This thesis experimentally studies quantum interference phenomena and quantum coherence in mesoscopic systems, and quantum transport as well as magnetotransport in various materials system. One overarching aim is exploring the different mechanisms that give rise to quantum phase decoherence in lithographically patterned mesoscopic structures, of importance in the field of quantum technologies and spintronics. Various mesoscopic structures, namely quantum stadia, quantum wires, and side-gated rings, were fabricated to function as quantum interference devices and platforms to study quantum coherence on two-dimensional electron systems in InGaAs/InAlAs heterostructures. The mesoscopic structures were fabricated by photolithography and electron-beam lithography. The dependence of quantum coherence on geometry or temperature is investigated for each of the quantum interference devices. In the case of quantum stadia, phase coherence lengths were extracted by universal conductance fluctuations, and the extracted phase coherence lengths show a dependence on both temperature and geometry. Phase coherence lengths decreased with increasing temperature, as expected. Moreover, phase coherence lengths also varied with the width-to-length ratio and length of the side wires connected to the stadia, where competition between Nyquist decoherence and environmental coupling decoherence mechanisms coexists. For the quantum wires studied, the phase coherence lengths were extracted from antilocalization signals. Antilocalization measurements provide a sensitive mean of probing the quantum mechanical correction to electronic transport. The phase coherence lengths increased as the wire length increased, due to reduction of the environmental coupling that induces decoherence at the ends of a wire; longer wires tend to have longer phase coherence lengths. In related work, the thesis shows that the spin coherence length, as limited by spin-orbit interaction, increases as the wire width decreases. Decoherence in side-gated rings was measured from the amplitudes of the quantum-mechanical Aharonov-Bohm oscillations. The side gates allow for an in-plane controllable electric field. Asymmetrically biased side-gate voltages allow for the breaking of the two-dimensional parity symmetry of the ring device, effectively resulting in reduced amplitude of the Aharonov-Bohm oscillations. The mechanism that contributes to decoherence in these rings appears to be related to the breaking of the spatial symmetry. Measurements of antilocalization and weak-localization as well as magnetotransport were used to probe interesting or unique quantum mechanical phenomena in the following two, quite different, materials system: bismuth iridate thin films, and Ge/AlAs heterostructures on GaAs or Si substrates. Both materials are of interest for future quantum technologies and devices. Measurements in bismuth iridate thin films reveal interesting transport characteristics such as logarithmic temperature dependence of the resistivity, multiple charge carriers, and antilocalization due to spin-orbit interaction in the system. Weak localization measurements in the Ge/AlAs heterostructure on GaAs or Si substrates show that single carrier transport is essentially located in the Ge layer only. Further, the weak localization results indicate the near-absence of spin-orbit interaction for carriers in the electronically active Ge layer, suggesting the potential use of this materials system as a promising candidate for future electronic device applications. In short, quantum transport and interference measurements probe the quantum-mechanical behavior of materials system for future quantum, spin and electronic technologies. Mesoscopic patterned geometries in InGaAs/InAlAs heterostructures offer a wide range of interesting and unique platforms to study quantum-mechanical phenomena, specifically quantum decoherence, in the solid state. The decoherence phenomena observed and the investigations to the underlying mechanisms studied and modeled in this thesis may be transferred to similar materials system, enriching the knowledge in the field of quantum technologies. Magnetotransport and quantum transport were also applied to Ge/AlAs heterostructures and bismuth iridate thin films, to study the properties of their carrier systems. / Ph. D. / The work present experimental results on electron quantum coherence and quantum interference phenomena in solid-state systems of small sizes, and on electronic charge transport under magnetic fields in various solid-state materials. More particularly, structures such as quantum stadia, quantum wires, and quantum rings were fabricated on InGaAs/InAlAs semiconductor heterostructures by photolithography and electron-beam lithography, and were characterized by electronic transport at low temperatures to investigate quantum phenomena. Also, electronic transport over variable temperatures and under high magnetic fields was used to characterize the electronic properties of bismuth iridate thin films and Ge/AlAs semiconductor heterostructures. In quantum stadia and in quantum wires, it was found that the electrons quantum coherence lengths decrease with increasing temperature, due to an increase in electron-electron scattering of the Nyquist type as temperature increases. In quantum stadia, the experiments show that quantum coherence lengths vary with the width-to-length ratio and with the lengths of side wires connected to the stadia, explained by a competition between decoherence due to the above-mentioned electron-electron scattering and decoherence due to coupling of the stadia to the external classical environment. In quantum wires, the quantum coherence length was observed to increase as the wire length increases, due to decreased decoherence from coupling of the wires to the external classical environment for longer wires. The dependence of quantum coherence on quantum system geometry described for quantum stadia and wires has a fundamental importance for future quantum device designs. In quantum rings, quantum coherence was investigated by the amplitude of Aharonov-Bohm oscillations. Asymmetric electric fields were applied to break the spatial symmetry of the ring, resulting in reduced amplitude of Aharonov-Bohm oscillations. The result is explained by an effectively reduced quantum coherence due to breaking of the spatial symmetry. Bismuth iridate thin films revealed interesting electronic transport properties, including a logarithmic temperature dependence of the resistivity, multiple charge carriers, and antilocalization due to spin-orbit interaction in the system. Ge/AlAs heterostructures showed single-carrier electronic transport, and weak-localization signals indicating the near-absence of spin-orbit interaction, and a carrier presence restricted to the Ge layer. The promising properties of these two solid-state materials imply potential applications in future electronic and quantum devices.

<p>This thesis addresses electron spin phenomena in semi-conductor quantum dots/anti-dots from a computational perspective. In the first paper (paper I) we have studied spin-dependent transport through open quantum dots, i.e., dots strongly coupled to their leads, within the Hubbard model. Results in this model were found consistent with experimental data and suggest that spin-degeneracy is lifted inside the dot – even at zero magnetic field.</p><p>Similar systems were also studied with electron-electron effects incorporated via Density Functional Theory (DFT) in paper III. Within DFT we found a significant spin-polarisation in the dot at low electron densities. As the electron density increases the spin polarisation in the dot gradually diminishes. These findings are consistent with available experimental observations. Notably, the polarisation is qualitatively different from the one found in the Hubbard model – this indicates that the simplified approach to electronelectron interaction in the Hubbard model might not always be reliable.</p><p>In paper II we propose a spin-filter device based on resonant backscattering of edge states against a quantum anti-dot embedded in a quantum wire. A magnetic field is applied and the spin up/spin down states are separated through Zeeman splitting. Their respective resonant states may be tuned so that the device can be used to filter either spin in a controlled way.</p> / Report code: LIU-Tek-Lic 2005:65

Mesoscopic effects in ferromagnets could be different from mesoscopic effects in normal metals. While normal metals with a short mean-free-path do not exhibit classical magnetoresistance, weakly disordered ferromagnets with a similar mean-free-path display magnetoresistance including domain wall resistance (DWR) and anisotropic magnetoresistance (AMR). Magnetoresistance could lead to novel mesoscopic effects because the wave function phase depends on the scattering potential. In this thesis, we present our measurements of mesoscopic resistance fluctuations in cobalt nanoparticles and study how the fluctuations with bias voltage, bias fingerprints, respond to magnetization-reversal processes. The resistance has been found to be very sensitive to the magnetic state of the sample. In particular, we observe significant wave-function phase shifts generated by domain walls, and it is explained by mistracking effect, where electron spins lag in orientation with respect to the moments inside the domain wall. Short dephasing length and dephasing time are found in our Co nanoparticles, which we attribute to the strong magnetocrystalline anisotropy.

Electron transport in mesoscopic systems, such as quantum point contacts and Aharonov-Bohm rings are investigated numerically in a tight-binding language with a recursive Green's function algorithm. The simulation reveals among other things the quantized nature of the conductance in point contacts, the Hall conductance, the decreasing sensitivity to scattering impurities in a magnetic field, and the periodic magnetoconductance in an Aharonov-Bohm ring. Furthermore, the probability density distributions for some different setups are mapped, making the transmission coefficients, the quantum Hall effect, and the cyclotron radius visible, where the latter indicates the correspondance between quantum mechanics and classical physics on the mesoscopic scale.

On s'est intéressé dans ce travail à la modélisation 2D et 3D du procédé CIC (Compaction Isostatique à Chaud) à l'échelle mésoscopique, en vue d'une compréhension approfondie des différents mécanismes physiques impliqués dans la densification de poudre. Le modèle est formulé dans un cadre eulérien, et est basé sur l'utilisation de la méthode level-set couplée avec une technique de génération et d'adaptation de maillages éléments finis afin de modéliser la déformation des particules de poudre sur un Volume Élémentaire Représentatif (VER). La génération des particules a été effectuée avec un générateur statistique de microstructures en tenant compte d'une distribution réelle de la taille des particules. Les conditions aux limites mécaniques ont été appliquées sur le VER, entraînant la déformation des particules et la densification du VER. Dans ce travail, la déformation viscoplastique des particules a été considérée comme le seul mécanisme de densification (mécanisme prépondérant pour une grande partie du temps du procédé). A partir de données issues de simulations macroscopiques du mécanisme CIC pour le cas de particule de poudre 316L, des simulations mésoscopiques ont été réalisées (approche macro-méso). Les résultats de ces simulations sont présentés et discutés à la lumière d'une étude expérimentale (microscopie optique, MEB, EBSD et EPMA) de la structure et microstructure des particules, obtenues à partir d'essais de compactions interrompus. De plus, des essais mécaniques ont été réalisés à température ambiante sur des lopins totalement denses issus du procédé CIC.Mots clés : CIC, 316L, compaction de poudre, fonction level-set, génération et adaptation de maillages, échelle mésoscopique, étude expérimentale, microstructure, EBSD, essais mécaniques. / A two and three-dimensional finite element simulation of HIP (Hot Isostatic Pressing) at mesoscopic scale is proposed, in view of an in-depth understanding of the different physical mechanisms involved in powder densification. The model is formulated in a eulerian framework, using level set formulation and adaptive meshing and remeshing strategy to identify particle interactions inside a representative elementary volume (REV). A statistical generator is in charge of the definition of the initial configuration under the constraint of accounting for the real particle size distribution. Mechanical boundary conditions are applied to the REV, resulting in the deformation of particles and densification of the REV. As a first approach, the power-law creep of particles is considered as the unique densification mechanism. Starting from data issued from macroscopic simulations of the HIPping of a part made of 316L powder, mesoscopic simulations in different locations of the part have been carried out (macro-to-meso approach). The results of these simulations are presented and discussed in the light of experimental studies (optical microscopy and SEM, EBSD, EPMA) of the structure and microstructure of the compact, which were obtained from interrupted compactions. Mechanical tests on fully densified 316L were also conducted.Keywords : HIP, 316L, powder densification, level-set function, meshing and remeshing technique, mesoscopic scale, experimental studies, microstructure, EBSD, mechanical tests.

Dans ce manuscrit, nous présentons des mesures haute résolution de chaleur spécifique Cp réalisées sur des anneaux mésoscopiques d'argent à très basses températures. Le but de cette expérience est de mettre en évidence une possible signature thermique due à la présence de courants permanents. Ce phénomène reste encore mal compris malgré de nombreuses expériences. Il existe en effet des contradictions entre les différents modèles théoriques et les résultats expérimentaux. L'approche thermique que nous exposons ici est un angle nouveau qui n'a jamais été exploré. Sous champ magnétique, une oscillation de période égale au quantum de flux Φ0=h/e (ou moitié) de Cp est attendue théoriquement. L'échantillon étudié est composé d'un grand nombre d'anneaux d'argent mésoscopiques déposés sur une membrane en silicium suspendue. Nous avons réalisé, à différentes températures, de multiples balayages en champ de la chaleur spécifique. Un traitement des signaux obtenus a ensuite été réalisé afin de repérer une éventuelle périodicité. Il a révélé la présence sur le signal de phase d'une oscillation de période h/2e n'apparaissant pas sur le module de la chaleur spécifique. Cette signature est maximale à 100mK. La fréquence et l'amplitude obtenues sont en accord avec les prédictions théoriques. La sensibilité atteinte est de ΔC~10^(-14)J/K sur le module (et de Δφ~10^(-2) degrés sur la phase) soit 10^(-21)J/K par anneau. Les résultats présentés ici apportent donc de nouveaux éléments dans l'analyse des courants permanents. / In this thesis, we report very high resolution specific heat measurements of normal metal mesoscopic silver rings at very low temperatures. The objective of this experiment is to measure the possible existence of thermal signatures due to the presence of persistent currents. This phenomenon is still misunderstood despite many measurements. Some contradictions exist between experimental results and the different theories. The thermal approach is a new point of view. Under magnetic field, the Cp variation is expected be periodic with the quantum of flux Φ0=h/e or half of the quantum of flux as it has been theoretically predicted. The studied samples is composed by a large number of silver rings with an electronic phase coherence length of few microns at low temperatures. They were deposited on the suspended membrane of a silicon sensor. We have realized, at different temperatures, a large number of identical scans of the heat capacity variation as a function of the applied field. A signal processing work on this data has then been realized to detect signal periodicities. It showed a difference between the module and the phase of the heat capacity. A peak can be observed on the FFT at a frequency corresponding to h/2e on the phase signal. The peak appears particularly at 100mK with a amplitude which is consistent with previous calculations. A sensitivity of ΔC~10^(-14)J/K on the module (Δφ~10^(-2) degrees on the phase) has been reached whether 10^(-21)J/K per ring. Thus, the presented experiences give new elements in the field of persistent currents.

Cette thèse est consacrée à l'étude du transport dans les nanostructures quantiques unidimensionnelles dont les propriétés sont étudiées en s'appuyant notamment sur le bruit en excès à fréquence finie. La première partie de cette thèse est consacrée à l'étude du transport à travers une impureté dans un liquide de Luttinger couplée à un environnement électromagnétique arbitraire. L'impureté est traitée dans deux limites de transmission, la limite tunnel et la limite de faible rétrodiffusion. Les calculs sont menés dans le formalisme de Keldysh. Nous montrons ainsi que la théorie du blocage de Coulomb dynamique, établie pour une jonction tunnel couplée à un environnement à l'équilibre, demeure valide pour un liquide de Luttinger. Par ailleurs nous montrons que les relations de fluctuation dissipation reliant le bruit à fréquence finie au courant reste valide. Nous montrons que cette théorie peut également s'étendre dans la limite de faible rétrodiffusion à condition de prendre en compte la rétro-action du liquide électronique sur l'environnement. En revanche, les relations de fluctuation dissipation ne sont respectées que pour le bruit en émission. Dans une seconde partie nous intéressons effets d'une modulation radiofréquence sur les propriétés de transport des mêmes systèmes. Nous montrons notamment que ces effets peuvent être décrit par une théorie du blocage de Coulomb dynamique effective en convoluant la statistique d'absorption de photon avec la statistique de Tien-Gordon. Notons cependant que les relations de fluctuation dissipation ne sont plus vérifiées. Ces prédictions théoriques sont comparées aux résultats expériments obtenus par une équipe du SPEC au CEA de Saclay. Enfin nous étudions les propriétés de transport lorsque l'environnement, ici un oscillateur harmonique, est maintenu dans un état excité. Nous montrons que la présence de photons autorise d'une part le processus photo-assistés mais favorise également l'absorption de photons par des processus de bunching. Nous montrons finalement que les propriétés du transport s'obtiennent en convoluant la loi de Poisson du blocage de Coulomb avec la fonction caractéristique de Glauber de l'état peuplant l'oscillateur, menant à des statistiques exotiques. Ce dernier point nous permet d'utiliser ce système comme un détecteur d'état quantique. / This thesis discusses electronic transport in uni-dimensional quantum systems whose properties are studied with an extensive use of the finite-frequency non symmetrised excess noise. The first part focuses on transport through an impurity embedded in a Luttinger liquid coupled to an arbitrary electromagnetic environment. The impurity is treated in two paradigmatic situations : The tunneling and the weak backscattering regime. The out-of-equilibrium situation is dealt with the Keldysh Formalism. We show that the dynamical Coulomb blockade theory, extends to the case of a a tunnel junction between Luttinger liquids. Besides, fluctuations dissipation relations that link noise noise and current remain valid. In the transparent regime, we show that the dynamical Coulomb blockade theory applies to the backscattering current albeit back-action effects of the electronic liquid on the electromagnetic environment that have to be taken into account. Fluctuation-dissipation relations remain valid only for the emission noise. The second part focuses on the effects of a micro-wave modulation on the transport properties of the transport properties of these systems. An effective dynamical Coulomb blockade can be obtained by convolving the statistic of absorption of the environment with the Tien-Gordon statistic. Yet, the fluctuation dissipation relations are not verified in this case. These predictions are compared to the experimental results obtained by a team of the SPEC at the CEA Saclay. Last, we study the transport properties of a tunnel junction coupled to a harmonic oscillator maintained in an excited state. We show that the photons within the cavity lead to two distinct processes: photo-assisted transport that enhance the conductance, and bunching effects that enhance the probability to absorbe a large number of photons. An effective dynamical Coulomb blockade theory can also be derived by convolving the Poisson distribution with Glauber characteristic function leading to exotic statistics. These can be probed by excess noise which can thus be used as a quantum state detector.

Hu, Jianliang. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (leaves 88-105). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Orientador: Guillermo Gerardo Cabrera Oyarzun / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-09-24T19:11:22Z (GMT). No. of bitstreams: 1 Dartora_CesarAugusto_D.pdf: 2101604 bytes, checksum: 3eb6940416ec56ede441909468db04be (MD5) Previous issue date: 2005 / Resumo: O interesse atual e crescente nos sistemas mesoscópicos se deve à miniaturização cada vez maior dos dispositivos eletrônicos e à produção de materiais com possibilidade de armazenar informação em altas densidades (Gbits e Terabits/pol 2 ). A Física Mesoscópica descreve fenômenos que ocorrem em uma escala de tamanhos intermediária entre o macroscópico e o microscópico. Esta região cinzenta permite interpolar entre o regime atômico-molecular e o limite macroscópico, dominado este último pelas propriedades de volume (bulk ), que são objetos usuais de estudo em Física da Matéria Condensada. Na escala de nanometros e dezenas de nanometros, os elétrons podem propagar-se sem sofrer espalhamento inelástico (regime balístico) e a fase da função de onda pode manter sua coerência em escala da ordem do tamanho do sistema, dando lugar aos típicos fenômenos de interferência quântica. Neste trabalho fazemos um estudo detalhado das propriedades de transporte quântico em sistemas mesoscópicos, onde as barreiras de tunelamento fazem parte de diversos dispositivos eletrônicos. Estes sistemas incluem barreiras isolantes entre eletrodos metálicos, nanocontatos metálicos e junções tipo Josephson entre supercondutores. As principais estruturas aqui estudadas são as junções magnéticas de tunelamento e os nanofios e nanocontatos ferromagnéticos. Em ambos o fenômeno da magnetorresistência gigante (GMR) está presente, porém as origens do fenômeno são diferentes. Em junções de tunelamento a GMR tem origem na densidade de estados dos elétrons de condução nos eletrôdos ferromagnéticos, entre os quais uma barreira isolante é colocada, bem como no tunelamento inelástico assistido por mágnons que surgem nas interfaces entre eletrodos e região isolante. Em nanocontatos e nanofios o fenômeno deve-se principalmente ao forte espalhamento de elétrons com dependência de spin na presença de paredes de domínio magnéticas / Abstract: The interest in mesoscopic systems has grown significantly due to the increasing miniaturization of electronic devices and the production of materials which makes possible to store information in higher densities (Gbits and Terabits/in 2 ). The Mesoscopic Physics describes phenomena that happen in an intermediary scale of sizes between the macroscopic and the microscopic world. This gray region allows to interpolate between the atomic-molecular regime and the macroscopic limit, the last one dominated by bulk properties which are the usual subject of Condensed Matter Physics. In the nanometer and tens of nanometers scale electrons can pro-pagate without suffering inelastic scattering (ballistic regime) and the phase of the wavefunction maintain its coherence in the scale of system¿s size, giving place to the typical phenomena of quantum interference. In this work a detailed study of quantum transport properties in mesoscopic systems, where the tunnelling barriers make part of many electronic devices, is done. These systems include insulating barriers between metallic electrodes, metallic nanocontacts and nanowires, and Josephson junctions between superconductors. The main structures here studied are magnetic tunnelling junctions and ferromag-netic nanowires and nanocontacts. In both cases the giant magnetoresistance phe-nomenon (GMR) is present, however the origins of it are quite different. In tun-neling junctions, where an insulating barrier is placed between two ferromagnetic electrodes, the GMR is due to both, density of states effects at the ferromagnetic elec-trodes, and inelastic tunneling from magnons at the interface regions. In nanowires and nanocontacts the transport is strongly in uenced by spin-dependent scattering in the presence of magnetic domain walls / Doutorado / Física da Matéria Condensada / Doutor em Ciências

The recently developed Shear-Force Acoustic Near-Field Microscope (SANM) is used to investigate the viscoelastic properties of a mesoscopic fluid layer confined between two trapping boundaries, one being a stationary substrate and the other the apex of a laterally oscillating tapered probe. Hardware improvements and evaluation of the SANM-probe robustness will be a major focus of this thesis. The investigation first discusses characterization and recent developments made to the microscope, including: modifications to the sensor head, conditioning of the Nano positioners electrical drive signal, and the assessment of the probe against eventual plastic deformation or compliance against interactions with samples (the latter comprising a solid substrate and its adhered fluid layer which is typically a few monolayers thick). Furthermore, this study includes an analysis of the adsorbed mesoscopic fluid's viscoelastic properties. This inquiry aims to better understand probe-sample interactions with the mesoscopic fluid. This includes adhesion, wetting, and to inquire the nature of the hydrophobic interaction, which is relevant in many areas of study such a protein folding, and interfacial friction which has wide ranging applications including desalination. This analysis will be performed using a Sheer force microscopy (implemented with quartz tuning fork QTF), and another recently introduced technique Whispering Gallery Acoustic Sensor (WGAS). The latter allows more direct monitoring of the QTF's mechanical displacement. These measurements will be supplemented by simultaneously monitoring the acoustic emission from the mesoscopic fluid under confinement between the probe and the substrate, which will be monitored using the SANM sensor positioned beneath the substrate.