46 research outputs found
Atomistic models and time-dependent simulations of spin dynamics in isolated and current-carrying systems
No full textTHESIS 10028Understanding and ultimately controlling the dynamics of electrons and their spins is an important aspect of spin-based electronics. Recent experimental advances open up new possibilities for probing spin dynamics with very high spatial and temporal resolution, i.e. in atomic-sized systems and at sub-picosecond time scales. At these lengths and times quantum effects and atomistic details can not be neglected and the applicability of classical models is limited. Therefore, there is a need to develop new methodologies and computational tools for atomistic modeling of spin dynamics, in particular in the presence of electric currents. In this Thesis we construct a computational scheme, capable of describing the time evolution of spin systems both in isolation and under current-carrying conditions. The method has been developed in the spirit of the tightbinding model but it is transferable to more accurate first-principles methods such as density functional theory. This computational tool is then used to investigate timedependent phenomena in the systems of interest
Excitonic instability in optically pumped three-dimensional Dirac materials
No full textRecently it was suggested that transient excitonic instability can be realized in optically pumped two-dimensional (2D) Dirac materials (DMs), such as graphene and topological insulator surface states. Here we discuss the possibility of achieving a transient excitonic condensate in optically pumped three-dimensional (3D) DMs, such as Dirac and Weyl semimetals, described by nonequilibrium chemical potentials for photoexcited electrons and holes. Similar to the equilibrium case with long-range interactions, we find that for pumped 3D DMs with screened Coulomb potential two possible excitonic phases exist, an excitonic insulator phase and the charge density wave phase originating from intranodal and internodal interactions, respectively. In the pumped case, the critical coupling for excitonic instability vanishes; therefore the two phases coexist for arbitrarily weak coupling strengths. The excitonic gap in the charge density wave phase is always the largest one. The competition between screening effects and the increase of the density of states with optical pumping results in a rich phase diagram for the transient excitonic condensate. Based on the static theory of screening, we find that under certain conditions the value of the dimensionless coupling constant screening in 3D DMs can be weaker than in 2D DMs. Furthermore, we identify the signatures of the transient excitonic condensate that could be probed by scanning tunneling spectroscopy, photoemission, and optical conductivity measurements. Finally, we provide estimates of critical temperatures and excitonic gaps for existing and hypothetical 3D DMs.</p
Dynamically Induced Excitonic Instability in Pumped Dirac Materials
No full textDriven and non-equilibrium quantum states of matter have attracted growing interest in both theoretical and experimental studies in condensed matter physics. Recent progress in realizing transient collective states in driven or pumped Dirac materials (DMs) is reviewed herein. In particular, the focus is on optically pumped DMs which are a promising platform for transient excitonic instabilities. Optical pumping combined with the linear (Dirac) dispersion of the electronic spectrum offers a knob for tuning the effective interaction between the photoexcited electrons and holes, and thus provides a way of reducing the critical coupling for excitonic instability. As a result, a transient excitonic condensate could be achieved in a pumped DM while it is not feasible in equilibrium. A unifying theoretical framework is provided for describing transient collective states in 2D and 3D DMs. The experimental signatures are described and numerical estimates of the size of the dynamically induced excitonic gaps and the values of the critical temperatures for several specific systems, are summarized. In addition, general guidelines for identifying promising material candidates are discussed. Finally, comments are provided regarding recent experimental efforts in realizing transient excitonic condensate in pumped DMs, and outstanding issues and possible future directions are outlined.</p
Effects of the Weight-to-Stress Principle in English Speakers Learning Japanese Prosody
No full textThis thesis explores the effect of the Weight-to-Stress Principle (WSP) in second language transfer of word-level accent from English stress accent to Japanese pitch accent within the framework of Optimality Theory. A model is proposed that predicts some English speakers will be more likely to insert pitch accents into unaccented Japanese words containing heavy syllables (Heavy words) than ones consisting only of light syllables (Light words), due to the effects of WSP. I performed an elicitation experiment on English-speaking learners of Japanese wherein participants produced unaccented Japanese nonce words of varying prosodic structure in a sentence. At least one participant was more likely to insert a pitch accent into Heavy words than Light words, supporting the proposed model. Two other participants showed a preference for inserting pitch accents on three-mora Light words, indicating that additional factors may be at work.Master of Art
Probing the wavefunction of the surface states in Bi<sub>2</sub>Se<sub>3</sub> topological insulator : a realistic tight-binding approach
No full textWe report on microscopic tight-binding modeling of surfacestates in BiSe three-dimensional topological insulator, based on a\textit{sp} Slater-Koster Hamiltonian, with parameters calculated fromdensity functional theory. The effect of spin-orbit interaction on theelectronic structure of the bulk and of a slab with finite thickness isinvestigated. In particular, a phenomenological criterion of band inversion isformulated for both bulk and slab, based on the calculated atomic- andorbital-projections of the wavefunctions, associated with valence and conductionband extrema at the center of the Brillouin zone. We carry out athorough analysis of the calculated bandstructures of slabs with varyingthickness, where surface states are identified using a quantitative criterionaccording to their spatial distribution. The thickness-dependent energy gap,attributed to inter-surface interaction, and the emergence of gapless surfacestates for slabs above a critical thickness are investigated. We map out thetransition to the infinite-thickness limit by calculating explicitly themodifications in the spatial distribution and spin-character of the surfacestates wavefunction with increasing the slab thickness. Our numerical analysisshows that the system must be approximately forty quintuple-layers thick toexhibit completely decoupled surface states, localized on the oppositesurfaces. These results have implications on the effect of external perturbationson the surface states near the Dirac point.</p
Thin films of a three-dimensional topological insulator in a strong magnetic field: a microscopic study
No full textThe response of thin films of BiSe to a strong perpendicular magnetic field is investigated by performing magnetic bandstructure calculations for a realistic multi-band tight-binding model. Several crucial features of Landau quantization in a realistic three-dimensional topological insulator are revealed. The Landau level is absent in ultra-thin films, in agreement with experiment. In films with a crossover thickness of five quintuple layers, there is a signature of the level, whose overall trend as a function of magnetic field matches the established low-energy effective-model result. Importantly, we find a field-dependent splitting and a strong spin-polarization of the level which can be measured experimentally at reasonable field strengths. Our calculations show mixing between the surface and bulk Landau levels which causes the character of levels to evolve with magnetic field
The peculiarities of the artistic time in V. Vynnychenko’s short story «Dark might»
No full text(uk) У статті розглядаються особливості організації часопросторовихзв’язків в оповіданні В. К. Винниченка «Темна сила» та їхнє значення для розуміння ідейного змісту твору. У формуванні цілісного уявлення про творчість письменника важливим є вивчення створеного ним хронотопу, адже організація часопростору впливає на ідейно-художню специфіку тексту. На межі світу автора і реальності утворюється художній простір оповідання, що є виразом світовідчуття письменника. Хронотоп – це особливе поєднання та взаємозв’язок просторових і часових характеристик у літературному творі. Час ніби переходить у простір, поєднуючись, вони створюють своєрідний фундамент розвитку подальших подій. Художній простір в оповіданні «Темна сила» не лише фон, на якому розгортається дія, він визначає поведінку та вчинки героїв. Отже, зовнішній простір впливає на внутрішній простір героїв, тобто на їх свідомість.Особливості організації часопростору у В. К. Винниченка пов’язані з кольором як емоційно-експресивним засобом художньої конкретизації. Особливо частотним є сірий, що відбиває внутрішній душевний стан героїв. Смислового акценту набуває така деталь хронотопу як вікно. У контексті твору вікно для героїв – це символ надії, можливість розширити зовнішній простір та зв’язатися з ним. Художній простір оповідання підпорядкований авторській ідеї відтворити реалістичний час і пояснити почуття, рефлексії героїв. Час і простір формують образ світу, представлений у творі.(en) The peculiarities of the time-space links in the story «Темна сила» / «Dark Force» by Volodymyr Vynnychenko and their importance for the understanding of the ideological content of the story are in the focus of the proposed paper.
To form the overall picture of the writer’s creativity it is essential to study the chronotope developed by the author, for, the organization of the time-space affects the ideological land artistic specificy of the text. Chronotope is a particular combination and correlation of the spatial and temporal characteristics of a literary work. Time goes into space, and when combined, they create a kind of a basis of future events development. At the turn of the author’s world and the reality, the created art space of the story is an expression of the writer’s perception of the world.
The art space in the story «Dark Force» is not only the background against which the story unfolds, but it determines the behavior and actions of the characters. Thus, the external space affects the internal space of the characters, their minds, in particular.
The features of the time-space organization in V. K. Vynnychenko story are associated with color as an emotionally expressive tool of artistic facet. I n particular frequency is gray, which reflects the inner side of the characters.
Therefore, such time-space detail as window acquires its semantic accent. I n the context of the story window for heroes becomes a symbol of hope and opportunity to expand and contact the external space. The art space of the story subordinates to the author' s idea to replicate realistic time in order to explain the feelings and reflections of the characters. Consequently, time and space form the image of the world presented in the book
Effects of short-range electron-electron interactions in doped graphene
No full textWe study theoretically the effects of short-range electron-electron interactions on the electronic structure of graphene, in the presence of substitutional impurities. Our computational approach is based on the π orbital tight-binding model for graphene, with the electron-electron interactions treated self-consistently at the level of the mean-field Hubbard model. The finite impurity concentration is modeled using the supercell approach. We compare explicitly noninteracting and interacting cases with varying interaction strength and impurity potential strength. We focus in particular on the interaction-induced modifications in the local density of states around the impurity, which is a quantity that can be directly probed by scanning tunneling spectroscopy of doped graphene. We find that the resonant character of the impurity states near the Fermi level is enhanced by the interactions. Furthermore, the size of the energy gap, which opens up at high-symmetry points of the Brillouin zone of the supercell upon doping, is significantly affected by the interactions. The details of this effect depend subtly on the supercell geometry. We use a perturbative model to explain these features and find quantitative agreement with numerical results.</p
Quantum Transport by Spin‐Polarized Edge States in Graphene Nanoribbons in the Quantum Spin Hall and Quantum Anomalous Hall Regimes
No full textUsing the non-equilibrium Green’s function method and the Keldysh formalism, we study the effects of spin–orbit interactions and time-reversal symmetry breaking exchange fields on non-equilibrium quantum transport in graphene armchair nanoribbons. We identify signatures of the quantum spin Hall (QSH) and the quantum anomalous Hall (QAH) phases in nonequilibrium edge transport by calculating the spin-resolved real space charge density and local currents at the nanoribbon edges. We find that the QSH phase, which is realized in a system with intrinsic spin–orbit coupling, is characterized by chiral counter-propagating local spin currents summing up to a net charge flow with opposite spin polarization at the edges. In the QAH phase, emerging in the presence of Rashba spin–orbit coupling and a ferromagnetic exchange field, two chiral edge channels with opposite spins propagate in the same direction at each edge, generating an unpolarized charge current and a quantized Hall conductance . Increasing the intrinsic spin–orbit coupling causes a transition from the QAH to the QSH phase, evinced by characteristic changes in the non-equilibrium edge transport. In contrast, an antiferromagnetic exchange field can coexist with a QSH phase, but can never induce a QAH phase due to a symmetry that combines time-reversal and sublattice translational symmetry
The peculiarities of the artistic time in V. Vynnychenko’s short story «Dark might»
No full text(uk) У статті розглядаються особливості організації часопросторовихзв’язків в оповіданні В. К. Винниченка «Темна сила» та їхнє значення для розуміння ідейного змісту твору. У формуванні цілісного уявлення про творчість письменника важливим є вивчення створеного ним хронотопу, адже організація часопростору впливає на ідейно-художню специфіку тексту. На межі світу автора і реальності утворюється художній простір оповідання, що є виразом світовідчуття письменника. Хронотоп – це особливе поєднання та взаємозв’язок просторових і часових характеристик у літературному творі. Час ніби переходить у простір, поєднуючись, вони створюють своєрідний фундамент розвитку подальших подій. Художній простір в оповіданні «Темна сила» не лише фон, на якому розгортається дія, він визначає поведінку та вчинки героїв. Отже, зовнішній простір впливає на внутрішній простір героїв, тобто на їх свідомість.Особливості організації часопростору у В. К. Винниченка пов’язані з кольором як емоційно-експресивним засобом художньої конкретизації. Особливо частотним є сірий, що відбиває внутрішній душевний стан героїв. Смислового акценту набуває така деталь хронотопу як вікно. У контексті твору вікно для героїв – це символ надії, можливість розширити зовнішній простір та зв’язатися з ним. Художній простір оповідання підпорядкований авторській ідеї відтворити реалістичний час і пояснити почуття, рефлексії героїв. Час і простір формують образ світу, представлений у творі.(en) The peculiarities of the time-space links in the story «Темна сила» / «Dark Force» by Volodymyr Vynnychenko and their importance for the understanding of the ideological content of the story are in the focus of the proposed paper.
To form the overall picture of the writer’s creativity it is essential to study the chronotope developed by the author, for, the organization of the time-space affects the ideological land artistic specificy of the text. Chronotope is a particular combination and correlation of the spatial and temporal characteristics of a literary work. Time goes into space, and when combined, they create a kind of a basis of future events development. At the turn of the author’s world and the reality, the created art space of the story is an expression of the writer’s perception of the world.
The art space in the story «Dark Force» is not only the background against which the story unfolds, but it determines the behavior and actions of the characters. Thus, the external space affects the internal space of the characters, their minds, in particular.
The features of the time-space organization in V. K. Vynnychenko story are associated with color as an emotionally expressive tool of artistic facet. I n particular frequency is gray, which reflects the inner side of the characters.
Therefore, such time-space detail as window acquires its semantic accent. I n the context of the story window for heroes becomes a symbol of hope and opportunity to expand and contact the external space. The art space of the story subordinates to the author' s idea to replicate realistic time in order to explain the feelings and reflections of the characters. Consequently, time and space form the image of the world presented in the book
