Nanoscale materials have many potential advantages because of their quantum confinement, cost and producibility by low-temperature chemical methods. Advancement of theoretical methods as well as the availability of modern high-performance supercomputers allow us to control and exploit their microscopic properties at the atomic scale, hence making it possible to design novel nanoscale molecular devices with interesting features (e.g switches, rectifiers, negative differential conductance, and high magnetoresistance). In this thesis, state-of-the-art theoretical calculations have been performed for the quantum transport properties of nano-structured materials within the framework of Density Functional Theory (DFT) and the Nonequilibrium Green's Function (NEGF) formalism. The switching behavior of a dithiolated phenylene-vinylene oligomer sandwiched between Au(111) electrodes is investigated. The molecule presents a configurational bistability, which can be exploited in constructing molecular memories, switches, and sensors. We find that protonation of the terminating thiol groups is at the origin of the change in conductance. H bonding at the thiol group weakens the S-Au bond, and thus lowers the conductance. Our results allow us to re-interpret the experimental data originally attributing the conductance reduction to H dissociation. Also examined is current-induced migration of atoms in nanoscale devices that plays an important role for device operation and breakdown. We studied the migration of adatoms and defects in graphene and carbon nanotubes under finite bias. We demonstrate that current-induced forces within DFT are non-conservative, which so far has only been shown for model systems, and can lower migration barrier heights. Further, we investigated the quantum transport behavior of an experimentally observed diblock molecule by varying the amounts of phenyl (donor) and pyrimidinyl (acceptor) rings under finite bias. We show that a tandem configuration of two dipyrimidinyl-diphenyl molecules improves the rectification ratio, and tuning the asymmetry of the tandem set-up by rearranging the molecular blocks greatly enhances it. It has been recently demonstrated that the large band gap of boronitrene can be significantly reduced by carbon functionalization. We show that specific defect configurations can result in metallicity, raising interest in the material for electronic applications. In particular, we demonstrate negative differential conductance with high peak-to-valley ratios, depending on the details of the material, and identify the finite bias effects that are responsible for this behavior. Also, we studied the spin polarized transport through Mn-decorated topological line defects in graphene. Strong preferential bonding is found, which overcomes the high mobility of transition metal atoms on graphene and results in stable structures. Despite a large distance between the magnetic centers, we find a high magnetoresistance and attribute this unexpected property to very strong induced π magnetism. Finally, the results obtained herein advance the field of quantum electronic transport and provide significant insight on switches, rectification, negative differential conductance, magnetoresistance, and current-induced forces of novel nanoscale materials.

This cumulative dissertation is based on the publications [P1–P6], covering various aspects in theoretical studies of isolated quantum many-body systems. The transport and relaxation dynamics in quantum lattice models are studied with a particular focus on (i) the effect of a mass imbalance between different particles on their relaxation dynamics as well as (ii) the influence of generic perturbations on different reference dynamics. As for (i), the dynamics of two mutually interacting fermionic particle species on a lattice are investigated for different mass ratios between the two species [P4]. Numerical studies of density dynamics show that diffusive transport which is expected for small mass imbalances persists also for moderate imbalances and becomes anomalous for stronger imbalances. On the other hand, while transport is suppressed in the limit of infinite imbalance, i.e., if one particle species is immobile, this effective localization is shown to give way to anomalous diffusion as soon as the heavy particle species gains a finite mobility. Regarding (ii), the effect of perturbations on dynamics is investigated from the perspective of projection-operator techniques [P6]. As a main result, it is demonstrated that simple exponential damping, which is expected in the overwhelming majority of cases, may only occur for the density matrix in the interaction picture. Within this approach, this simple damping carries over to the time dependence of standard correlation functions only in certain cases. In particular, the possibility of nontrivial damping in physically relevant perturbation scenarios is discussed. A considerable portion of this work is concerned with the implementation of powerful numerical and (semi-)analytical tools to overcome the enhanced computational complexity in numerical studies of quantum many-body systems. This includes the concept of dynamical quantum typicality [P2, P3], numerical linked-cluster expansions [P5], and projection-operator techniques, as well as the combined use of available symmetries [P1].

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18776号 / 理博第4034号 / 新制||理||1581(附属図書館) / 31727 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 武末 真二, 教授 佐々 真一, 教授 早川 尚男 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Two-dimensional (2D) carrier systems built from semiconductor heterostructures have been at the center of a wide variety of experimental and theoretical research over the past decades. The quality improvement of GaAs/AlGaAs systems has allowed the observation of several peculiar ground states stabilized by the subtle interplay between carrier-carrier interaction, disorder and magnetic field. More recently, 2D systems in semiconductor heterostructures have also been used as a prime substrate for further confinement of the carriers to mesoscopic systems of major interest for the emerging fields of quantum computing and spintronics. This thesis addresses both magnetotransport measurements in hole open quantum dots (QDs) and thermopower studies of 2D holes in (311)A GaAs heterostructures. In the first part of this thesis, we describe the fabrication process for hole GaAs open QDs and investigate their magnetotransport properties at very low temperature T. Below 500 mK, the magnetoconductance of the open QDs exhibits clear signatures of coherent transport, namely magnetoconductance fluctuations and weak anti-localization. From these effects, we extract a T dependence for the dephasing time, together with an upper limit for the spin-orbit scattering time using the random matrix theory. Both the dephasing time and the spin-orbit scattering time are found to be much smaller than for electrons in similar systems. In the second part of this work, we report low-T thermopower measurements in the parallel magnetic field-induced metal-insulator transition (MIT) of 2D GaAs hole heterojunctions with different interface-dependent mobilities. When the magnetic field is increased, the diffusion thermopower decreases across the MIT. The reduction of the diffusion thermopower is more pronounced for the lower mobility sample where it reverses its sign. This behaviour indicates that the system does not undergo any ground state modification through the MIT but rather that the parallel magnetic field induces a dramatic change of the dominant hole scattering mechanisms. Finally, the last part of this thesis is devoted to the thermopower study of the insulating phase (IP) observed in 2D GaAs bilayer hole systems around the total Landau level filling factor n = 1. Our measurements show that the diffusion thermopower diverges with decreasing T in the IP. This divergence of the diffusion thermopower at low T indicates the opening of an energy gap in the system's ground state and suggests the formation of a pinned bilayer hole Wigner crystal around n = 1.

Dans cette thèse, nous étudions le transport quantique d’ondes de matière avec des atomes ultrafroids. Ces systèmes d’atomes ultrafroids fournissent un très bon contrôle et une grande flexibilité pour les paramètres du système tels que les interactions, sa dimensionnalité et les potentiels externes. Cela les rend un excellent outil pour l’étude de plusieurs concepts fondamentaux de la physique de la matière condensée. Nous nous concentrons sur le transport quantique dans les milieux désordonnés. Il diffère du transport classique par le rôle fondamental joué par les phénomènes d’inférence, qui peuvent éventuellement conduire à la suppression du transport; connu comme la Localisation d’Anderson. Nous étudions l’expansion d’un condensat de Bose-Einstein dans un désordre fort et montrons des signes de localisation d’atomes ultrafroids à trois dimensions. Dans la dernière partie de ce manuscrit, nous discutons l’observation de la rétrodiffusion cohérente d’atomes ultrafroids, ce qui est un signal direct du rôle de la cohérence quantique dans le transport quantique dans les milieux désordonnés. Nous observons l’évolution temporelle de la distribution d’impulsions d’un nuage de atomes ultrafroids, lancé avec une distribution de vitesse étroite dans un potentiel désordonné. Un pic émerge dans le sens rétrograde, correspondant au signal de CBS. / In this thesis we study the quantum transport of matter waves with ultracold atoms. Such ultracold atom systems provide a very good control and a high flexibility of the parameters of the systems like the interactions, its dimensionality and the external potentials. This makes them a great tool for the investigation of several fundamental concepts of condensed matter physics. We focus on the quantum transport in disordered media. It differs to classical transport by the fundamental role played by inference phenomena, which can eventually lead to the suppression of transport; known as Anderson Localization. Observing the expansion of a Bose-Einstein condensate in a strong light disorder, we show evidence for Localization of ultracold atoms in three dimensions. In the last part of this manuscript we discuss the observation of Coherent Backscattering of ultracold atoms, which is a direct signal of the role of quantum coherence in quantum transport in disordered media. We observe the time evolution of the momentum distribution of a cloud of ultra-cold atoms, launched with a narrow velocity distribution in a disordered potential. A peak emerges in the backwards direction, corresponding to the CBS signal.

Orientador: Guillermo Gerardo Cabrera Oyarzun / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-09-24T18:53:24Z (GMT). No. of bitstreams: 1 CorreaJunior_Clovis_M.pdf: 2475896 bytes, checksum: c134096367f5dc832f4c32ec8a9e224b (MD5) Previous issue date: 2009 / Resumo: Nesta dissertação de mestrado são descritas as características fundamentais dos condutores mesoscópicos, e as origens e propriedades das várias fontes de ruído em condutores. Primeiramente, descrevemos o ruído com distintos métodos e enfatizamos as propriedades de cada tipo de ruído. Em seguida, apresentamos a abordagem da matriz de espalhamento para condutores com coerência de fase, a qual permite-nos tratar as propriedades de transporte de forma unificada. Estudamos um modelo proposto para descrever as propriedades de transporte de nanofios e nanocontatos dos metais magnéticos de transição. É assumido que os orbitais de condução são do tipo s, o que permite a existência de dois canais de condução devido ao spin. Da mesma forma, consideramos os orbitais d como fontes de momentos de dipolos magnéticos locais. O modelo é aplicado ao caso de nanocontatos constituídos de dois átomos, os quais estão acoplados a dois eletrodos magnéticos. Usando um pequeno campo externo, é possível controlar os estados de polarização dos eletrodos: paralelamente e anti-paralelamente. Nesse nanocontato, são estudados as propriedades do coe½ciente de transmissão, da condutância, do ruído shot quântico, do fator de Fano e da magnetoresistência / Abstract: This dissertation describes the fundamental caracteristics of mesoscopic conductors, and the origins and properties of the sources of noise in conductors. Firstly, we describe the noise through different methods and emphasize the properties of each kind of noise. In the following, we present the scattering approach for coherent phase conductors, which allows us to get the transport properties from a unified picture. It is studied a particular model to describe the transport properties of magnetic transsition metal nanowires and nanocontacts. It is assumed that conduction orbitals are s-like, with the occurrence of only two conductions channel due to spin. In turn, d-like orbitals are sources of local magnetic moments. The model is applied to a simple nanocontact built of two atoms, which are coupled to two magnetic electrodes. Using small external fields, one can handle the polarization state of the electrods: in parallel or antiparallel alignment. From that nanocontact, we investigate the properties of the transmission coe°cient, the conductance, the quantum shot noise, the Fano factor and the magnetoresistance / Mestrado / Física da Matéria Condensada / Mestre em Física

In ‘clean' confined conductors (the so-called mesoscopic systems), the electronic phase and momentum can be preserved over very long distances compared to the system dimensions. This gives rise to peculiar transport properties, bearing signatures of electron interferences, ballistic electron trajectories, electron-electron interactions, regular-chaotic electron dynamics and (in some cases) spin-orbit coupling. Examples of such effects are the Universal Conductance Fluctuations (UCFs) and the Weak Localization observed in the low-temperature magnetoconductance of many confined electronic systems. Of central importance, the electronic phase coherence time and the spin-orbit coupling time determine the amplitude of these quantum effects. In the first part of this thesis, we use UCFs to extract these characteristic timescales in open ballistic quantum dots (QDs) fabricated from InGaAs heterostructures. We observe an intrinsic saturation of the coherence time at low temperature in the InGaAs QDs. The origin of this phenomenon has been intensely debated during the last decade. Based on our observations and previous experimental data in QDs, we propose an explanation: the dwell time becomes the limiting factor for electron interferences in QDs at low temperature. Then, we report on magnetoconductance measurements in a bismuth ballistic nano-cavity. The cavity is found to be zero-dimensional for phase coherent processes at low temperature. We evidence an anomalous reduction of the phase coherence time in the cavity with respect to data obtained in thin Bi films, while the spin-orbit coupling time is similar in both systems. Finally, we examine the current-voltage characteristics of asymmetric InGaAs nano-junctions in the nonlinear regime. We observe a new tunable rectification effect, whose amplitude and sign are governed by the conductances of the junctions' channels. We show that the effect is ballistic and exhibits new features with respect to predictions of available models.

Atomically two-dimensional (2-D) graphene, as well as the hexagonal boron nitride dielectric have been and are continuing to be widely investigated for the next generation nanoelectronic devices. More recently, other 2-D materials and electronic systems including the surface states of topological insulators (TIs) and monolayers of transition metal dichalcogenides (TMDs) have also attracted considerable interest. In this work I have focused on these latter two material systems on possible device applications. TIs are characterized by an insulating bulk band gap and metallic Dirac surface states which are spin-polarized. Here, the electronic structures of bulk and thin film TIs are studied using ab-initio density functional theory (DFT). Band inversion, an essential characteristic of TIs, is shown in the bulk band structures. Properties of TI surface bands in thin film such as the critical film thickness to induce a gap, the thickness dependent gap size, and the localization length of surface states are reported. Effects of crystalline dielectric materials on TI surface states are also addressed by ab-initio calculations. I discuss the sensitivity of Dirac point degeneracy and linear band dispersion of TI with respect to different dielectric surface terminations as well as different relative atom positions of the dielectric and TI. Additionally, this work presents research on exciton condensation in TI using a tight-binding model combined with self-consistent non-local Hartree-Fock mean-field theory. Possibility of exciton condensation in the TI Bi₂Se₃ thin film is assessed. Non-equilibrium Green's function (NEGF) simulations with the atomistic tight-binding (TB) Hamiltonian are carried out to explore the performance of metal-oxide-semiconductor field-effect-transistor (MOSFET) and tunnel field-effect-transistor (TFET) based on the Bi₂Se₃ TI thin film. How the high dielectric constant of Bi₂Se₃ affects the performance of MOSFET and TFET is presented. Bulk TMDs such as MoS₂, WS₂ and others are the van der Waals-bonded layered material, much like graphite, except monolayer (and Bulk) TMDs have a large band gap in-contrast to graphene (and graphite). Here, the performance of nanoscale monolayer MoS₂ n-channel MOSFETs are examined through NEGF simulations using an atomistic TB Hamiltonian. N- and p-channel MOSFETs of various monolayer TMDs are also compared by the same approach. I correlate the performance differences with the band structure differences. Finally, ab-initio calculations of adatom doping effects on the monolayer MoS₂ is shown. I discuss the most stable atomic configurations, the bonding type and the amount of charge transfer from adatom to the monolayer MoS₂. / text

There are many semiconductors with nominally superior electronic properties compared to silicon. However, silicon became the material of choice for MOSFETs due to its robust native oxide. With Moore's observation as a guiding principle, the semiconductor industry has come a long way in scaling the silicon MOSFETs to smaller dimensions every generation with engineering ingenuity and technological innovation. As per the 2012 International Technology Roadmap for Semiconductors (ITRS), the MOSFET is expected to be scaled to near 6 nm gate length by 2025. However, materials, design and fabrication capabilities aside, basic physical considerations such as source to drain quantum mechanical tunneling, channel to gate tunneling, and thermionic emission over the channel barrier suggest an end to the roadmap for CMOS is on the horizon. The semiconductor industry is already aggressively looking for the next switch which can replace the silicon FET in the long term. My Ph.D. research is part of the quest for the next switch. The promises of process compatibility with existing CMOS technologies, fast carriers with high mobilities, and symmetric conduction and valence bands have led to graphene being considered as a possible alternative to silicon. This work looks at three devices based on graphene using first principles atomistic transport simulations and compact models capturing essential physics: the large-area graphene RF FET, the Bilayer pseudoSpin FET, and the double electron layer resonant tunneling transistor. The characteristics and performance of each device is explored with a combination of SPICE simulations and atomistic quasi static transport simulations. The BiSFET device was found to be a promising alternative to CMOS due to extremely low power dissipation. Finally, I have presented formalism for efficient simulation of time dependent transport in graphene for beyond quasi static performance analysis of the graphene based devices explored in this work. / text

Rectification phenomena in two specific mesoscopic systems are reviewed. The phenomenon is called ratchet effect, and such systems are called ratchets. In this thesis, particularly a rocked quantum-dot ratchet, and a tunneling ratchet are considered. The origin of the name is explained in a brief historical background. Due to rectification, there is a net non-vanishing electronic current, whose direction can be reversed by changing rocking amplitude, the Fermi energy, or applying magnetic field to the devices (for the rocked ratchet), and tuning the temperature (for the tunneling ratchet). In the last part, a theoretical examination based on the Landauer-B&uuml / ttiker formalism of mesoscopic quantum transport is presented.