Solid-state magnetic traps and lattices

We propose and analyze magnetic traps and lattices for electrons in semiconductors. We provide a general theoretical framework and show that thermally stable traps can be generated by magnetically driving the particle's internal spin transition, akin to optical dipole traps for ultra-cold atoms. Next we discuss in detail periodic arrays of magnetic traps, i.e. magnetic lattices, as a platform for quantum simulation of exotic Hubbard models, with lattice parameters that can be tuned in real time. Our scheme can be readily implemented in state-of-the-art experiments, as we particularize for two specific setups, one based on a superconducting circuit and another one based on surface acoustic waves.Comment: 18 pages, 8 figure

Electronic and Optical Properties of Silicon Nanowires: Theory and Modeling

Narrow silicon nanowires host a rich set of physical phenomena. Understanding these phenomena will open new opportunities for applications of silicon nanowires in optoelectronic devices and adds more functionality to silicon especially in those realms that bulk silicon may not operate remarkably. Compatibility of silicon nanowires with the mainstream fabrication technology is also advantageous. The main theme of this thesis is finding the possibility of using silicon nanowires in light sources; laser and light emitting diodes. Using Tight Binding (TB) and ab-initio Density Functional Theory (DFT) methods it was shown that axial strain can induce significant changes in the effective mass, density of states and bandgap of silicon nanowires. Generality of the observed effects was proven by investigating nanowires of different crystallography, diameter and material (e.g. germanium nanowires). The observed direct to indirect bandgap conversion suggests that strain is able to modulate the light emission properties of silicon nanowires. To investigate this possibility, spontaneous emission time was formulated using perturbation theory including Longitudinal Optical (LO) and Acoustic (LA) phonons. It was observed that corresponding to bandgap conversion, the spontaneous emission time can be modulated by more than one order of magnitude. This emanates from bandgap conversion and symmetry change of wave function in response to strain. A mechanism for population inversion was proposed in the thesis which is based on the Ensemble Monte Carlo (EMC) study of carrier statistics in direct and indirect conduction sub bands. By calculating all possible electron-phonon scattering mechanisms which may deplete the already populated indirect subband, it was shown that at different temperatures and under different electric fields there is a factor of 10 difference between the population of indirect and direct sub bands. This suggests that population inversion can be achieved by biasing an already strained nanowire in its indirect bandgap state. The light emission is possible then by releasing or inverting the strain direction. A few ideas of implementing this experiment were proposed as a patent application. Furthermore the photo absorption of silicon nanowires was calculated using TB method and the role of diameter, optical anisotropy and strain were investigated on band-edge absorption

Plasmonic Properties of Nanoparticle and Two Dimensional Material Integrated Structure

Recently, various groups have demonstrated nano-scale engineering of nanostructures for optical to infrared wavelength plasmonic applications. Most fabrication technique processes, especially those using noble metals, requires an adhesion layer. Previously proposed theoretical work to support experimental measurement often neglect the effect of the adhesion layers. The first finding of this work focuses on the impact of the adhesion layer on nanoparticle plasmonic properties. Gold nanodisks with a titanium adhesion layer are investigated by calculating the scattering, absorption, and extinction cross-section with numerical simulations using a finite difference time domain (FDTD) method. I demonstrate that a gold nanodisk with an adhesive layer significantly shifts the plasmon resonance relative to one without adhesion material. In addition, the adhesive layer also introduces stronger damping and decay time. Next, I investigate the plasmonic properties and effects of dielectric environment of black phosphorene (BP), a newly discovered anisotropic 2D material. Results suggest that the surface plasmon properties of a black phosphorene nanoribbon could be exploited to probe the efficiency of edge plasmonic enhanced absorption. Furthermore, the enhanced absorption of periodic BP nanoribbons is affected strongly by high density free carriers in BP nanoribbon geometries from mid-infrared to high infrared regime. Also when adding a thin dielectric shielding layer, such as hexagonal boron nitride, in addition to preserving the edge mode plasmonic nature of BP, it also allows for an unprecedented control of the absorption resonance energy. Finally, I also show monolayer graphene surface plasmon hybridization with hyperbolic phonon polarization local density of state of hyperbolic ferroelectric LiNbO3. The results show that the dispersion mode hybridization process is significantly regulated by a electrostatic gated single graphene and double graphene layer in addition to the ferroelectric layer size. The spontaneous emission (SE) rate the hyperbolic band contribution of LiNbO3 with graphene integrated system elucidated enhancement and inhibit spontaneous emission. Specially, the SE rate between in hybrid system is always smaller than that of the bulk in the hyperbolic band region with higher chemical potential

Spin dynamics in semiconductors

This article reviews the current status of spin dynamics in semiconductors which has achieved a lot of progress in the past years due to the fast growing field of semiconductor spintronics. The primary focus is the theoretical and experimental developments of spin relaxation and dephasing in both spin precession in time domain and spin diffusion and transport in spacial domain. A fully microscopic many-body investigation on spin dynamics based on the kinetic spin Bloch equation approach is reviewed comprehensively.Comment: a review article with 193 pages and 1103 references. To be published in Physics Reports

Semiconductor-based electron lattices for quantum information processing

Scalable physical systems that enable trapping and coherent manipulation of quantum matter lie at the heart of quantum information processing (QIP). Solid-state approaches benefit from rapidly evolving nanotechnology and provide a way towards efficient on-chip quantum devices. In this thesis we show how key ideas from quantum optics can inspire novel setups and implementations for QIP in solid-state settings. To this end, we develop strategies for the realization of well-defined lattices for electrons and other quasiparticles in semiconductors. The theoretical proposals presented in this thesis may serve as novel platforms for controlling and studying quantum many-body systems. In an introductory passage, we provide a brief summary of goals, recent advances and future directions in quantum information science, highlighting the significant progress that has been made in related fields during the past years. The core of the thesis consists of two parts, one dedicated to quantum systems interacting with elastic waves in solids, and another related to a novel class of two-dimensional semiconductors. In the first part, we theoretically investigate how surface acoustic waves (SAWs) may be used to create well-defined potentials for mobile electrons and other semiconductor quasiparticles. We develop an effective description of electrons coupled to SAW-driven time-dependent electromagnetic fields by modelling their dynamics within a Floquet framework. The underlying physical coupling mechanisms can be based on piezoelectric, piezomagnetic or strain fields, respectively, and we discuss the implications of each. We show that these systems bear striking similarities with atomic, molecular and optical implementations, such as trapped ions and cold neutral atoms in optical lattices. Specific to the solid-state environment are couplings to various sorts of impurities and bulk phonons, which possibly degrade the quality of SAW-based traps. We thus take into account these effects, and investigate the influence of thermal bulk phonons by deriving an effective description of the electronic motion based on quantum master equations. These results provide a recipe for a near-term realization of acoustic traps and lattices for semiconductor particles. The versatility of the presented theoretical approach allows a thorough examination of various materials, heterostructures and quasiparticles. Several case studies of suitable host materials are presented, and connections to possible future experimental work are established. With a projected lattice spacing on the scale of ∼ 100nm, acoustically defined electron lattices allow for relatively large energy scales in the realization of fermionic Hubbard models, and a parameter regime very different from the one typically obtained in other systems. The ultimate prospect of entering the low-temperature, strong-interaction regime may be crucial for a better understanding of high-temperature superconductivity. In the second part of this thesis, we focus on realization and detection of self-assembled electron lattices in transition-metal dichalcogenides (TMDs). TMDs have remarkable mechanical, optical and electronic properties and are ideally suited for the study of quantum Wigner crystals (WCs). WCs are prime candidates for the realization of regular electron lattices under minimal requirements on external control and electronics. However, several technical challenges have prevented their detailed experimental investigation and applications to date. Based on scattering theory, we theoretically analyze the optical response of TMD-based WCs. We show that TMDs allow for minimally invasive all-optical detection schemes of charge order inherent in WCs, and that optical selection rules of TMDs provide direct access to spin measurements via Faraday rotation. Experimental signatures of WCs are presented, and disorder-induced imperfections are considered. We highlight their potential as a platform for the quantum simulation of geometrically frustrated magnetism with adjustable and self-assembled lattice structures. Future research directions, that are related to the results presented here, are discussed at the end of the thesis.Kontrollierbare und skalierbare Quantensysteme bilden die Grundlage für Quanteninformationsverarbeitung. Festkörpersystemen kommt dabei eine besondere Rolle zu, da diese von industriell verfügbaren Nanofabrikationstechniken profitieren und somit einen möglichen Weg zur Bereitstellung von Chip-basierten Quantentechnologien bieten. In dieser Arbeit zeigen wir auf, wie Schlüsselkonzepte aus der Quantenoptik neuartige Implementierungen festkörperbasierter Quantensysteme ermöglichen können, die im Rahmen der Quanteninformationsverarbeitung relevant sind. Zu diesem Zweck entwickeln wir Strategien zur Realisierung wohldefinierter Fallen und Gitter zum Fangen von Elektronen und anderen Quasiteilchen in Halbleitern. Die theoretischen Ausarbeitungen in dieser Arbeit eröffnen Möglichkeiten für neue Plattformen zur Kontrolle und zum Studium von Quantenvielteilchensystemen. In der Einleitung stellen wir eine Übersicht der Ziele, jüngsten Fortschritte und Zukunftsvisionen im Bereich der Quanteninformationswissenschaften vor. Der Hauptteil dieser Arbeit besteht aus zwei Teilen. Zuerst widmen wir uns Quantensystemen, die in Festkörpern mit akustischen Wellen wechselwirken, und danach setzen wir uns mit einer neuartigen Klasse zweidimensionaler Halbleiter auseinander. Im ersten Teil untersuchen wir, wie akustische Oberflächenwellen wohldefinierte Potentiallandschaften für bewegliche Elektronen und andere Quasiteilchen in Halbleitern erzeugen können. Wir entwickeln eine effektive Beschreibung von Elektronen, die an akustisch getriebene, zeitabhängige elektromagnetische Felder koppeln. Hierfür modellieren wir die Dynamik der freien Ladungsträger im Rahmen eines Floquet-Formalismus. Die zu Grunde liegenden physikalischen Kopplungsmechanismen können piezoelektrischen oder piezomagnetischen Ursprungs sein oder von mechanischer Spannung herrühren. Wir zeigen, dass diese Systeme Ähnlichkeit mit atomaren, molekularen und optischen Quantensystemen haben, z. B. mit gefangenen Ionen und ultrakalten Atomen in optischen Gittern. Spezifisch für die festkörperbasierten Systeme sind Kopplungen der Elektronen an Störstellen und phononische Freiheitsgrade, welche die Funktionsweise von akustischen Fallen beeinträchtigen können. Daher berücksichtigen wir diese Effekte und untersuchen den Einfluss von thermischen Phononen auf die elektronische Bewegung auf Basis einer Quantenmastergleichung. Diese Resultate bilden die Basis für eine ausführliche Untersuchung verschiedener möglicher experimenteller Umsetzungen. Hierzu berücksichtigen wir diverse Materialien, Heterostukturen und Quasiteilchen. Verschiedene konkrete Fallbeispiele werden diskutiert. Da die Gitterkonstanten von akustischen Gittern im Bereich von ∼ 100nm liegen können, erlauben akustisch getriebene Elektronengitter die Verwirklichung relativ großer Energieskalen in fermionischen Hubbardmodellen, und damit das Erreichen von Parameterkonstellationen, die mit anderen Systemen typischerweise nicht realisiert werden können. Damit ermöglichen uns diese Systeme gleichzeitig starke Wechselwirkungen und tiefe Temperaturen, um zum Beispiel Neues über Hochtemperatursupraleiter zu erfahren. Im zweiten Teil beschäftigen wir uns mit der Realisierung und Detektion von selbstorganisierten Elektronengittern in zweidimensionalen Halbleitern. Die hier untersuchten Halbleiter haben beeindruckende mechanische, optische und elektronische Eigenschaften und eignen sich in besonderer Weise zur Untersuchung von Wignerkristallen. Wignerkristalle stellen Elektronengitter dar, die ohne hohe Anforderungen an externe Kontrollparameter auskommen. Allerdings haben sich diese bisher aufgrund einiger technischer Schwierigkeiten der detaillierten experimentellen Untersuchung entzogen. Aufbauend auf einer Streutheorie des Lichts untersuchen wir optische Eigenschaften von Wignerkristallen in zweidimensionalen Halbleitern. Insbesondere zeigen wir, dass diese Systeme minimalinvasive optische Detektion der Ladungsträgeranordnung in Wignerkristallen erlauben. Des Weiteren ermöglichen es die optischen Auswahlregeln dieser Halbleiter, anhand des gestreuten Lichts auch Informationen über den Spinfreiheitsgrad der Elektronen zu gewinnen. Experimentell beobachtbare Signale werden vorgestellt und Imperfektionen der Elektronengitter werden untersucht. Wir zeigen, dass sich diese Systeme auch für die Quantensimulation von frustriertem Magnetismus eignen. Künftige Forschungsfragen, die im Zusammenhang mit dieser Arbeit stehen, werden im abschließenden Kapitel diskutiert