding of the involved phenomena have come a long way. In its original form the technique involved the study of the steady-state response of the nuclear magnetic moment to a driving field. This continuous wave NMR had the basic limitation of exciting resonances in a given sample, serially. In due course of time, this technique was replaced by the Fourier transform NMR (FTNMR) [4]. This technique differed from the continuous wave NMR in its study of the transient response of the system in contrast to the steady-state response in the former. The advantage of this method is the parallel observation of all the resonances present in the system ( within the band-width of the excitation). In addition to the bias created by the external field, other internal molecular fields produce additional bias which in turn produce interesting signatures on the spectrum of the system, which are potential carriers of information about the molecular state. The fact that the spins are not isolated from the molecular environment, produces a striking effect on the ideal spectrum of the system. These effects contain in them, the signatures of the molecular local environment and are hence of immense interest to physicists, chemists and biologists.

Quantum computing has been a new and challenging area of research since the concept was put forward in 1980s. A quantum computer is a computer that processes information encoded in systems that exhibit quantum properties and is proved in theory to be more powerful than classical computers. Various approaches to the implementation of the quantum computers have been studied over the decades, each of them having their own advantages and disadvantages in terms of the lifetime of the quantum information, processing time, and scalability of the implementation. Proposals for hybrid quantum processors are interesting because they benefit from the advantages of each comprising system, and thus providing a promising approach to a practical quantum computer. In this thesis, I demonstrate experimentally the principle of utilizing electron spin ensembles as a quantum memory for hybrid quantum processors. I demonstrate the storage and on-demand retrieval of multiple bits of quantum information into and from a single electron spin ensemble by applying magnetic field gradient pulses. I then study the coupling between an electron spin ensemble and a three-dimensional microwave cavity, in the aim of discussing the condition for the coherent information transfer between the excitations in solid-state matter and photons. As an alternative to the high power pulses in electron paramagnetic resonance (EPR), I study the possibility of controlling the electron spin states via adiabatic processes. I demonstrate the implementation of adiabatic geometric phase gates in electron spins and compare their performances to other phase gates achieved with microwave pulses in both simulation and experiment, verifying the robustness of the adiabatic gates against certain type of noises. Finally I present the simulation method developed for simulating the pulsed EPR experiments in this thesis, using a model more general than some currently-existing simulation packages.

A microwave reflection method has been used to measure the spin excitations corresponding to the translational mode of magnetic vortices in samples containing either one or two vortices. Experimental findings are complemented by micromagnetic simulations. One-vortex systems are investigated in micron-sized circular and elliptical cylinders. For ellipses, the resonance frequency can effectively be tuned by applying static magnetic fields and the field dependence of the frequency is significant for fields applied along the short axes but negligible when applied along the long axes of the ellipses. This is contrary to the circular case, where virtually no field dependence was found. This can be understood by considering the shape of the vortex potential well. Further, it is found that the resonance frequency is independent on the direction of the excitation field for the one-vortex systems. Ellipses containing two interacting vortices are also investigated. It is shown that the relative vortex core polarizations dominate the vortex translational mode and cause, in the case of opposite polarizations, a dependence on the excitation field direction. For parallel core polarizations, no dependence on the excitation field direction is found. The dependence of the resonance frequencies on applied static fields along the long and short axes are also experimentally mapped out and compared with micromagnetic simulations, where the possible eigenmodes are determined. Another section of the thesis introduces the dawning of a device based on patterned magnetic elliptical elements for the manipulation and movement of magnetic particles on a surface. The controlled movement and separation of individual particles are successfully demonstrated. Contributions to micromagnetic standard problems and simulations on magnetization switching in nanoscale particles have also been performed. The standard problems highlight some important aspects of choosing the discretization cell sizes and the finite temperature simulations show that thermal fluctuations can alter the magnetization reversal paths.

The first part of this thesis is an experimental study of the spin-dependent transport in magnetic point contacts. Nano-contacts are produced micromechanically, by bringing a sharpened non-magnetic (N) tip into contact with a ferromagnetic (F) film. The magnetic and magneto-transport properties of such N/F nanocontacts are studied using transport spectroscopy, spanning the ballistic, diffusive, and thermal transport regimes. Single N/F interfaces can exhibit current driven magnetic excitations, which are often manifest as peaks in the differential resistance of a point contact defining the N/F interface. Our experiments show that such surface magnetization excitations, and thus the single-interface spin torques, are observed for diffusive and thermal transport regimes where the conduction electrons experience strong scattering near the N/F interface, and are absent for purely ballistic contacts. We conclude that the single-interface spin torque effect is due to impurity scattering at N/F interfaces. Single N/F interfaces can also exhibit hysteretic conductivity, which is qualitatively similar to the spin-valve effect found in F/N/F trilayers. Based on our measurements of N/F point contacts in the size range of 1-30 nm, we propose two mechanisms of the observed hysteresis. The first mechanism relies on a non-uniform spin distribution near the contact core and is magnetoelastic in origin. This interpretation is in good agreement with some of our experiments on larger point contacts as well as with a numerical micromagnetic model we have developed, where a stress-induced anisotropy creates a non-uniform, domain-wall-like spin distribution in the contact core. The second mechanism we propose is a surface effect which relies on a difference between the surface and interior spins in the ferromagnet in terms of their exchange and anisotropy properties. The surface spin-valve mechanism is in good agreement with the hysteretic magnetoresistance observed for our smallest contacts (~1 nm) and for contacts to nanometer thin ferromagnetic films. This interpretation means that the surface magnetization can be reduced and weakly coupled to the interior spins in the ferromagnet. We find that this surface spin layer can be affected by both external fields and the spin torque of a transport current. The surface magnetization can even form nano-sized spin vorticies at the interface. The nature of the magnetic excitations induced by by nominally unpolarized currents through single N/F interfaces was probed directly using microwave irradiation. We observed two characteristic high-frequency effects: a resonant stimulation of spin-wave modes by microwaves, and a rectification of off-resonant microwave currents by spin-wave nonlinearities in the point contact conductance. These experiments demonstrate that the effects observed are spin-dynamic in nature. In the second part of the thesis we study the spin-dynamics in spin-flop tunnel junctions used in toggle magnetic random access memory. Current pulses in the range of 100 ps used to excite the magnetic moments of the two coupled Py free layers into an oscillatory state, in both the antiparallel and scissor states of the cell. These oscillations are detected directly by measuring the junction resistance in real time with a 6 GHz measurement bandwidth. The junctions had the shape of an ellipse, with lateral size ranging from 350x420 to 400x560 nm. The optical and acoustical precession modes of the the spin-flop trilayer are observed in experiment, as expected from single-domain model. The experimental spectra contain additional features, which are explained using numerical micromagnetic simulations, as originating from magnetic state transitions between different magnetization states with non-uniform spin distributions. / QC 20100818

Ce travail de thèse est une contribution à l'étude de la dynamique de spin des porteurs dans des structures semiconductrices III-V en vue d’applications possibles dans le domaine émergent de la spintronique dans les semiconducteurs. Deux approches différentes on été envisagées afin de pouvoir obtenir une polarisation en spin des porteurs longue et robuste : i) le confinement spatial dans des nano-structures 0D (boîtes quantiques), ii) l’ingénierie des centres paramagnétiques dans des couches massives.Pour la première approche, nous avons étudié les propriétés de polarisation de spin d’excitons confinés dans des boîtes quantiques de GaN/AlN insérées dans des nano-fils. Nous avons d’abord mis en évidence un taux important de polarisation de la photoluminescence (15 %) à basse température sous excitation quasi-résonante et nous avons démontré que cette polarisation est temporellement constante pendant la durée de vie des excitons. Grâce à des mesures en température, nous avons aussi démontré que cette polarisation n’est aucunement affectée jusqu’à 300 K. Nous avons aussi développé un modèle détaillé basé sur la matrice densité pour décrire le dégré de polarisation de la photoluminescence et sa dépendance angulaire.Pour la deuxième approche, nous avons réalisé un dispositif prototype de filtrage de spin basé sur l’implantation de centres paramagnétiques dans des couches massives de InGaAs. Le principe repose sur la création de défauts interstitiels paramagnétiques comme précédemment démontré dans notre groupe pour les nitrures dilués tels que GaAsN. Le but de ce travail a été le développement d’un procédé de création de ces défauts qui puisse surmonter les inconvénients liés à l’insertion de l’azote dans les semiconducteurs de type GaAs : a) la dépendance de l’efficacité du filtrage de spin avec de l’énergie de photoluminescence, b) l’impossibilité de créer des zones actives avec des motifs spécifiques.Dans ce travail, nous démontrons que des régions actives de filtre à spin peuvent être créées par implantation ionique de défauts paramagnétiques avec une densité et des motifs spatiaux prédéfinis. Grâce à des études par photoluminescence, nous avons d’une part mis en évidence des taux de recombinaison dépendant en spin pouvant aller jusqu’à 240 % dans les zones implantées. D’autre part, nous avons déterminé la dose d’implantation la plus favorable grâce à une étude systématique sur différents échantillons implantés avec des densités ioniques étendues sur quatre ordres de grandeurs. Nous avons également observé que l’application d’un champ magnétique externe produit une augmentation significative du taux de recombinaison dépendant en spin due à la polarisation en spin des noyaux implantés / This thesis work is a contribution to the investigation by photoluminescence spectroscopy of the spin properties of III-V semiconductors with possible applications to the emerging semiconductor spintronics field. Two approaches have been explored in this work to achieve a long and robust spin polarization: i) Spatial confinement of the carriers in 0D nanostructured systems (quantum dots). ii) Defect engineering of paramagnetic centres in a bulk systems. Concerning the first approach, we have investigated the polarization properties of excitons in nanowire-embedded GaN/AlN quantum dots. We first evidence a low temperature sizeable linear polarization degree of the photoluminescence (~15 %) under quasi-resonant excitation with no temporal decay during the exciton lifetime. Moreover, we demonstrate that this stable exciton spin polarization is unaffected by the temperature up to 300 K. A detailed theoretical model based on the density matrix approach has also been developed to account for the observed polarization degree and its angular dependence.Regarding the second approach, we have demonstrated a proof-of-concept of conduction band spin-filtering device based on the implantation of paramagnetic centres in InGaAs epilayers. The principle relies on the creation of Ga interstitial defects as previously demonstrated in our group in dilute nitride GaAsN compounds. The driving force behind this work has been to overcome the limitations inherent to the introduction of N in the compounds: a) The dependence of the photoluminescence energy on the spin-filtering efficiency. b) The lack of spatial patterning of the active regions.In this work we show how the spin-filtering defects can be created by ion implantation creating a chosen density and spatial distribution of gallium paramagnetic centers in N-free epilayers. We demonstrate by photoluminescence spectroscopy that spin-dependent recombination (SDR) ratios as high as 240 % can be achieved in the implanted areas. The optimum implantation conditions for the most efficient SDR are also determined by the systematic analysis of different ion doses spanning four orders of magnitude. We finally show how the application of a weak external magnetic field leads to a sizable enhancement of the SDR ratio from the spin polarization of the implanted nuclei