• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 3
  • 2
  • Tagged with
  • 5
  • 5
  • 5
  • 2
  • 2
  • 2
  • 2
  • 2
  • 2
  • 2
  • 2
  • 2
  • 2
  • 2
  • 2
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Estudo analítico e soluções exatas da equação de spin / Analytical study and exact solutions of the spin equation

Baldiotti, Mário César 01 July 2005 (has links)
O presente trabalho se destina a um estudo detalhado da chamada equação de spin, a qual pode ser utilizada para descrever o comportamento de sistemas de dois níveis. Para campos externos dados por funções reais, esta equação pode ser identificada com uma redução da equação de Pauli para o caso 0+1 dimensional. Inicialmente, demonstraremos a relação entre esta equação de spin e várias outras equações relacionadas com diversos problemas em física. Com estas relações, podemos construir novas soluções da equação de spin a partir do conhecimento de soluções exatas destes outros problemas e, por outro lado, estender a aplicação das soluções obtidas. Em seguida, descrevemos a forma geral da solução desta equação, construímos o operador de evolução e resolvemos o problema inverso, i.e., a determinação do campo externo supondo o conhecimento de uma solução. Finalizando, para o importante caso de campos externos reais, desenvolvemos um método de construção de novas soluções a partir de uma solução previamente conhecida, utilizando a chamada transformação de Darboux. Em particular, demonstramos a existência de operadores de entrelaçamento de Darboux, que não violam a estrutura específica dos sistemas de dois níveis, e permitem construir novos campos externos também dados por funções reais. Como resultado destes desenvolvimentos, apresentamos uma série de novas soluções exatas para a equação de spin. / The aim of the present work is to study in detail the so called spin equation, which can be used to describe the behavior of two-level systems. We recall that, for real external fields, this equation can be treated as a reduction of the Pauli equation to the 0+1 dimensional base. Initially, we present the relation between the spin equation and some other equations related to diferent physical problems. With these relations, we construct new solutions to the spin equation from the knowledge of the exact solutions of these other problems and, on the other hand, extend the applicability of the obtained solutions. After that, we describe the general solution of the spin equation, construct the evolution operator and solve the inverse problem, i.e., the construction of the external field from a given supposed solution. Finally, for the important case of real fields, we develop a method to construct new solutions from a previously known one, by the application of the so called Darboux transformation. In particular, we demonstrate the existence of Darboux intertwining operators which do not violate the specific structure of the two-level systems and allow the construction of external fields which are also given by real functions. As a result of all these developments, we present several new sets of exact solutions for the spin equation.
2

Processus stochastiques et non-linéaires dans les systèmes nano-électro-mécaniques / Stochastic and non-linear processes in nano-electro-mechanical systems

Maillet, Olivier 26 March 2018 (has links)
Dans cette thèse, nous étudions des systèmes nano-électro-mécaniques (NEMS) en conditions cryogéniques (de 30 mK à 30 K) sensibles à des conditions de fluctuations ou de désordre. Les phénomènes fondamentaux étudiés sont omniprésents dans la physique des NEMS, et pour certains vont même au-delà avec des analogies vers d’autres disciplines de la physique, comme les transitions de phase ou la RMN.Dans la première partie de cette thèse, nous nous intéressons ainsi au bruit d'amplitude du NEMS, fournissant un exemple de mouvement Brownien dans un potentiel de confinement. Du fait de la non-linéarité géométrique intrinsèque au système, l'anharmonicité du potentiel transduit le mouvement Brownien de chaque mode mécanique en fluctuations des fréquences propres de résonance. Ainsi, nous observons expérimentalement un phénomène de diffusion spectrale, se traduisant par un élargissement et un décalage de la raie de résonance non-triviaux rendant compte de la compétition entre la diffusion de la phase de la réponse du mode due à la transduction, et les mécanismes de relaxation du mode fluctuant. Une approche par intégrale de chemin de la diffusion capture l'effet analytiquement. Un tel mécanisme altère la résonance d'un mode mécanique sans influer sur les échanges d'énergie avec le bain thermodynamique du mode. En outre, l'introduction d'une forte excitation sinusoïdale agit en retour sur les fluctuations hors équilibre via la non-linéarité, ralentissant la dynamique du système et comprimant ses fluctuations pour certains points critiques de l'espace des paramètres, près du ou dans le régime de la réponse bistable permise par la non-linéarité. Enfin, des expériences-modèles ont été réalisées afin de comprendre en détail la décohérence mécanique classique à l’aide d’un bruit en fréquence extrinsèque, réalisé à l’aide d’une grille couplée au NEMS.La deuxième partie de cette thèse explore plus en détail certains mécanismes microscopiques de relaxation d'énergie ou du bruit en fréquence interne d’un mode mécanique, encore partiellement incompris pour les NEMS. Nous considérons d’abord le cas d’une contribution extérieure, mais universelle, qui a pour origine le transfert d’impulsion entre le NEMS et le gaz présent dans la cellule expérimentale, ici l’hélium 4. Dans la limite des faibles densités, la théorie cinétique décrit la dissipation dans le gaz ballistique. De façon inattendue, nous observons aux plus basses pressions atteignables une déviation à la théorie. Nous montrons pour plusieurs températures et plusieurs échantillons que cette déviation s’échelonne avec le rapport entre le libre parcours moyen des atomes dans le gaz et la hauteur du NEMS vis-à-vis du fond de l’échantillon. Ce résultat est justifié par un modèle phénoménologique prenant en compte la réflexion diffusive des atomes du gaz sur le mur du fond, qui présente à petite échelle une structure désordonnée. Cette réflexion résulte en une déviation à la Maxwellienne près du fond, et donc en l’établissement d’un gradient de densité du gaz sur une longueur de l’ordre du libre parcours moyen, qui renormalise le taux de relaxation d’énergie mécanique. Ainsi, le NEMS agit comme une sonde non-invasive d’un milieu hors équilibre du fait de ses très petites dimensions transverses. Enfin, nous mesurons la dissipation intrinsèque du NEMS jusqu’à 30 milliKelvin. Nous mettons en évidence le rôle des excitations de basse énergie couplées à la déformation du NEMS dans la relaxation d’énergie mécanique. Ces excitations, permises par la structure désordonnée des matériaux constitutifs du NEMS, sont modélisées comme des atomes se déplaçant par effet tunnel entre deux positions équivalentes du réseau atomique (TLS). Nous obtenons également le bruit en fréquence intrinsèque en développant une nouvelle technique de mesure utilisant la non-linéarité du NEMS. L’étude poussée nous permet de lier phénoménologiquement les deux phénomènes. / In this thesis we address cryogenic nano-electro-mechanical systems (NEMS) from 30 mK to 30 K sensitive to conditions involving fluctuations or disorder. The fundamental aspects studied are ubiquitous in NEMS physics, and for some of them go beyond, with possible analogies with phase transitions or NMR.In the first part of this work we focus on the NEMS position noise, which is a good example of Brownian motion within a confinement potential. Owing to the system’s intrinsic geometric nonlinearity, the potential anharmonicity translates each mode’s Brownian motion into fluctuations of the structure’s resonance eigenfrequencies. As a result we observe experimentally a spectral diffusion phenomenon that manifests through a linewidth broadening and a frequency shift of the resonance line: they account non-trivially for the competition between the probed mode’s response’s phase diffusion due to the transduction mechanism and the fluctuating modes relaxation mechanisms. A path integral approach to diffusion encompasses analytically the effect. Such a mechanism alters a mechanical mode’s resonance without changing energy transfers to the mode’s thermal bath. Furthermore, adding a strong sinusoidal excitation acts back on the out-of-equilibrium fluctuations through the nonlinearity: the system dynamics is slowed down, with its fluctuations squeezed, in peculiar points of the parameters space, near or within the non-linearity induced bistable regime. Finally, model experiments are realized so as to understand classical mechanical decoherence, through the use of an extrinsic frequency noise, artificially crafted thanks to a gate electrode coupled to the NEMS.In a second part, some microscopic mechanisms leading to mechanical damping and internal frequency noise of a mechanical mode are investigated, as they are still elusive to date for NEMS. We first consider the case of an external but universal source of damping, which originates from the momentum transfer between the NEMS and the gas flowing in the experimental cell, here Helium 4. In the rarefied limit, dissipation in a ballistic gas is well described by kinetic theory. Yet, unexpectedly, we observe at our lowest pressures a discrepancy between our measurements and theory. We show for several temperatures and samples that this deviation scales with the ratio between the gas atoms mean free path and the gap between the NEMS and the sample’s bottom trench. This result is modelled phenomenologically as arising from diffusive scattering of gas atoms at the bottom’s wall, which at small lengthscales has a disordered landscape. Diffusive scattering results in a deviation to the Maxwellian distribution, leading to a gas density gradient in the vicinity of the wall, established over a distance comparable with the mean free path, and which renormalizes the mechanical energy relaxation rate. Therefore, the NEMS acts as a non-invasive probe in a nonequilibrium medium due to its small cross-section. Finally, we investigate the NEMS intrinsic dissipation down to 30 milliKelvin. We highlight the role of low-energy excitations coupled to the NEMS deformation in damping mechanisms. These excitations, allowed by the disordered structure of the NEMS constitutive materials, are modelled as atoms tunneling between two equivalent positions of the atomic lattice (also referred to as TLS). Using a new technique which relies on the NEMS non-linearity, we measure the intrinsic frequency noise, and we show that it can be linked phenomenologically to the damping due to the TLS.
3

Estudo analítico e soluções exatas da equação de spin / Analytical study and exact solutions of the spin equation

Mário César Baldiotti 01 July 2005 (has links)
O presente trabalho se destina a um estudo detalhado da chamada equação de spin, a qual pode ser utilizada para descrever o comportamento de sistemas de dois níveis. Para campos externos dados por funções reais, esta equação pode ser identificada com uma redução da equação de Pauli para o caso 0+1 dimensional. Inicialmente, demonstraremos a relação entre esta equação de spin e várias outras equações relacionadas com diversos problemas em física. Com estas relações, podemos construir novas soluções da equação de spin a partir do conhecimento de soluções exatas destes outros problemas e, por outro lado, estender a aplicação das soluções obtidas. Em seguida, descrevemos a forma geral da solução desta equação, construímos o operador de evolução e resolvemos o problema inverso, i.e., a determinação do campo externo supondo o conhecimento de uma solução. Finalizando, para o importante caso de campos externos reais, desenvolvemos um método de construção de novas soluções a partir de uma solução previamente conhecida, utilizando a chamada transformação de Darboux. Em particular, demonstramos a existência de operadores de entrelaçamento de Darboux, que não violam a estrutura específica dos sistemas de dois níveis, e permitem construir novos campos externos também dados por funções reais. Como resultado destes desenvolvimentos, apresentamos uma série de novas soluções exatas para a equação de spin. / The aim of the present work is to study in detail the so called spin equation, which can be used to describe the behavior of two-level systems. We recall that, for real external fields, this equation can be treated as a reduction of the Pauli equation to the 0+1 dimensional base. Initially, we present the relation between the spin equation and some other equations related to diferent physical problems. With these relations, we construct new solutions to the spin equation from the knowledge of the exact solutions of these other problems and, on the other hand, extend the applicability of the obtained solutions. After that, we describe the general solution of the spin equation, construct the evolution operator and solve the inverse problem, i.e., the construction of the external field from a given supposed solution. Finally, for the important case of real fields, we develop a method to construct new solutions from a previously known one, by the application of the so called Darboux transformation. In particular, we demonstrate the existence of Darboux intertwining operators which do not violate the specific structure of the two-level systems and allow the construction of external fields which are also given by real functions. As a result of all these developments, we present several new sets of exact solutions for the spin equation.
4

Transport de phonons dans le régime quantique / Phonon transport in the quantum regime

Tavakoli-Ghinani, Adib 14 December 2017 (has links)
Ce travail de thèse est consacré à la mesure de transport de chaleur par les phonons dans le régime quantique dans des systèmes confinés à très basse température.Le contexte de ce sujet est de soumettre ces systèmes à deux conditions extrêmes : basse température et faibles dimensions et de comprendre les propriétés thermiques fondamentales issues de ces limites.Les échantillons étudiés au cours de cette thèse sont des structures suspendues (membrane ou nanofil) ; elles sont élaborées à partir de nitrure de silicium amorphe (SiN).En abaissant la température, les longueurs caractéristiques des phonons comme le libre parcours moyen ou la longueur d'onde dominante des phonons augmentent. Lorsque ces longueurs caractéristiques dépassent les dimensions latérales du système, la diffusion sur les surfaces (boundary scattering) régira les propriétés thermiques. Dans cette limite de diffusion, le transport des phonons va de la diffusion aux surfaces (régime de Casimir) au régime balistique (limite quantique). Dans ce régime balistique, le courant de chaleur peut être exprimé en utilisant le modèle de Landauer. La conductance thermique est alors exprimée par: K=N_α q T où, N_α est le nombre de modes vibratoires peuplés, q=((π²k_B^2)T)⁄3h est la valeur universelle du quantum de conductance thermique et T est le coefficient de transmission.Dans ce travail, les mesures de conductance thermique de nanofils suspendus ont été effectuées jusqu'à très basse température. Une plate-forme de mesure ayant une sensibilité sans précédent a été développée pour mesurer la variation d'énergie inférieure à l'attojoule. Ces nouveaux capteurs permettent de mesurer les propriétés thermiques du guide d'onde de phonon 1D dans le régime quantique du transport de chaleur. Nous montrons que le coefficient de transmission est le facteur dominant qui définit la valeur de conductance thermique. Ce coefficent dépend de la dimension et de la forme des réservoirs ainsi que de la nature du matériau utilisé ce qui rend difficile la mesure du quantum de conductance thermique. Nous montrons que dans toutes les structures de SiN mesurées, le transport thermique pourrait être dominé par des excitations de faible énergie qui existent dans les solides amorphes (a-solides).Le deuxième ensemble important d'expériences concerne la chaleur spécifique. Nous avons étudié les propriétés thermiques de membranes suspendues de SiN très minces que l'on pense être des cavités de phonon 2D. Nous montrons que la dépendance en température de la chaleur spécifique s'écarte du comportement quadratique comme prévu à très basse température. Les modèles pertinents donnant une explication quantitative des résultats sont encore à l'étude. La présence de systèmes à deux niveaux dans les matériaux amorphes pourrait être une explication possible de la valeur absolue élevée de la chaleur spécifique observée. / This PhD entitles Phonon heat transport in the quantum regime is based on the analysis of the thermal properties of confined systems at very low temperature.The context of this subject is putting the systems in two extreme conditions (low temperature and low dimensions) and understand the fundamental thermal properties coming from these limits.The studied samples during this PhD that are suspended structures (membrane or nanowire) are elaborated from amorphous silicon nitride.By lowering the temperature, the phonon characteristic lengths like the mean free path or the phonon dominant wavelength increase. When these characteristic lengths exceed lateral dimensions of the system, the boundary scattering will govern the thermal properties. In the boundary scattering, phonon transport goes from boundary limited scattering (Casimir regime) to ballistics regime (quantum limit). In this ballistic regime, the heat current can be expressed using the Landauer model. The thermal conductance is then expressed as: K=N_α q T where N_α is the number of populated vibrational modes, q=((π²k_B^2)T)⁄3h is the universal value of quantum of thermal conductance, and T is the transmission coefficient.In this work, thermal conductance measurements of suspended nanowires have been performed down to very low temperature. A measurement platform having an unprecedented sensitivity have been developed that can measure a variation of energy smaller than the attojoule. These new sensors allow the measurement of thermal properties of 1D phonon waveguide in the quantum regime of heat transport. We show that the transmission coefficient is the dominant factor that set the thermal conductance value. It depends on the dimension and the shape of the reservoirs, and the nature of the material in use rendering difficult the measurement of the quantum of thermal conductance. We show that in all of the SiN structures, the thermal transport could be dominated by low energy excitations that exist in amorphous solids (a-solids).The second important set of experiments concerns the specific heat. We have studied suspended the thermal properties of very thin SiN membranes that are thought to be 2D phonon cavities. We show that the temperature dependence of the specific heat departs from the quadratic behavior as expected at very low temperature. The true models giving a quantitative explanation of the results is still under consideration. The presence of tunneling two-level systems in amorphous materials could be one possible explanation for the high absolute value of specific heat that has been measured.
5

Nonlinear optical responses in strongly correlated electron systems / 強相関電子系における非線形光学応答

Kofuji, Akira 25 March 2024 (has links)
京都大学 / 新制・課程博士 / 博士(理学) / 甲第25099号 / 理博第5006号 / 新制||理||1714(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)講師 PETERSRobert, 教授 柳瀬 陽一, 教授 田中 耕一郎 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DGAM

Page generated in 0.0369 seconds