Dissertations / Theses on the topic 'Impurity models'
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Glossop, Matthew T. "Theoretical studies of Anderson impurity models." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.365734.
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Dickens, Nigel L. "Quantum impurity models : a local moment approach." Thesis, University of Oxford, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.270010.
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Simpson, James. "Theoretical studies of Jahn-Teller impurity ion complexes in III-V semiconductors." Thesis, University of Nottingham, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.329837.
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Holzner, Andreas Michael. "DMRG studies of Chebyshev-expanded spectral functions and quantum impurity models." Diss., lmu, 2012. http://nbn-resolving.de/urn:nbn:de:bvb:19-139333.
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Sandven, Håkon. "Evaluation of Distribution Function Models for ICRH-induced Impurity Transport in Tokamaks." Thesis, KTH, Skolan för elektro- och systemteknik (EES), 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-192460.
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Fusion power is the utilization of the energy released in nuclear fusion reactions. It has the potential to become an energy source which is more sustainable, safer and cleaner than the primary energy sources today. The fundamental problem for fusion power is energy confinement. Impurity ions are a major source of energy confinement loss in magnetic confinement fusion devices. Hence, impurity transport in fusion plasma is an important field to study. Ion cyclotron resonance heating (ICRH) has been shown both experimentally and theoretically to influence impurity transport in tokamaks. A poloidal asymmetry in the minority ions produces an electric potential, which causes impurity accumulation on the inboard side of the tokamak. Poloidal asymmetry in the impurity density induces a net radial impurity flux over a flux surface. This project has compared the ICRH-induced impurity transport for four approximate minority ion distribution function models with numerical results from the SELFO code. This has been done computationally for JET-like, concentric tokamak geometry with deuterium plasma, hydrogen minority ions, and tungsten impurities. Two models, the so-called bi-Maxwellian and LFS bi-Maxwellian model, are used in existing literature. Two further models are introduced, called the tri-Maxwellian and LFS tri-Maxwellian model. Unlike the bi-Maxwellian models, these models take into account the existence of thermal and fast ions in the minority population. The results show that there are noticeable differences between the different models, in particular when the resonant surface is on the inboard side. The tri-Maxwellian models offer a clear improvement over the bi-Maxwellian models compared with SELFO. However, there are some features in the SELFO results that none of the approximate models predict, this is because the models neglects wide orbits. A possible barrier in the radial transport has also been identified at flux surfaces where the impurity density asymmetry closely resembles the magnetic field strength asymmetry. The LFS bi-Maxwellian model predicts the radial position of the barrier most accurately and reliably compared with SELFO.Fusionsenergi är utnyttjandet av energi som frigörs i kärnfusionsreaktioner, och har potential för att bli en energikälla som är mer hållbar, säkrare och renare än de primära energikällorna idag. Det grundläggande problemet för fusionskraft är energiinneslutning. Förorenande joner är en viktig källa för förlust av energiinneslutning i fusionsanläggningar med magnetisk inneslutning. Därför är föroreningstransport i fusionsplasma ett viktigt ämnesområde. Joncyklotronresonansupphettning (ICRH) har visats både experimentellt och teoretiskt att påverka föroreningstransport i tokamaker. En poloidal asymmetri i minoritetsjonerna ger en elektrisk potential, som orsakar förorening samlas på den inre sidan av fusionsanläggningen. Poloidal asymmetri i föroreningsdensiteten på ett fluxyta inducerar en netto radialflux över fluxytan. Detta projekt har jämfört ICRH-inducerad föroreningstransport för fyra approximativa distributionsfunktionsmodeller för minoritetsjon med numeriska resultat från SELFO koden. Detta har gjorts med beräkningar för JET-liknande, kon-centrisk tokamak-geometri med deuterium-plasma, väte som minoritetsjoner, och volfram som föroreningsjoner. Två modeller, s.k. bi-Maxwellian- och LFS bi-Maxwellianmodellen, används i existerande litteratur. Ytterligare två modeller introduceras, kallad tri-Maxwellian- och LFS tri-Maxwellianmodellen. Dessa modeller tar hänsyn till förekomsten av termiska och snabba joner i minoritetsbefolkningen. Resultaten visar att det finns tydliga skillnader mellan de olika modellerna, särskilt när resonansytan är på den inre sidan. Tri-Maxwellianmodellerna visar en klar förbättring över bi-Maxwellianmodellerna jämfört med SELFO. Det finns dock vissa särdrag i resultaten från SELFO som ingen av de approximative modellerna förutsäger, eftersom modellerna försummar breda banor. En möjlig barriär i den radiella transporten har också blivit identifierat på fluxytor där asymmetrin i föroreningsdensiteten liknar asymmetrin i den magnetiska fältstyrkan. LFS bi-Maxwellianmodellen förutsäger den radiella positionen av barriären mest noggrant och tillförlitligt jämfört med SELFO.
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Sindel, Michael. "Numerical Renormalization Group studies of Quantum Impurity Models in the Strong Coupling Limit." Diss., lmu, 2005. http://nbn-resolving.de/urn:nbn:de:bvb:19-31150.
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Münder, Wolfgang. "Matrix product state clculations for one-dimensional quantum chains and quantum impurity models." Diss., lmu, 2011. http://nbn-resolving.de/urn:nbn:de:bvb:19-135224.
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Hanl, Markus Johannes. "Optical and transport properties of quantum impurity models - an NRG study of generic models and real physical systems." Diss., Ludwig-Maximilians-Universität München, 2014. http://nbn-resolving.de/urn:nbn:de:bvb:19-174608.
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This thesis contributes to the understanding of impurity models. It is divided into two main parts, with a general introduction given in Part I and the research related to it presented in Part II, with the second part being subdivided into two main projects.
In the first project, the influence of two many-body effects, the Kondo effect and the Fermi edge singularity, on the absorption and emission spectra of self-assembled quantum dots (QDs) is examined. Whereas the Kondo effect so far was always examined with transport experiments, we show that it has been observed with optical methods for the first time, by comparing experimental data for the absorption line shapes of QDs to calculations with the numerical renormalization group. We continue by examining a QD with strong optical coupling of the energy levels. The resulting interplay of Rabi-oscillations and Kondo effect leads to a new many-body state, a secondary, outer Kondo effect, with Kondo-like correlations between the spin-Kondo and the trion state. The last work regarding optics at QDs addresses the Fermi edge singularity. We show that for QDs this phenomenon can be described numerically on a quantitative level.
The second project concerns transport properties of impurity models. First, we present a comprehensive study of the Kondo effect for an InAs-nanowire QD, a system for which the Kondo effect was observed only a few years ago. The second study regarding transport concerns the Kondo effect in bulk metals with magnetic impurities. Although nowadays the Kondo effect is often studied with QDs, it was discovered for iron impurities in noble metals like gold and silver. However, it was unknown for a long time which exact realization of Kondo model describes these systems. We identify the model by comparing numerical calculations for the magnetoresistivity and the dephasing rate for different models to experimental results. The third work about transport concerns the phenomenon that for a fixed type of Kondo model quantities like the magnetoresistivity or the conductivity, respectively, can be scaled onto a universal curve for different parameters, when energies are rescaled with the the Kondo temperature $T_K$, since it is the only relevant low energy scale of the problem. For finite bandwidth, however, different definitions of $T_K$ (which coincide in the limit of infinite bandwidth) lead to different $T_K$-values. We show that with a very common definition of $T_K$, finite bandwidth, which is always present at numerical calculations, can deteriorate the universality of rescaled curves, and we offer an alternative definition of $T_K$ which ensures proper scaling. In the last study presented in this thesis we calculate the Fermi-liquid coefficients for fully screened multi-channel Kondo models. For temperatures below $T_K$, these models show Fermi-liquid behavior, and the impurity density of states and certain quantities which depend on it, like resistivity, show quadratic dependencies on parameters like temperature or magnetic field, described by the Fermi-liquid coefficients. We calculate these coefficients both analytically and numerically.
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De, Leo Lorenzo. "Non-Fermi liquid behavior in multi-orbital Anderson impurity models and possible relevance for strongly correlated lattice models." Doctoral thesis, SISSA, 2004. http://hdl.handle.net/20.500.11767/4016.
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Bidzhiev, Kemal. "Out-of-equilibrium dynamics in a quantum impurity model." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS352/document.
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Le domaine des problèmes quantiques à N-corps à l'équilibre et hors d'équilibre sont des sujets majeurs de la Physique et de la Physique de la matière condensée en particulier. Les propriétés d'équilibre de nombreux systèmes unidimensionnels en interaction sont bien comprises d'un point de vue théorique, des chaînes de spins aux théories quantiques des champs dans le continue. Ces progrès ont été rendus possibles par le développement de nombreuses techniques puissantes, comme, par exemple, l'ansatz de Bethe, le groupe de renormalisation, la bosonisation, les états produits de matrices ou la théorie des champs invariante conforme. Même si les propriétés à l'équilibre de nombreux modèles soient connues, ceci n'est en général pas suffisant pour décrire leurs comportements hors d'équilibre, et ces derniers restent moins explorés et beaucoup moins bien compris. Les modèles d'impuretés quantiques représentent certains des modèles à N-corps les plus simples. Mais malgré leur apparente simplicité ils peuvent capturer plusieurs phénomènes expérimentaux importants, de l'effet Kondo dans les métaux aux propriétés de transports dans les nanostructures, comme les points quantiques. Dans ce travail nous considérons un modèle d'impureté appelé "modèle de niveau résonnant en interaction" (IRLM). Ce modèle décrit des fermions sans spin se propageant dans deux fils semi-infinis qui sont couplés à un niveau résonant -- appelé point ou impureté quantique -- via un terme de saut et une répulsion Coulombienne. Nous nous intéressons aux situations hors d'équilibre où un courant de particules s'écoule à travers le point quantique, et étudions les propriétés de transport telles que le courant stationnaire (en fonction du voltage), la conductance différentielle, le courant réfléchi, le bruit du courant ou encore l'entropie d'intrication. Nous réalisons des simulations numériques de la dynamique du modèle avec la méthode du groupe de renormalisation de la matrice densité dépendent du temps (tDMRG), qui est basée sur une description des fonctions d'onde en terme d'états produits de matrices. Nous obtenons des résultats de grande précision concernant les courbes courant-voltage ou bruit-voltage de l'IRLM, dans un grand domaine de paramètres du modèle (voltage, force de l'interaction, amplitude de saut vers le dot, etc.). Ces résultats numériques sont analysés à la lumière de résultats exacts de théorie des champs hors d'équilibre qui ont été obtenus pour un modèle similaire à l'IRLM, le modèle de Sine-Gordon avec bord (BSG). Cette analyse est en particulier basée sur l'identification d'une échelle d'énergie Kondo et d'exposants décrivant les régimes de petit et grand voltage. Aux deux points particuliers où les modèles sont connus comme étant équivalents, nos résultats sont en accord parfait avec la solution exacte. En dehors de ces deux points particuliers nous trouvons que les courbes de transport de l'IRLM et du modèle BSG demeurent très proches, ce qui était inattendu et qui reste dans une certaine mesure inexpliquéThe fields of in- and out-of-equilibrium quantum many-body systems are major topics in Physics, and in condensed-matter Physics in particular. The equilibrium properties of one-dimensional problems are well studied and understood theoretically for a vast amount of interacting models, from lattice spin chains to quantum fields in a continuum. This progress was allowed by the development of diverse powerful techniques, for instance, Bethe ansatz, renormalization group, bosonization, matrix product states and conformal field theory. Although the equilibrium characteristics of many models are known, this is in general not enough to describe their non-equilibrium behaviors, the latter often remain less explored and much less understood. Quantum impurity models represent some of the simplest many-body problems. But despite their apparent simplicity, they can capture several important experimental phenomena, from the Kondo effect in metals to transport in nanostructures such as point contacts or quantum dots. In this thesis consider a classic impurity model - the interacting resonant level model (IRLM). The model describes spinless fermions in two semi-infinite leads that are coupled to a resonant level -- called quantum dot or impurity -- via weak tunneling and Coulomb repulsion. We are interested in out-of-equilibrium situations where some particle current flows through the dot, and study transport characteristics like the steady current (versus voltage), differential conductance, backscattered current, current noise or the entanglement entropy. We perform extensive state-of-the-art computer simulations of model dynamics with the time-dependent density renormalization group method (tDMRG) which is based on a matrix product state description of the wave functions. We obtain highly accurate results concerning the current-voltage and noise-voltage curves of the IRLM in a wide range parameter of the model (voltage bias, interaction strength, tunneling amplitude to the dot, etc.).These numerical results are analyzed in the light of some exact out-of-equilibrium field-theory results that have been obtained for a model similar to the IRLM, the boundary sine-Gordon model (BSG).This analysis is in particular based on identifying an emerging Kondo energy scale and relevant exponents describing the high- and low- voltage regimes. At the two specific points where the models are known to be equivalent our results agree perfectly with the exact solution. Away from these two points, we find that, within the precision of our simulations, the transport curves of the IRLM and BSG remain very similar, which was not expected and which remains somewhat unexplained
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Schwarz, Frauke [Verfasser], and Jan von [Akademischer Betreuer] Delft. "Nonequilibrium steady-state transport in quantum impurity models / Frauke Schwarz ; Betreuer: Jan von Delft." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2017. http://d-nb.info/1169572251/34.
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Holzner, Andreas Michael [Verfasser], and Jan von [Akademischer Betreuer] Delft. "DMRG studies of Chebyshev-expanded spectral functions and quantum impurity models / Andreas Michael Holzner. Betreuer: Jan von Delft." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2012. http://d-nb.info/1019479299/34.
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Krones, Jörn [Verfasser], Götz Silvester [Akademischer Betreuer] Uhrig, and Frithjof [Gutachter] Anders. "Effective models for the single impurity Anderson and Kondo model from continuous unitary transformations / Jörn Krones. Betreuer: Götz Silvester Uhrig. Gutachter: Frithjof Anders." Dortmund : Universitätsbibliothek Dortmund, 2014. http://d-nb.info/1101476389/34.
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Hanl, Markus Johannes [Verfasser], and Jan von [Akademischer Betreuer] Delft. "Optical and transport properties of quantum impurity models - an NRG study of generic models and real physical systems / Markus Johannes Hanl. Betreuer: Jan von Delft." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2014. http://d-nb.info/1059351544/34.
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Münder, Wolfgang [Verfasser], and Jan von [Akademischer Betreuer] Delft. "Matrix product state clculations for one-dimensional quantum chains and quantum impurity models / Wolfgang Münder. Betreuer: Jan von Delft." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2011. http://d-nb.info/1015925227/34.
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Basirat, Farzad. "Process Models for CO2 Migration and Leakage : Gas Transport, Pore-Scale Displacement and Effects of Impurities." Doctoral thesis, Uppsala universitet, Luft-, vatten och landskapslära, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-315490.
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Geological Carbon Storage (GCS) is considered as one of the key techniques to reduce the rate of atmospheric emissions of CO2 and thereby to contribute to controlling the global warming. A successful application of a GCS project requires the capability of the formation to trap CO2 for a long term. In this context, processes related to CO2 trapping and also possible leakage of CO2 to the near surface environment need to be understood. The overall aim of this thesis is to understand the flow and transport of CO2 through porous media in the context of geological storage of CO2. The entire range of scales, including the pore scale, the laboratory scale, the field experiment scale and the industrial scale of CO2 injection operation are addressed, and some of the key processes investigated by means of experiments and modeling. First, a numerical model and laboratory experimental setup were developed to investigate the CO2 gas flow, mimicking the system in the near-surface conditions in case a leak from the storage formation should occur. The system specifically addressed the coupled flow and mass transport of gaseous CO2 both in the porous domain as well as the free flow domain above it. The comparison of experiments and modelling results showed a very good agreement indicating that the model developed can be applied to evaluate monitoring and surface detection of potential CO2 leakage. Second, the field scale CO2 injection test carried out in a shallow aquifer in Maguelone, France was analyzed and modeled. The results showed that Monte Carlo simulations accounting for the heterogeneity effects of the permeability field did capture the key observations of the monitoring data, while a homogeneous model could not represent them. Third, a numerical model based on phase-field method was developed and model simulations carried out addressing the effect of wettability on CO2-brine displacement at the pore-scale. The results show that strongly water-wet reservoirs provide a better potential for the dissolution trapping, due to the increase of interface between CO2 and brine with very low contact angles. The results further showed that strong water-wet conditions also imply a strong capillary effect, which is important for residual trapping of CO2. Finally, numerical model development and model simulations were carried out to address the large scale geological storage of CO2 in the presence of impurity gases in the CO2 rich phase. The results showed that impurity gases N2 and CH4 affected the spatial distribution of the gas (the supercritical CO2 rich phase), and a larger volume of reservoir is needed in comparison to the pure CO2 injection scenario. In addition, the solubility trapping significantly increased in the presence of N2 and CH4.
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Dutreix, Clément. "Impurity and boundary modes in the honeycomb lattice." Thesis, Paris 11, 2014. http://www.theses.fr/2014PA112217/document.
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La présente thèse s’articule autour de deux sujets. Le premier concerne la localisation des électrons en présence d’impuretés ou d’interfaces dans le réseau hexagonal. Le deuxième, en revanche, traite de l’accumulation de spin dans un supraconducteur hors-Équilibre de type s.Le graphène est la principale motivation de la première partie. Ce matériau bidimensionnel consiste en un feuillet d’atomes de carbones et peut être décrit comme un réseau hexagonal, c’est-à-dire un réseau de Bravais triangulaire avec un motif diatomique. La structure de bande électronique révèle alors l’existence d’électrons de Dirac sans masse et chiraux à basse énergie.D’une part, il est possible d’annihiler ces fermions chiraux en étirant de façon uni-Axiale le matériau. Pour une valeur seuil de l’étirement, les électrons deviennent massiques et non-Relativistes, ce qui définit une transition de phase dite de Lifshitz. Afin de caractériser cette transition, nous étudions la diffusion des électrons sur des impuretés en fonction de l’étirement. Une impureté localisée induit des interférences quantiques dans la densité électronique, connues sous le nom d’oscillations de Friedel. Etant sensibles à la nature chirale des électrons, nous montrons que ces oscillations décroissent selon des lois de puissances qui permettent de caractériser chacune des phases de la transition. La même étude est réalisée dans le cas limite où le diffuseur est une lacune.D’autre part, le motif diatomique du réseau hexagonal propose aussi une incursion dans le monde des isolants et supraconducteurs topologiques. Pour ces systèmes, la caractérisation topologique de la structure de bande électronique permet de prédire l’existence d’états de bord aux interfaces. Nous développons notamment un modèle de supraconducteur topologique basé sur le réseau hexagonal du graphène, en présence de supraconductivité de type singulet (s ou d). Lorsque la symétrie par renversement du temps est brisée par un champ Zeeman, et en présence de couplage spin-Orbit Rashba, nous donnons une prescription qui permet de caractériser les différentes phases topologiques possibles et de prédire l’apparition d’états de bord (états de Majorana) dans des nano-Rubans de graphène.La seconde partie discute l’accumulation de spin dans un supraconducteur hors-Équilibre, joint à un ferromagnétique. Lorsqu’il est à l’équilibre, le supraconducteur est composé de quasiparticules et d’un condensat. L’injection de particules polarisées en charge et en spin, à savoir des électrons polarisés en spin, induit une accumulation de spin et de charge à l’intérieur du supraconducteur. Si l’injection cesse, les populations de spin et de charge vont relaxer vers l’équilibre, mais pas nécessairement sur des échelles de temps identiques. Récemment, la réalisation d’une expérience a mis en évidence que le la charge pouvait relaxer bien plus rapidement que le spin. Afin de confirmer cet effet, une nouvelle expérience a été réalisée grâce à des mesures établies dans le domaine fréquentiel. Ici, nous adressons un model relatif à cette dernière expérience, dans le but d’extraire le temps caractéristique de relaxation du spin qui s’avère être de l’ordre de quelques nanosecondesTwo fields of research define the framework in which the present thesis can be apprehended. The first one deals with impurity and boundary modes in the hexagonal lattice. The second one concerns a spin accumulation in an out-Of-Equilibrium superconductor.Two fields of research define the framework in which the present thesis can be apprehended. The first one deals with impurity and boundary modes in the hexagonal lattice. The second one concerns a spin accumulation in an out-Of-Equilibrium superconductor.Graphene is the main motivation of the first part. From a crystallographic perspective, the carbon atoms in graphene, a graphite layer, design a triangular Bravais lattice with a diatomic pattern. This gives rise to an extra degree of freedom in the electronic band structure that crucially reveals chiral massless Dirac electrons at low-Energy. First of all, it is possible to make these chiral fermions annihilate when a uniaxial strain stretches the graphene layer. For a critical value of the strain, all the fermions become massive and nonrelativistic, which defines a Lifshitz transition. We study the impurity scattering as a function of the strain magnitude. A localised impurity yields quantum interferences in the local density of states that are known as Friedel oscillations. Because they are affected by the chiral nature of the electrons, we show that the decaying laws of these oscillations are specific to the phase the system belongs to. Thus, the impurity scattering offers the possibility to fully characterise the transition.Second, the diatomic pattern of the graphene lattice can also be considered as an invitation to the world of topological insulators and superconductors. The existence of edge states in such systems relies on the topological characterization of the band structure. Here we especially introduce a model of topological superconductor based on the honeycomb lattice with induces spin-Singlet superconductivity. When a Zeeman field breaks the time-Reversal invariance, and in the presence of Rashba spin-Orbit interactions, we give a prescription to describe the topological phases of the system and predict the emergence of Majorana modes (edge states) in strained and doped nanoribbons.The second part discusses the study of a spin accumulation in an out-Of-Equilibrium s-Wave superconductor. At the equilibrium, the superconductor is made of particles coupled by a s-Wave pairing, as well as unpaired quasiparticles. Injecting spin-Polarised electrons into the superconductor induces charge and spin imbalances. When the injection stops, it may happen that charge and spin do not relax over the same time-Scale. The first experiment that points out such a spin-Charge decoupling has recently been realised. In order to confirm this chargeless spin-Relaxation time, a new experiment has been developed [96], based on measurements in the frequency domain. Here, we address a model that fits the experimental data and thus enables the extraction of this characteristic time that is of the order of a few nanoseconds
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Ekeberg, Lukas, and Alexander Fahnehjelm. "Maskininlärning som verktyg för att extrahera information om attribut kring bostadsannonser i syfte att maximera försäljningspris." Thesis, KTH, Skolan för elektroteknik och datavetenskap (EECS), 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-240401.
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The Swedish real estate market has been digitalized over the past decade with the current practice being to post your real estate advertisement online. A question that has arisen is how a seller can optimize their public listing to maximize the selling premium. This paper analyzes the use of three machine learning methods to solve this problem: Linear Regression, Decision Tree Regressor and Random Forest Regressor. The aim is to retrieve information regarding how certain attributes contribute to the premium value. The dataset used contains apartments sold within the years of 2014-2018 in the Östermalm / Djurgården district in Stockholm, Sweden. The resulting models returned an R2-value of approx. 0.26 and Mean Absolute Error of approx. 0.06. While the models were not accurate regarding prediction of premium, information was still able to be extracted from the models. In conclusion, a high amount of views and a publication made in April provide the best conditions for an advertisement to reach a high selling premium. The seller should try to keep the amount of days since publication lower than 15.5 days and avoid publishing on a Tuesday.Den svenska bostadsmarknaden har blivit alltmer digitaliserad under det senaste årtiondet med nuvarande praxis att säljaren publicerar sin bostadsannons online. En fråga som uppstår är hur en säljare kan optimera sin annons för att maximera budpremie. Denna studie analyserar tre maskininlärningsmetoder för att lösa detta problem: Linear Regression, Decision Tree Regressor och Random Forest Regressor. Syftet är att utvinna information om de signifikanta attribut som påverkar budpremien. Det dataset som använts innehåller lägenheter som såldes under åren 2014-2018 i Stockholmsområdet Östermalm / Djurgården. Modellerna som togs fram uppnådde ett R²-värde på approximativt 0.26 och Mean Absolute Error på approximativt 0.06. Signifikant information kunde extraheras from modellerna trots att de inte var exakta i att förutspå budpremien. Sammanfattningsvis skapar ett stort antal visningar och en publicering i april de bästa förutsättningarna för att uppnå en hög budpremie. Säljaren ska försöka hålla antal dagar sedan publicering under 15.5 dagar och undvika att publicera på tisdagar.
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Derry, Philip. "Quasiparticle interference in strongly correlated electronic systems." Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:f487c821-dbbb-4ebe-8b05-c13807379c2c.
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We investigate the manifestation of strong electronic correlations in the quasiparticle interference (QPI), arising from the scattering of conduction electrons from defects and impurities in an otherwise translationally-invariant host. The QPI may be measured experimentally as the Fourier transform of the spatial modulations in the host surface density of states that result, which are mapped using a scanning tunnelling microscope. We calculate the QPI for a range of physically relevant models, demonstrating the effect of strong local electronic correlations arising in systems of magnetic impurities adsorbed on the surface of non-interacting host systems. In the first instance the effect of these magnetic impurities is modelled via the single Anderson impurity model, treated via numerical renormalization group (NRG) calculations. The scattering of conduction electrons, and hence the QPI, demonstrate an array of characteristic signatures of the many-body state formed by the impurity, for example due to the Kondo effect. The effect of multiple impurities on the QPI is also investigated, with a numerically-exact treatment of the system of two Anderson impurities via state-of-the-art NRG calculations. Inter-impurity interactions are found to result in additional scattering channels and additional features in the QPI. The QPI is then investigated for the layered transition metal oxide Sr2RuO4, for which strong interactions in the host conduction electrons give rise to an unconventional triplet superconducting state at Tc ∼ 1.5K. The detailed mechanism for this superconductivity is still unknown, but electron-electron or electron-phonon interactions are believed to play a central role. We simulate the QPI in Sr2RuO4, employing an effective parametrized model consisting of three conduction bands derived from the Ru 4d t2g orbitals that takes into account spin orbit coupling and the anisotropy of the Ru t2g orbitals. Signatures of such interactions in the normal state are investigated by comparing these model calculations to experimental results. We also calculate the QPI in the superconducting state, and propose how experimental measurements may provide direct evidence of the anisotropy and symmetry of the superconducting gap, and thus offer insight into the pairing mechanism and the superconducting state.
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Kim, Tae-Suk. "Various Kondo Effects for a Model Cerium Impurity in Normal Metals /." The Ohio State University, 1995. http://rave.ohiolink.edu/etdc/view?acc_num=osu148793030468492.
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Fiebig, Anke. "Prediction of crystal morphology for a limited range of impurity concentrations." Aachen Shaker, 2007. http://d-nb.info/988385546/04.
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Motahari, Sareh [Verfasser]. "Kondo physics and thermodynamics of the Anderson impurity model by distributional exact diagonalization / Sareh Motahari." Halle, 2017. http://d-nb.info/1147380651/34.
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Rosa, Priscila Ferrari Silveira 1988. "Dinâmica quântica de estados de impurezas em cadeias de spin." [s.n.], 2010. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277877.
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Orientador: Guillermo Gerardo Cabrera OyarzúnDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física "Gleb Wataghin"
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Resumo: A descrição da dinâmica quântica de sistemas de muitos corpos é um ingrediente chave para a computação quântica. No presente projeto, propomos o estudo da dinâmica em cadeias finitas de spin 1/2 na presença de impurezas ou defeitos. O modelo adotado será o de Ising quântico com campo transverso, do qual é possível obter o espectro de forma exata na presença de uma impureza. A dinâmica do sistema é movida exclusivamente por flutuações quânticas, cuja origem é o Princípio da Incerteza. Investigamos a relaxação de estados iniciais caracterizados por uma magnetização espacialmente não homogênea e que não possuam hipóteses sobre a proximidade com o estado de equilíbrio. Dessa forma, a matriz densidade inicial será dependente apenas de uma única coordenada espacial. A investigação então é realizada através da grandeza de interesse, a saber, a evolução temporal do valor médio das componentes de Fourier da magnetização, (SzQ)t. Soluções exatas, tanto analíticas quanto numéricas, são obtidas. Um dos objetivos iniciais deste trabalho consiste na busca de processos de relaxação lentos. Para os casos de solução analítica (impurezas periódica e antiperiódica) observamos relaxações oscilatórias e amortecidas por uma lei de potência no tempo do tipo (t/tQ)-vQ, onde tQ e vQ são dois parâmetros livres e Q é o número de onda associado a cada componente de Fourier. Há uma criticalidade no expoente vQ, o qual muda de 3/2 para 1/2 para certos valores de Q críticos. Por outro lado, para os casos de solução numérica (impurezas arbitrárias), os processos de relaxação são distintos daqueles citados acima. Os estados iniciais analisados são do tipo produto direto ferromagnético com um único spin virado, próximo ou distante da impureza. Neste caso, as evoluções temporais oscilam em torno deum valor médio não nulo e há uma larga faixa de valores de Q na qual os modos não se extinguem completamente
Abstract: The description of many body systems quantum dynamics is a key ingredient for quantum computation. In the present project we study finite spin-1/2 chains dynamic properties in the presence of impurities or defects. We adopt the quantum Ising model with transverse field, of which it is possible to obtain the energy spectrum by exact calculations in the presence of one impurity. The system dynamics is driven exclusively by quantum fluctuations, whose origin is the Uncertainty Principle. We investigate the relaxation of initial states characterized by spatially inhomogeneous magnetization without any hypothesis about the proximity with the equilibrium state. Thus, the initial density matrix will be dependent of only one spatial coordinate. The investigation then is realized through the temporal evolution of the magnetization's Fourier components. Exact solutions, analytical and numerical, are obtained. One of the goals of this work consist in the search of slow relaxation processes. For the analytical cases (periodic and anti-periodic impurities) we observe oscillatory relaxations with a decay given by a power law in time (t/tQ)-vQ, where tQ and vQ are two free parameters and Q is the wave number associated to a Fourier component. There is a criticality in the exponent vQ: its value changes from 3/2 to 1/2 for certain critical values of Q. On the other hand, for the numerical cases (arbitrary impurities), the relaxation processes are distinct from the cases cited above. The initial state analyzed is a ferromagnetic direct product with only one flipped spin, near or far from the impurity. In this case, the temporal evolutions oscillate around a finite mean value and there is a large interval of Q values in which the modes do not extinguish completely
Mestrado
Física da Matéria Condensada
Mestra em Física
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Freire, MÃrcio de Melo. "Teoria de funÃÃes de Green para uma impureza isolada localizada intersticialmente em sistemas ferromagnÃticos." Universidade Federal do CearÃ, 2017. http://www.teses.ufc.br/tde_busca/arquivo.php?codArquivo=18807.
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Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgicoUm formalismo da funÃÃo de Green à usado para calcular o espectro de excitaÃÃes associadas com uma impureza magnÃtica localizada intersticialmente em diferentes estruturas ferromagnÃticas descritas pelo modelo de Ising e de Heisenberg. No capÃtulo 3, descrevemos um ferromagneto de rede cÃbica simples semi-infinita atravÃs do modelo de Ising. Neste caso, as excitaÃÃes nÃo-ressonantes (isto Ã, os modos de defeito fora da regiÃo das ondas de spin de volume e de superfÃcie) e as excitaÃÃes ressonantes (os modos de defeito dentro da regiÃo das ondas de spin de volume) sÃo calculadas numericamente para a fase de alta-temperatura. Duas situaÃÃes sÃo analisadas, dependendo da posiÃÃo da impureza em relaÃÃo a seus vizinhos: a impureza està na superfÃcie; a impureza està na regiÃo de volume. Nos demais capÃtulos, usamos o modelo de Heisenberg/Ising (onde passamos do modelo de Heisenberg para o de Ising atravÃs do controle de um parÃmetro) para descrever os seguintes sistemas: ferromagneto de rede quadrada infinita (capÃtulo 4), ferromagneto de rede quadrada centrada infinita (capÃtulo 5), ferromagneto de rede cÃbica de corpo centrado infinita (capÃtulo 6) e rede favo de mel infinita (capÃtulo 7), todos contendo uma impureza magnÃtica localizada intersticialmente. Nos trÃs primeiros casos, sÃo calculados apenas os modos de defeito acima da banda de volume do material puro (modos Ãpticos). No capÃtulo 7, sÃo analisados apenas os modos de defeito abaixo da banda de volume do material puro (modos acÃsticos).
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Kinza, Michael Wolfgang [Verfasser]. "Single impurity Anderson model and dynamical mean field theory : a functional renormalization group study / Michael Wolfgang Kinza." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2014. http://d-nb.info/105154369X/34.
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Rocha, Francisco Manoel Bezerra e. "O método do grupo de renormalização de teoria de campos aplicado ao modelo de Anderson de uma impureza." Universidade Federal de Goiás, 2012. http://repositorio.bc.ufg.br/tede/handle/tde/2906.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
We apply the perturbative eld-theoretical renormalization group (RG) implemented within an approach which considers the calculation for the e ective couplings up to one loop and the computation of the self-energy up to two loops of the single-impurity Anderson model with particle-hole symmetry. To this end, we follow Feynman's diagrammatic method applied to the model and we begin our analysis by calculating the so-called vertex corrections up to one loop. The e ect of correlations on the single-particle excitations is viewed most clearly by means of the computation of the self-energy and its closely-related quantity: the quasiparticle weight. Moreover, to determine the nature of the ground state of the model, we also perform the RG calculation of the so-called uniform spin susceptibility. Then we apply the RG technique, adapting it conveniently to our problem at hand. The next step consists of deriving analytically and solving numerically the coupled di erential RG ow equations for the e ective couplings, the quasiparticle weight and the uniform spin susceptibility. We show that our results agree qualitatively with other analytical works available in the literature, such as, e.g., the functional RG. To benchmark our method, we compare our results with Wilson's numerical RG data. This latter method provides highly accurate numerical results for the quantities analyzed here and, for this reason, it will be an important check for our analytical method. Since the eldtheoretical RG turns out to be a exible technique and also simpler to be implemented at higher orders if compared to some versions of the functional RG method, we argue here that the present methodology could potentially o er a possible alternative to other analytic RG methods to describe eletronic correlations within the single-impurity Anderson model.
Nesta dissertação, aplicamos o método do grupo de renormalização (GR) perturbativo construído a partir de uma abordagem mista que mescla um cálculo até um loop para os acoplamentos efetivos e um cálculo até dois loops para a auto-energia do modelo de Anderson de uma impureza com simetria partícula-buraco. Para isso, utilizamos o método diagramático de Feynman aplicado ao modelo e iniciamos nossa análise calculando todos os chamados diagramas de correção ao vértice até um loop. Os efeitos de correlação nas excitações de uma partícula são vistos mais claramente por meio do cálculo da auto-energia e de uma quantidade física diretamente relacionada que é o peso da quasipartícula. Além disso, para determinarmos a natureza do estado fundamental desse modelo, efetuamos também o cálculo de GR da chamada susceptibilidade uniforme de spin. Desenvolveremos, em seguida, a técnica do GR, adaptando-a convenientemente ao nosso problema de interesse. O próximo passo consistiu em derivar analiticamente e então resolver numericamente as equações diferenciais acopladas para os acoplamentos efetivos, o peso da quasipartículas e a susceptibilidade uniforme de spin. Mostramos que os nossos resultados concordam qualitativamente com outros trabalhos analíticos disponíveis na literatura como, por exemplo, o método do grupo de renormalizalização funcional. Procuramos também comparar nossos resultados com o chamado método do GR numérico de Wilson. Esse é o último método fornece resultados numéricos altamente precisos para as grandezas aqui calculadas em nosso modelo, de modo que essa técnica serviria como uma referência-padrão para nossa abordagem analítica. Nesse sentido, como o método do GR de teoria de campos se revela uma técnica flexível e mais simples de ser utilizada em cálculos perturbativos de ordens superiores se comparada com algumas implementações do método do grupo de renormalização funcional, vamos argumentar que a presente metodologia do GR pode oferecer uma abordagem analítica alternativa para descrever as correlações eletrônicas contidas no modelo de Anderson de uma impureza
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Chaves, Neto Antonio Maia de Jesus. "Modelo de Anderson para duas impurezas : metodo dos campos efetivos." [s.n.], 2004. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277076.
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Orientadores: Roberto Eugenio Lagos Monaco, Guillermo Gerardo Cabrera OyarzunTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: O Hamiltoniano do modelo de Anderson para duas impurezas é estudado via um desacoplamento das funções de Green. Neste caso resulta ser equivalente a aproximação de potencial coerente (CPA) que também, neste caso, coincide com a aproximação Hubbard-I. Consideram-se todos os termos de interação impureza-impureza, tanto os denominados termos de um corpo como os de dois corpos. Os parâmetros associados às interações mencionadas acima incluem: repulsão coulombiana intra e intersítio, hopping direto, hopping correlacionado e o termo de troca (exchange). Todos estes são modelados via orbitais atômicos de Slater, e neste caso se considera o modelo mais simples, o caso não degenerado, ou seja, um nível por impureza. Nesta modelagem, incluindo o metal hospedeiro, os parâmetros independentes resultam ser: a constante de hibridização eletrônica impureza-metal, o número de portadores do metal hospedeiro, o vetor de onda de Fermi associado ao mesmo, a largura dos orbitais atômicos das impurezas e a distância impureza-impureza. Para o caso particular de temperatura nula e no regime denominado de banda semicheia (a metade dos níveis das impurezas são preenchidos considerando os valores esperados) são calculadas as densidades espectrais (densidade de estados) associadas às impurezas, as funções de correlações de spin e carga, suscetibilidades magnéticas e de carga, e a energia de correlação associada as impurezas. Os resultados são discutidos no contexto dos modelos já a existentes na literatura, assim como os casos limites para os quais existem resultados exatos. Os resultados encontrados estão de acordo com os casos limites conhecidos e são interessantes. Mostram também a importância de se considerar todas as interações impureza-impureza, fato negligenciado até agora na literatura
Abstract: We study the two impurity Anderson Model Hamiltonian via a Greens function decoupling scheme. This case turns out to be equivalent to the Coherent Potential Approximation (CPA) and furthermore equivalent to the Hubbard-I approximation. We consider all one and two body impurity-impurity interactions. The parameters associated to the latter include: the intra and intersite Coulomb repulsion, direct (band) hopping, correlated hopping and the exchange term. All of the above are modeled via Slater atomic orbitals, and here we consider the simplest model, non degenerate single impurity level. Including the host metal the resulting independent parameters are: The impurity-metal hybridization constant, the metal host carrier density, the associated Fermi wavevector, the atomic orbital width and the impurity-impurity distance. For the zero temperature case and in the so called band half called regime (impurities levels half called, in the mean) we compute the impurities spectral densities (density of states), spin and charge correlation functions, their respective susceptibilities and the correlation energy. We discuss our results considering the existing literature as well as the exact results for particular limiting cases. Our results agree with the latter and also yield interesting consequences, among others: the importance of including all impurity-impurity interactions, hitherto not considered.
Doutorado
Física da Matéria Condensada
Mestre em Física
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Herazo, Warnes Jesus Maria 1982. "Transições de fases quânticas em sistemas bosônicos fortemente correlacionados." [s.n.], 2011. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277709.
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Orientador: Eduardo MirandaTese (doutorado) - Universidade Estadual de Campinas, Instituto de Física "Gleb Wataghin"
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Resumo: A questão da natureza das transições de fases de sistemas de redes de bósons tem se tornado cada vez mais urgente à vista da capacidade de carregamento de átomos ultrafrios em redes ópticas. Nesta tese, tentamos avançar este conhecimento através do estudo de 3 modelos básicos de redes de bósons interagentes. Inicialmente, determinamos o diagrama de fases c as propriedades físicas do modelo bosônico de impureza única de Anderson. Este modelo é interessante tanto em si mesmo quanto por causa de sua relação com outras abordagens teóricas tais como a teoria dinâmica de campo médio bosônica. Usamos como estratégia a inclusão de um pequeno campo externo acoplado ao parâmetro de ordem superfluido, que quebra a simetria global de calibre do modelo. Desta forma, foi possível estudar a transição de condensação de Base-Einstein através do critério de quebra espontânea de simetria global de calibre. Outras quantidades como a ocupação da impureza, o desvio padrão da ocupação e a susceptibilidade com respeito ao campo externo também foram calculadas, caracterizando a transição de fase do modelo. Alguns desses resultados foram comparados com aqueles já obtidos na literatura através do grupo de renormalização numérico. Encontramos bom acordo entre os dois métodos. O segundo estudo realizado nesta tese refere-se ao comportamento crítico do modelo de Bose-Hubbard desordenado através da chamada teoria de campo médio estocástica. O objeto central dessa teoria de campo médio é a distribuição de parâmetros de ordem P(?). Estudos numéricos estabelecem que perto da linha crítica que separa as fases superfluida e vidro de Base do modelo, essa distribuição exibe uma grande região com comportamento de lei de potência P(?) ~ ? ^-(1+ß_c), onde ß_c < 1. Usando esse comportamento como tentativa, obtivemos analiticamente tanto a fronteira de fases quanto o valor do expoente crítico da lei de potência ß_c , encontrando um razoável acordo com os resultados numéricos e avançando o entendimento da natureza da transição de fase específica ao modelo desordenado. Finalmente, o modelo de Bose-Hubbard desordenado para partículas de spin-1 foi estudado dentro da teoria de campo médio estocástica. As distribuições de probabilidade de várias quantidades físicas como o parâmetro de ordem superfluido, o desvio padrão da ocupação por sítio, a fração do condensado, o quadrado do operador de spin, bem como seus valores médios, foram determinados para as três fases do modelo, a saber, o superfluido polar, o isolante de Mott e o vidro de Bose. Uma completa caracterização das propriedades físicas dessas fases e das transições de fase entre elas foi estabelecida
Abstract: The question of the nature of phase transitions of systems of lattice bosons has become increasingly more pressing in view of the capability of loading ultracold atoms in opticallattices. In this thesis we try to advance this understanding through the study of 3 basic models of interacting lattice bosons. Initially, we determined the phase diagram and physical properties of the bosonic singleimpurity Anderson model. This model is interesting both in its own right and because of its relation to other theoretical approaches such as the bosonic dynamical field theory method. We used as strategy the inclusion of a small external field coupled to the superfluid order parameter, which breaks the global gauge symmetry of the model. Thus, it was possible to study the Base-Einstein condensation transition through the criterion of the onset of spontaneous broken global gauge symmetry. Other quantities such as the occupation of the impurity, the standard deviation of the occupation and the susceptibility with respect to the external! Field were calculated characterizing the phase transition in the model. Some of the results were compared with those already reported in the literature, obtained with tic numerical renormalization group. We found good agreement between the two methods. The second study carried out in this thesis concerned the critical behavior of the disordered Bose-Hubbard model within the so-called stochastic mean-field theory. The central object of this mean-field theory is the distribution of order parameters P(?). Numerical studies establish that near the critical line separating the superfluid and Bose glass phases of this model, this distribution shows a wide region of power-law behavior P(?) ~ ? ^-(1+ß_c), where ß_c < 1. Using this behavior as an Ansatz, we obtained analytically both the phase boundary and the value of the critical power-law exponent ß_c, finding a reasonably good agreement with the numerical results and thus shedding new light on the nature of this phase transition specific to disordered model. Finally, the disordered Bose-Hubbard model for spin-1 particles was studied within the stochastic mean-field theory. The probability distributions of various physical quantities, such as the superfluid order parameter, the standard deviation of the occupation per site, the condensate fraction, the square of the spin operator, as well as their average values, were determined for the three phases of the model, namely, the polar superfluid, the Mott insulating and the Bose glass phases. A complete characterization of the physical properties of these phases and the phase transitions between them was then established
Doutorado
Física da Matéria Condensada
Doutor em Ciências
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Fornara, Pascal. "Modélisation et simulation numérique de la croissance des siliciures pour la microélectronique." Université Joseph Fourier (Grenoble), 1996. http://www.theses.fr/1996GRE10082.
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La miniaturisation des transistors mos n'est plus envisageable sans une etape de metallisation auto-alignee des contacts source/drain et grille (procede salicide: self aligned silicide). Dans les technologies mos actuelles, le siliciure le plus couramment utilise est le disiliciure de titane: tisi#2. La siliciuration ayant une influence a la fois sur les dopants et sur la topographie, elle a un impact evident sur les dispositifs. Afin de garantir des simulations electriques precises, la modelisation des procedes technologiques submicroniques doit donc etre capable de prendre en compte cette etape. Dans ce travail, nous avons developpe un modele general de croissance des siliciures et nous l'avons implemente dans un simulateur de procede bidimensionnel. Nous avons demontre qu'il etait possible de simuler la croissance d'un siliciure aussi bien par diffusion du silicium que par diffusion du metal, ainsi que la siliciuration par depot selectif en phase vapeur. Les parametres du modele ont ete ajustes afin d'obtenir des cinetiques de croissance du tisi#2 en bon accord avec les resultats experimentaux. Nous avons montre que notre modele permettait de rendre compte d'effets topographiques bidimensionnels (absence de croissance sous l'espaceur, courbure de la couche de tisi#2 sur les grilles de polysilicium,), a condition de faire dependre certains parametres des contraintes mecaniques generees pendant la croissance de la couche. L'influence de la siliciuration sur la redistribution des dopants a ete analysee ; la simulation a confirme que la segregation du bore a l'interface tisi#2/si etait responsable de la degradation des resistances d'acces des contacts siliciures sur silicium dope au bore. L'injection de lacunes pendant la siliciuration a egalement ete calibree grace aux resultats experimentaux et, par des simulations de procedes et de dispositifs, nous avons montre que ce phenomene pouvait induire une redistribution des dopants a longue distance. L'outil que nous avons developpe apporte donc une aide a l'optimisation du procede de siliciuration dans les technologies silicium
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Rush, Jeremy Richard. "Crystal growth, guest ordering and ferroelastic properties of urea inclusion compounds." Diss., Manhattan, Kan. : Kansas State University, 2007. http://hdl.handle.net/2097/526.
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Schwanke, Peter. "Implementation into DIVIMP of a drift-kinetic model derived from the Fokker-Planck equation to examine the parallel-to-B velocity component of impurity ions in divertor-Tokamak plasmas." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/MQ62896.pdf.
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Stadler, Katharina Maria [Verfasser], and Jan von [Akademischer Betreuer] Delft. "A model study of strong correlations in Hund metals : the Numerical Renormalization Group as efficient multi-band impurity solver for Dynamical Mean-Field Theory / Katharina Maria Stadler ; Betreuer: Jan von Delft." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2019. http://d-nb.info/1182228690/34.
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Matychak, Yaroslav, A. Prytula, Viktor Fedirko, and I. Pohreljuk. "Theoretical model for an interaction between titanium and binary rarified gas medium with taking into account the segregation of the impurity on the interface and in the bulk of the metal." Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-194617.
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Matychak, Yaroslav, A. Prytula, Viktor Fedirko, and I. Pohreljuk. "Theoretical model for an interaction between titanium and binary rarified gas medium with taking into account the segregation of the impurity on the interface and in the bulk of the metal." Diffusion fundamentals 3 (2005) 8, S. 1-18, 2005. https://ul.qucosa.de/id/qucosa%3A14296.
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Bastide, Christophe. "Etude de quelques problèmes théoriques dans les phases normales et supraconductrices des fermions lourds." Grenoble 1, 1987. http://www.theses.fr/1987GRE10155.
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Etude de la dilatation thermique negative de ceal::(3) associee au comportement de la chaleur specifique sous pression, en utilisant des resultats de methodes variationnelles appliquees au reseau d'anderson. Etude des proprietes de la phase supraconductrice des fermions lourds en relation avec les differentes symetries possibles pour le parametre d'ordre. Calcul de l'effet d'impuretes non magnetiques sur la temperature critique des phases de symetrie s-etendu et d. Etude du reseau d'anderson dans la limite d'une faible amplitude de saut
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Dargel, Piet. "Spectral functions of low-dimensional quantum systems." Doctoral thesis, Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2012. http://hdl.handle.net/11858/00-1735-0000-000D-F1A3-6.
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Maryasin, Vladimir. "Ordre par le désordre structural et les effets du champ magnétique dans les systèmes frustrés." Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GREAY051/document.
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La compétition des interactions est une caractéristique essentielle des systèmes frustrés, elle est à l'origine d'une large dégénérescence des états fondamentaux classiques ou, obtenus par une théorie de champ moyen.Fréquemment la dégénérescence peut être levée par des fluctuations thermiques ou quantiques, ce qui constitue la base du mécanisme appelé textit{ordre par le désordre}.Les systèmes magnétiques étudiés expérimentalement contiennent une quantité inévitable de désordre structural.Dans cette thèse de doctorat, l'influence des défauts, créé par des sites inoccupés ou par un désordre des liens sur l'espace dégénéré des états fondamentaux est étudiée pour des systèmes frustrés divers.Nous avons trouvé qu'un désordre structural est, lui aussi, capable de lever systématiquement la dégénérescence; par ailleurs, la tendance est inverse par rapport au le mécanisme d'ordre par le désordre produit par les fluctuations.Pour chacun des modèles considérés, les corrections à l'énergie ont été calculées sous la forme de termes anisotropes effectifs qui agissent sur l'espace dégénéré des états fondamentaux.Ces arguments analytiques ont été confirmés par des calculs numériques que nous avons effectués par minimisation de l'énergie, ainsi que par simulation de type Monte-Carlo classique. La séquence des états ordonnés que nous avons détectée est attribuée à la compétition entre l'effet d'ordre induit par les fluctuations et celui induit par les défauts structuraux. L'effet observé peut ouvrir des possibilités supplémentaires de contrôler la structure magnétique des systèmes.Enfin, les effets d'un champ magnétique externe ont été étudiés pour le système antiferromagnétique pyrochlore pur avec anisotropie de plan facile.Nous avons observé des transitions de phases qui dépendent de l'orientation du champ et qui n'existent pas dans la description de type champ moyen du système. Elles constituent une généralisation des transitions de type spin-flop pour le cas de la symétrie discrète $mathbb{Z}_k$ brisée avec $k > 2$Competing interactions is an essential feature of frustrated systems, they stand behind the large degeneracy of classical or mean-field ground states.%produce degeneracy of classical mean-field ground states.In many cases the degeneracy can be lifted by thermal and quantum fluctuations, such mechanism is commonly called textit{order from disorder}.Experimentally studied magnetic systems inevitably contain a finite amount of structural disorder.In this work the influence of defects, namely vacancies and bond disorder, on a degenerate ground state manifold is studied for various frustrated systems.We find that quenched disorder is also capable of consistently lifting the degeneracy, moreover, it has%in a wide range of frustrated systems.%Moreover, the effect of quenched disorder leads toan opposite tendency, compared to the order by disorder mechanism, produced by fluctuations.For every considered model, analytic energy corrections are derived in the form of effective anisotropic terms, which act on the manifold of degenerate ground states.Analytical arguments are confirmed by numerical calculations, which include energy minimization and classical Monte Carlo simulations.The detected sequences of ordered states is attributed to competition of fluctuations and structural disorder.The observed effect can open additional possibilities in tuning magnetic structure of the system.Finally, the effect of external magnetic field is investigated for the pure $XY$ pyrochlore antiferromagnet.Depending on the field orientation we observe phase transitions, which do not exist within the mean-field description of the system.They are generalizations of the spin-flop transition for the case of broken discrete $mathbb{Z}_k$ symmetry with $k > 2$
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Bertrand, Corentin. "Algorithme Monte-Carlo pour les systèmes quantiques à fortes interactions et hors d'équilibre en nanoélectronique." Thesis, Université Grenoble Alpes (ComUE), 2019. http://www.theses.fr/2019GREAY030.
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Les problèmes quantiques à plusieurs corps hors d'équilibre concentrent de plus en plus d'attention en physique de la matière condensée. Par exemple, les systèmes d'électrons en interaction soumis à un champ électrique externe (constant ou variable) sont un sujet d'étude important en nanoélectronique, mais aussi plus récemment en science des matériaux, afin d'identifier de nouveaux états de la matière hors d'équilibre. Dans cette thèse, une nouvelle méthode numérique et générique a été conçue pour ces systèmes, et appliquée au modèle d'impureté d'Anderson. Ce modèle représente fidèlement un point quantique couplé à une ou plusieurs électrodes, et rends compte à l'équilibre de l'effet Kondo : une manifestation des interactions Coulombiennes au sein du point quantique. Cette méthode a permis d'observer la disparition de l'effet Kondo lorsque le point quantique est conduit hors d'équilibre par une différence de potentiel. Le cœur de la méthode utilise un algorithme Monte-Carlo Quantique diagrammatique. Il s'agit d'une version optimisée de l'algorithme de Profumo et al. [Phys. Rev. B 91, 245154 (2015)], qui calcule des observables dépendantes du temps ou des fonctions de corrélations à travers leurs séries de perturbation en puissances de la force de l'interaction U. Le problème de la divergence de ces séries à grand U est traité par une méthode de resommation robuste. Elle analyse la structure analytique des séries dans le plan complexe en U afin de proposer une régularisation sur mesure par transformation conforme du plan complexe. En post-traitement, une technique Bayésienne permet d'inclure des informations non perturbatives pour réduire les barres d'erreurs qui ont été exacerbées par la resommation. Cette méthode pourrait être appliquée à l'étude de matériaux hors d'équilibre grâce aux algorithmes de "quantum embedding", comme la théorie dynamique de champs moyen, qui permettent l'étude de modèles de réseaux par la résolution d'un problème d'impureté autocohérentNon-equilibrium quantum many-body problems are attracting increasingly more attention in condensed matter physics. For instance, systems of interacting electrons submitted to an external (constant or varying) electric field are studied in nanoelectronics, and more recently in materials, for the search of novel non-equilibrium states of matter. In this thesis, we developed a new numerical generic method for these problems, and apply it to the Anderson impurity model. This model is a good representation of a quantum dot coupled to one or several leads, and gives rise at equilibrium to the Kondo effect --- a manifestation of Coulomb interactions within the dot. We apply our method to compute the collapse of the Kondo effect when the quantum dot is driven out of equilibrium by a voltage bias. Our method is based on a diagrammatic Quantum Monte Carlo (QMC) algorithm. The QMC is an optimized version of the algorithm of Profumo et al. [Phys. Rev. B 91, 245154 (2015)], which computes time-dependent observables or correlation functions as perturbation series in the interaction strength U. To address the problem of diverging series at large U, we constructed a robust resummation scheme which analyses the analytical structure of the series in the U complex plane, for proposing a tailor-made regularization method using a conformal transform of the complex plane. As a post-treatment, a Bayesian technique allows to introduce non-perturbative information to tame the exacerbation of error bars caused by the resummation. We emphasize the potential application to study non-equilibrium materials through "quantum embedding" schemes, such as the Dynamical Mean Field Theory (DMFT), which allow to study lattice models through solving a self-consistent impurity model
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Brum, José Antonio. "Etude theorique des proprietes electroniques des heterostructures de semiconducteurs." Paris 7, 1987. http://www.theses.fr/1987PA077006.
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Relations de dispersion des porteurs dans le plan des couches, avec attention particuliere aux sous-bandes de valence issues des extrema gamma ::(8) des materiaux hotes; etude des problemes coulombiens avec resolution du probleme de l'exciton et etude de la raie d'emission associee aux recombinaisons electron-trou piege dans les puits quantiques gaas/algaas. Effets d'un champ electrique longitudinal sur les niveaux d'energie a une et deux particules; interpretations de la stabilite de l'exciton et etude des niveaux d'energie d'impurete et des super reseaux "dents de scie". Etude de la capture des porteurs depuis le continuum vers les etats lies du puits quantique (emission de phonon ou niveau-relais quasidiscret). Niveaux d'energie dans les fils quantiques
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Tomey, Lopez Estanislao Pablo. "Contribution à l'étude des propriétés structurales et magnétiques des composés intermétalliques de terre rare RFe10,5Mo1,5 : effets des éléments interstitiels H, C et N." Université Joseph Fourier (Grenoble), 1994. http://www.theses.fr/1994GRE10245.
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La stabilisation de la structure cristalline thmn#1#2 dans les alliages r(fe-m')#1#2 exige une reduction de la densite d'electrons de valence par rapport a celle apportee par 12 atomes de fer. Ceci est fait en substituant le fer par le molybdene. L'insertion des atomes interstitiels tels que z=h, c et n dans les alliages rfe#1#0#,#5mo#1#,#5 provoque des modifications importantes de leurs proprietes physiques. Si pour les alliages, la taille de l'element r se manifeste dans l'evolution du parametre a de la maille elementaire, au fur et a mesure du remplissage du site interstitiel c'est le parametre c qui traduit la variation de taille de l'element r. La temperature d'ordre des alliages et le moment magntique moyen du fer augmentent apres l'insertion de z. Les contributions a l'anisotropie magnetocristalline de l'element r et du fer sont fortement modifiees par la presence de z. En particulier, avec l'azote ou le carbone les deux contributions sont completement changees en signe et amplitude. L'etude par diffraction neutronique sur poudre a permis d'une part, de localiser l'hydrogene et d'autre part, de suivre l'evolution de la hierarchie des moments magnetiques pour les differents sous-reseaux de fer. Des effets magnetoelastiques notables accompagnent la reorientation de spin dans l'alliage ou r=dy. La justification d'une configuration ferro ou ferrimagnetique oblique, requiert une reduction de symetrie selon l'analyse des representations. Une methode numerique a ete mise au point afin de reproduire les courbes d'aimantation mesures sur poudre orientee tout en tenant compte du degre de desorientation des cristallites, pour une bonne determination des constantes d'anisotropie k#1 et k#2. Les courbes d'aimantation mesurees sur monocristal de dyfe#1#0#,#5mo#1#,#5 ont ete reproduites par un calcul auto-coherent selon un modele d'interaction d'echange et de champ cristallin. On en deduit un ensemble de parametres de champ cristallin a#m#1 qui permettent de reproduire de facon raisonnable les phases magnetiques stables a basse temperature pour la serie des alliages
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Hedden, Ralf. "Single Particle Dynamics of Anderson-like Impurity Models: A Functional Renormalization Group Study." Doctoral thesis, 2007. http://hdl.handle.net/11858/00-1735-0000-0006-B453-C.
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Sindel, Michael [Verfasser]. "Numerical renormalization group studies of quantum impurity models in the strong coupling limit / Michael Sindel." 2004. http://d-nb.info/974061166/34.
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Schmidt, Thomas [Verfasser]. "Correlation effects in the charge and spin transport statistics of quantum impurity models / vorgelegt von Thomas Schmidt." 2007. http://d-nb.info/986507261/34.
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Hedden, Ralf [Verfasser]. "Single particle dynamics of Anderson-like impurity models : a functional renormalization group study / vorgelegt von Ralf Hedden." 2007. http://d-nb.info/985696222/34.
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Palacios, Guillaume [Verfasser]. "Exactly solvable models of strongly correlated systems : application to one-dimensional cold gases and quantum impurity problems / von Guillaume Palacios." 2007. http://d-nb.info/987030949/34.
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Chen, Kun-Pao, and 陳崑寶. "Impurity effect on the phase transition behavior of model cell membrane." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/06028382663591638144.
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碩士國立中興大學
物理學系所
100
As minute amount of fluorescent dye Rhodamine.B-DHPE(RB) was added to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine(DPPC) giant Unilamellar vesicle(GUV) system, we found the major morphological response to DPPC phase transition changes from shrink to burst. More than 70% of GUVS with RB burst at the DPPC liquid to gel transition induced by temperature decreasing, and the growth of burst probability exceeds 7 times. Our data also demonstrates size dependence burst probability; larger diameter GUVs have higher tendency towards burst, regardless of the RB addition. Accordingly, the maximum flux calculated from measuring GUV volume change at the transition also reveals linear relationship with the vesicle diameter. Conferring to these tendencies, we speculate the deviance between demanded flux and permeability constraint provokes the burst. In the meanwhile, exclusion flux of DPPC GUV with RB displays a saturated trend as the vesicle diameter increased. We propose this saturated flux phenomenon is a consequence of permeability reduction due to RB addition and this decreased GUV permeability further enlarge the difference such that the burst probability is elevated. When solutions contain either sucrose or ions were injected into the GUV incubation cell, this buffer solution replacement represses the vesicle burst. This suppression is conjectured as an outcome of osmotic effect. Since all the vesicles are incubated in pure water, so introducing solutions with sucrose or ion solute should cause osmotic pressure differences across lipid membranes. We anticipate the pressure differences will induce an universal compression in vesicle volume upon addition. Cautious observations confirm the occurrence of the predicted pre-compression before the DPPC phase transition. This two-stage shrinkage results in an intriguing effect that the required volume reduction rates at each stage are dropped. This turns out to diminish the inconsistency between GUV effluxes and permeation, thus the vesicle dis-integration is suppressed. Phase contrast images show a pulse like intensity surge response in GUVs due to injection of high solute concentration solutions. The initial intensity increase is attributing to refraction index change as the surrounding solution is replaces in the injection. The recovery of the GUV intensity is believed as an equilibrate process which is suspected to be the solute equilibration across membrane via permeation. Vesicles with RB do always exhibit longer relaxation time as conclude by fitting the intensity relaxation curves. This data provides evidence in supporting the hypothesis of minute RB addition will cause reduction in membrane permeation.
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Nase, Carsten [Verfasser]. "Dynamic density matrix renormalization for the symmetric single impurity Anderson model / vorgelegt von Carsten Raas." 2005. http://d-nb.info/976561298/34.
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Feng, Qingguo [Verfasser]. "Study of single impurity Anderson model and dynamical mean field theory based on equation-of-motion method / von Qingguo Feng." 2009. http://d-nb.info/998896241/34.
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Lin, Kuo-Kai, and 林國凱. "Particle Swarm Optimization-based Impurity Function Band Prioritization Using Multiple Attribute Decision Making Model for Band Selection of High Dimensional Data Sets." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/58e9r9.
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碩士國立臺北科技大學
電機工程系所
102
In recent years, with the progress in remote sensing technologies, the numbers of data and dimension are increased in remote sensing imagery. For solve the huge data and high dimension computational complexity problem in remote sensing imagery can use band selection methods to reduced dimension and avoid Hughes Phenomena because of increased quantity of bands. Some scholars proposed many kinds of algorithms for band selection to reduce dimensionality, but those algorithms were inefficient and led to the dimensionality reduction effects couldn’t be significantly. Therefore, in this paper, using a band selection algorithm based on particle swarm optimization (PSO) combine with correlation coefficients matrix (C.C Matrix) to cluster the highly correlated bands together and to obtain greedy modular eigenspace (GME) and represented as analytic hierarchy process (AHP) module to observe the relationship with modular hierarchically. To combine AHP with impurity function class overlapping (IFCO) to calculate high correlation bands weights. Finally, according to the bands weighting scores statistics result to select representative bands for obtaining greatly dimensionality reduction effect. The effectiveness of the proposed method is evaluated by MASTER and AVIRIS remote sensing images for testing the variance and correlation of dimension reduction rate and classification accuracy. The experimental results show the dimension reduction rate is 90.91% and classification accuracy is 98.44% in Au-Ku; the dimension reduction rate is 85.00% and classification accuracy is 96.48% in NTC.
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Yuan, Haw, and 袁顥. "Particle Swarm Optimization-based Hotspot Analysis and Impurity Function Band Prioritization Using Multiple Attribute Decision-Making Model for Band Selection of High Dimensional Data Sets." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/428q69.
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碩士國立臺北科技大學
電機工程研究所
105
In recent years, the satellite technique has a tremendous progress. The images captured by satellites contain larger data and dimensions. To solve the problem caused by the huge datasets, selecting the representative bands to achieve the dimensionality reduction. Then it will prevent the Hughes Phenomena. Some scholars proposed many algorithms for band selection, but the effects of dimensionality reduction are not obvious. A researcher proposed combining particle swarm optimization with correlation coefficients matrix to cluster the highly correlated bands and to obtain the greedy modular eigenspaces. According to the analytic hierarchy process model to present and rate the eigenspaces. It can obtain excellent rate in both dimensionality reduction and accuracy. Therefore, this paper proposed a more advanced rating method called hotspot analysis. This method can calculate the weightings and analyze the correlation between each clustered blocks in each eigenspace. Finally, selecting the representative bands with higher weighting scores and achieving a better result of dimensionality reduction. In the result, this paper uses MASTER and NTC remote sensing images as the experimental datasets. To test the correlation and variation between dimensionality reduction and accuracy. Comparing the advanced method with the original method. In Au-Ku, the dimensionality reduction rate are both 90.91% but the original accuracy is 95.37% and the advanced method is 99.99%. In NTC, the dimensionality reduction rate are both 87.7% but the original accuracy is 95.22% and the advanced method is 96.48%. The result can show the advanced method has the better effects.