The concept of Berry phase, since its proposition in 1984, has found numerous applications and appears in almost every branch of physics today. In this work, we study several physical effects in ferromagnetic metal materials which are manifestations of the Berry phase. We first show that when a domain wall in a ferromagnetic nanowire is undergoing precessional motion, it pumps an electromotive force which follows a universal Josephson-type relation. We discover that the integral of the electromotive force over one pumping cycle is a quantized topological invariant equal to integer multiples of h/e, which does not depend on the domain wall geometry nor its detailed dynamic evolution. In particular, when a domain wall in a nanowire is driven by a constant magnetic field, we predict that the generated electromotive force is proportional to the applied field with a simple coefficient consisting of only fundamental constants. Our theoretical prediction has been successfully confirmed by experiments. Similar effect known as spin pumping occurs in magnetic multilayer heterostructures, where a precessing free magnetic layer pumps a spin current into its adjacent normal metal layers. Based on this effect, we propose two magnetic nanodevices that can be useful in future spintronics applications: the magnetic Josephson junction and the magneto-dynamic battery. The magnetic Josephson junction has a drastic increase in resistance when the applied current exceeds a critical value determined by the magnetic anisotropy. The magneto-dynamic battery acts as a conventional charge battery in a circuit with well-defined electromotive force and internal resistance. We investigate the condition under which the power output and efficiency of the battery can be optimized. Finally we study the side jump contribution in the anomalous Hall effect of a uniformly magnetized ferromagnetic metal. The side jump contribution, although arises from disorder scattering, was believed to be independent of both the scattering strength and the disorder density. Nevertheless, we find that it has a sensitive dependence on the spin structure of the disorder potential. We therefore propose a classification scheme of disorder scattering according to their spin structures. When two or more classes of disorders are present, the value of side jump is no longer fixed but depends on the relative disorder strength between classes. Due to this competition, the side jump contribution could flow from one class dominated limit to another class dominated limit when certain system control parameter changes. Our result indicates that the magnon scattering plays a role distinct from the normal impurity scattering and the phonon scattering in the anomalous Hall effect, because they belong to different scattering classes. / text

Techn. Universiẗat, Diss., 2003--Darmstadt.

Cette thèse porte sur l’étude des cristaux appelés semi-métaux relativistes dans lesquels les électrons se comportent comme des particules relativistes sans masse.Le premier exemple historique d’un tel matériau est le graphène.Dans cet assemblage planaire d’atomes de carbone, les bandes électroniques de valence et de conduction se touchent en deux points distincts du réseau réciproque: il s'agit d'un conducteur de gap nul, un semi-métal.Les électrons proches du niveau de Fermi ont une dynamique relativiste décrite par une équation de Dirac,bien que leur vitesse soit cent fois inférieure à celle de la lumière dans le vide. Des semi-métaux analogues ont récemment été identifiés :les semi-métaux de Weyl et de Dirac à 3D, et des phases plus exotiques décrites par des croisements à plus de deux bandes.Cette diversité de matériaux relativistes pose la question de leurs propriétés communes. Une première partie de la thèse présente les travaux reliés à l’étude de la stabilité de ces phases, c’est à dire du croisement de bandes électroniques. Nous avons étudié cette stabilité d’abord en la reliant à des propriétés topologiques, puis en évaluant l’effet du désordre, tel que des impuretés dans le matériau. Dans la deuxième partie, nous nous intéressons à la manifestation dans le transport de la nature relativiste de ces électrons. Dans une première étude, nous étudions la condition d’existence d’une conductivité finie exactement au croisement des bandes, due à une contribution d’états évanescents. Une deuxième étude porte sur le transport anormal sous champ magnétique dans les semi-métaux de Weyl, comme manifestation de l’anomalie chirale, propriété unique de fermions relativistes. / This thesis adresses the study of crystals called relativistic semi-metals, in which electrons behave like massless relativistic particles.The first historical example of such a material is graphene.In this planar arrangement of carbon atoms, electronic valence and conduction bands touch at two distincts points in the reciprocal lattice. Thus, graphene is a zero-gap semiconductor, a semi-metal.The dynamics of electrons close to the Fermi level is relativistic, described by a Dirac equation, although their velocity is a hunder times lower than the velocity of light in vacuum. Analogous semi-metallic phases have recently been identified: 3D Weyl and Dirac semimetals, as well as more exotic phases described by crossings with more than two bands. This variety of relativistic materials raises the question of their common properties. A first part of this thesis presents work related to the study of the stability of these phases, i.e. of the electronic band crossing. We have investigated this stability first by relating it to topological properties, then by evaluating the effect of disorder, such as the presence of impurities in the material. In the second part, we focus on the manifestation of the relativistic nature of these electrons in transport. In a first study, we examine the condition of existence of a finite conductivity exactly at the band crossing, due to the contribution of evanescent states. A second study concerns the anomalous transport under a magnetic field in Weyl semi-metals, as a manifestation of the chiral anomaly, a unique property of massless relativistic fermions.

Photonics, the technological field that encompasses all aspects of light, has been rapidly growing and increasingly useful in uncovering fundamental truths about nature. It has helped detect gravitational waves, allowed for a direct measurement of the quantum wave function, and has helped realize the coldest temperatures in the universe. But photonics has also had an enormous impact on day-to-day life as well; it has enabled high capacity and/or high speed telecommunication, offered cancer treatment solutions, and has completely revolutionized display and scanning technology. All of these discoveries and applications have required a superb understanding of light, but also a high degree of control over the sometimes abstract properties of light. The work contained in this thesis explores two novel means of controlling and manipulating two different abstract properties of light. In Part I, the property under investigation is the polarization state of light – a property that is paramount to all light-matter interactions, and even some light-light interactions such as interference. Here, a liquid crystal on silicon spatial light modulator (LCOS-SLM)’s capabilities in manipulating the polarization state of light is theoretically examined and experimentally exploited, tested, and reported on. It is found through experimentation that, for an appropriate range of beam sizes and input polarizations, a single LCOS-SLM can be used to produce any light field with an arbitrary, spatially varying polarization profile. In Part II, the property under investigation loosely corresponds to light’s spatial degree of freedom – how light can move from one spot in space to another in a non-trivial manner. Here, control over light’s position through a waveguide array through the use of quantum geometric phase is theoretically examined, simulated, and experimentally designed. It is found through simulation that a threewaveguide array is capable of implementing two dimensional unitary transformations. The common theme between Part I and Part II is manipulating these properties of light to realize classes of general transformations. Moreover, if the light field is treated as a quantum state in the basis of either property under investigation, a two dimensional computational basis ensues. This is precisely the right cardinality for applications in quantum information.

Abstract After the discovery of topological insulators (TIs), it has come to be widely recognized that topological states of matter can actually be widespread. In this sense, TIs have established a new paradigm about topological materials. Recent years have seen a surge of interest in topological semimetals, which embody two different ways of generalizing the effectively massless electrons to bulk materials. Dirac and, particularly, Weyl semimetals should support several transport and optical phenomena that are still being sought in experiments. A number of promising experimental results indicate superconductivity in members of half-Hesuler semimetals which realize the mixing singlet and triplet pairing symmetry. We now turn to results we got through the work on topological semimetals. This work presents quantum high field transports on Dirac and Weyl topological semimetals including Sr1-yMn1-zSb2 (y, z < 0.1), YbMnBi2 and TaP. In case of Sr1-yMn1-zSb2 (y, z < 0.1), massless relativistic fermion was reported with m* = 0.04-0.05m0. This material presented a ferromagnetic order for in 304 K < T < 565 K, but a canted antiferromagnetic order with a net ferromagnetic component for T < 304 K. These are considered striking features of Dirac fermions For YbMnBi2, we reported the unusual interlayer quantum transport behavior in magnetoresistivity, resulting from the zeroth LL mode observed in this time reversal symmetry breaking type II Weyl semimetal. Also, for Weyl semimetal TaP the measurements probed multiple Fermi pockets, from which nontrivial π Berry phase and Zeeman splitting were extracted. Our ultra-low penetration depth measurements on half-Heuslers YPdBi and TbPdBi revealed a power- law behavior with n= 2.76 ± 0.04 for YPdBi samples and n=2.6 ± 0.3 for TbPdBi sample. We may conclude the exponent n > 2 implies nodless superconducting gap in our samples. Also, we found that despite the increase in magnetic correlations from YPdBi to TbPdBi, superconductivity remains robust in both systems which indicates that AF fluctuations do not play a major role in superconducting mechanism.

Some special features of polaritons, quasi-particles being normal modes of system of excitons interacting with photons in so called strong coupling regime, are theoretically and numerically analyze in low dimensional systems. In Chapter 1 is given a brief overview of 0D, 1D and 2D semiconductor structures with a general introduction to the polariton field. Chapter 2 is devoted to micro / nano wires. The so called whispering gallery modes are studied in the general case of an anisotropic systems as well as polariton formation in ZnO wires. Theoretical model is compared with an experiment. In the Chapter 3 Josephson type dynamics with Bose-Einstein condensates of polaritons is analyzed taking into account pseudospin degree of freedom. Chapter 4 start with an introduction to Aharonov-Bohm effect, as the best known represent of geometrical phases. An another geometrical phase - Berry phase, occurring for a wide class of systems performing adiabatic motion on a closed ring, is main subject of this section. We considered one proposition for an exciton polariton ring interferometer based on Berry phase effect. Chapter 5 concerns one 0D system : strongly coupled quantum dot exciton to cavity photon. We have discussed possibility of obtaining entangled states from a quantum dot embedded in a photonic crystal in polariton regime.

This thesis presents some theoretical studies dealing with quantum interference effects in electron transport through single molecule magnet transistors and a study on optical non-conserving spin transitions in the Nitrogen-vacancy center in diamond. The thesis starts with a brief general introduction to the physics of quantum transport through single electron transistors. Afterwards, the main body of the thesis is divided into three studies: (i) In chapter (2) we describe the properties of single molecule magnets and the Berry phase interference present in this nanomagnets. We then propose a way to detect quantum interference experimentally in the current of a single molecule magnet transistor using polarized leads. We apply our theoretical results to the newly synthesized nanomagnet Ni4. (ii) In chapter (3) we review the Kondo effect and present a microscopic derivation of the Kondo Hamiltonian suitable for full and half integer spin nanomagnets. We then calculate the conductance of the single molecule magnet transistor in the presence of the Kondo effect for Ni4 and show how the Berry phase interference becomes temperature dependent. (iii) We conclude in chapter (4) with a theoretical study of the single Nitrogen vacancy defect center in diamond. We show that it is possible to have spin non-conserving transitions via the hyperfine interaction and propose a way to write and read quantum information using circularly polarized light by means of optical Lambda transitions in this solid state system.

No description available.

Optical microcavities (OMs) are receiving increasing attention owing to their potential applications ranging from cavity quantum electrodynamics, optical detection to photonic devices. Recently, rolled-up structures have been demonstrated as OMs which have gained considerable attention owing to their excellent customizability. To fully exploit this customizability, asymmetric and topological rolled-up OMs are proposed and investigated in addition to conventional rolled-up OMs in this thesis. By doing so, novel phenomena and applications are demonstrated in OMs. The fabrication of conventional rolled-up OMs is presented in details. Then, dynamic mode tuning by a near-field probe is performed on a conventional rolled-up OM. Next, mode splitting in rolled-up OMs is investigated. The effect of single nanoparticles on mode splitting in a rolled-up OM is studied. Because of a non-synchronized oscillating shift for different azimuthal split modes induced by a single nanoparticle at different positions, the position of the nanoparticle can be determined on the rolled-up OM. Moreover, asymmetric rolled-up OMs are fabricated for the purpose of introducing coupling between spin and orbital angular momenta (SOC) of light into OMs. Elliptically polarized modes are observed due to the SOC of light. Modes with an elliptical polarization can also be modeled as coupling between the linearly polarized TE and TM mode in asymmetric rolled-up OMs. Furthermore, by adding a helical geometry to rolled-up structures, Berry phase of light is introduced into OMs. A -π Berry phase is generated for light in topological rolled-up OMs so that modes have a half-integer number of wavelengths. In order to obtain a deeper understanding for existing rolled-up OMs and to develop the new type of rolled-up OMs, complete theoretical models are also presented in this thesis.

Inspired by graphene, I investigate the properties of several different real and artificial Dirac materials. Firstly, I consider a two-dimensional honeycomb lattice of metallic nanoparticles, each supporting localised surface plasmons, and study the quantum properties of the collective plasmons resulting from the near field dipolar interaction between the nanoparticles. I analytically investigate the dispersion, the effective Hamiltonian and the eigenstates of the collective plasmons for an arbitrary orientation of the individual dipole moments. When the polarisation points close to normal to the plane the spectrum presents Dirac cones, similar to those present in the electronic band structure of graphene. I derive the effective Dirac Hamiltonian for the collective plasmons and show that the corresponding spinor eigenstates represent chiral Dirac-like massless bosonic excitations that present similar effects to those of electrons in graphene, such as a non-trivial Berry phase and the absence of backscattering from smooth inhomogeneities. I further discuss how one can manipulate the Dirac points in the Brillouin zone and open a gap in the collective plasmon dispersion by modifying the polarisation of the localized surface plasmons, paving the way for a fully tunable plasmonic analogue of graphene. I present a phase diagram of gapless and gapped phases in the collective plasmon dispersion depending on the dipole orientation. When the inversion symmetry of the honeycomb structure is broken, the collective plasmons become gapped chiral Dirac modes with an energy-dependent Berry phase. I show that this concept can be generalised to describe many real and artificial graphene-like systems, labeling them Dirac materials with a linear gapped spectrum. I also show that biased bilayer graphene is another Dirac material with an energy dependent Berry phase, but with a parabolic gapped spectrum. I analyse the relativistic phenomenon of Klein Tunneling in both types of system. The Klein paradox is one of the most counter-intuitive results from quantum electrodynamics but it has been seen experimentally to occur in both monolayer and bilayer graphene, due to the chiral nature of the Dirac quasiparticles in these materials. The non-trivial Berry phase of pi in monolayer graphene leads to remarkable effects in transmission through potential barriers, whereas there is always zero transmission at normal incidence in unbiased bilayer graphene in the npn regime. These, and many other 2D materials have attracted attention due to their possible usefulness for the next generation of nano-electronic devices, but some of their Klein tunneling results may be a hindrance to this application. I will highlight how breaking the inversion symmetry of the system allows for results that are not possible in these system's inversion symmetrical counterparts.