Academic literature on the topic 'Systèmes quantiques désordonnés'
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Dissertations / Theses on the topic "Systèmes quantiques désordonnés":
Couëdo, François. "Transitions de phase quantiques dans les systèmes désordonnés de basse dimension." Phd thesis, Université Paris Sud - Paris XI, 2014. http://tel.archives-ouvertes.fr/tel-00990782.
Hainaut, Clément. "Effets des symétries sur la localisation dans des systèmes quantiques désordonnés." Thesis, Lille 1, 2017. http://www.theses.fr/2017LIL10082/document.
In this thesis, we use the Kicked Rotor, paradigm of quantum chaos, to study new physical aspects of disordered systems.We thus present the first experimental observation with atomic matter wave of a phenomenon directly linked to weak localization which is the Enhanced Return to the Origin. We show that this effect can be used as a tool to measure accuratly the decoherence in the system. We present a novel, outstandingly simple, experimental method to control symmetry properties of the Kicked Rotor. This allows us to study a disordered system in presence of a non-trivial artificial Aharonov-Bohm flux in a synthetic dimension. This gives us the opportunity to break the time reversal symmetry and then to study the physics of Anderson localization in two different symmetry classes : the orthogonal class and the unitary class. We have investigated the effect of this symmetry breaking on physical properties of 1D disordered systems by looking two signatures of quantum transport. We observe thus experimentally, for the first time, the Coherent Forward Scattering effect, predicted recently and which represents a novel genuine signature of Anderson localization. We show its distinctive signatures and a good agreement with theoretical predictions. Finally, we realise the first experimental measurements of the (G) scaling function, characteristic of transport in disordered medium, in two symmetry classes and we demonstrate their universality
Hainaut, Clément. "Effets des symétries sur la localisation dans des systèmes quantiques désordonnés." Electronic Thesis or Diss., Lille 1, 2017. http://www.theses.fr/2017LIL10082.
In this thesis, we use the Kicked Rotor, paradigm of quantum chaos, to study new physical aspects of disordered systems.We thus present the first experimental observation with atomic matter wave of a phenomenon directly linked to weak localization which is the Enhanced Return to the Origin. We show that this effect can be used as a tool to measure accuratly the decoherence in the system. We present a novel, outstandingly simple, experimental method to control symmetry properties of the Kicked Rotor. This allows us to study a disordered system in presence of a non-trivial artificial Aharonov-Bohm flux in a synthetic dimension. This gives us the opportunity to break the time reversal symmetry and then to study the physics of Anderson localization in two different symmetry classes : the orthogonal class and the unitary class. We have investigated the effect of this symmetry breaking on physical properties of 1D disordered systems by looking two signatures of quantum transport. We observe thus experimentally, for the first time, the Coherent Forward Scattering effect, predicted recently and which represents a novel genuine signature of Anderson localization. We show its distinctive signatures and a good agreement with theoretical predictions. Finally, we realise the first experimental measurements of the (G) scaling function, characteristic of transport in disordered medium, in two symmetry classes and we demonstrate their universality
Van, Den Berg Tineke. "Conductivité de spin et effets magnétiques dans les systèmes quantiques désordonnés." Thesis, Aix-Marseille, 2012. http://www.theses.fr/2012AIXM4812/document.
Spintronics is a research area that is concerned with the storage and transfer of information by means of electron spins. In the first part we investigated the intrinsic spin Hall effect in the presence of disordered magnetic impurities in a paramagnetic state in a two dimensional electron gas with Rashba spin-orbit coupling. In the presence of weak magnetic disorder the spin Hall conductivity stays close to its universal (clean system) value, as shown by analytical linear response calculations and numerical simulations. Heavy spin conductivity fluctuations are observed, that increase with disorder strength. To investigate the spreading of a wavepacket on a lattice we measure the wavepacket width, the inverse participation ratio and the (2)-fractal dimension. It is shown the system undergoes a localization transition at a critical disorder strength. In the localized regime the local density of states is not uniform anymore. An anti-ferromagnetic correlation between electron spins and impurity magnetic moments is observed. Beyond the localization transition the spin conductivity increases significantly. The first quantum (Cooperon) corrections in the linear response formalism are shown to contribute positively to the spin Hall conductivity. In the second part the double exchange Hubbard model for correlated electron systems is studied using dynamical mean field theory (DMFT) with the non-crossing approximation (NCA). Around quarter filling an orbital polaron is observed, numerically and in an effective Hamiltonian. Double exchange in dilute magnetic semiconductors is studied using the coherent potential approximation (CPA)
Van, Den Berg Tineke. "Conductivité de spin et effets magnétiques dans les systèmes quantiques désordonnés." Electronic Thesis or Diss., Aix-Marseille, 2012. http://www.theses.fr/2012AIXM4812.
Spintronics is a research area that is concerned with the storage and transfer of information by means of electron spins. In the first part we investigated the intrinsic spin Hall effect in the presence of disordered magnetic impurities in a paramagnetic state in a two dimensional electron gas with Rashba spin-orbit coupling. In the presence of weak magnetic disorder the spin Hall conductivity stays close to its universal (clean system) value, as shown by analytical linear response calculations and numerical simulations. Heavy spin conductivity fluctuations are observed, that increase with disorder strength. To investigate the spreading of a wavepacket on a lattice we measure the wavepacket width, the inverse participation ratio and the (2)-fractal dimension. It is shown the system undergoes a localization transition at a critical disorder strength. In the localized regime the local density of states is not uniform anymore. An anti-ferromagnetic correlation between electron spins and impurity magnetic moments is observed. Beyond the localization transition the spin conductivity increases significantly. The first quantum (Cooperon) corrections in the linear response formalism are shown to contribute positively to the spin Hall conductivity. In the second part the double exchange Hubbard model for correlated electron systems is studied using dynamical mean field theory (DMFT) with the non-crossing approximation (NCA). Around quarter filling an orbital polaron is observed, numerically and in an effective Hamiltonian. Double exchange in dilute magnetic semiconductors is studied using the coherent potential approximation (CPA)
Sabri, Mostafa. "Etude de la localisation pour des systèmes désordonnés sur un graphe quantique." Paris 7, 2014. http://www.theses.fr/2014PA077022.
This work is devoted to the study of some spectral properties of random Schrödinger operators. It is divided into two parts : 1. A study of localization for multi-particle systems on quantum graphs. 2. An abstract formulation of some Wegner estimates, followed by a list of applications for concrete models. In Chapter 1 we try to introduce the problems and the results of this thesis in an elementary way. The first part occupies chapters 2 and 3. Chapter 2 essentially reproduces our article "Anderson Localization for a multi-particle quantum graph" [97] on this subject. In Chapter 3 we discuss some additional properties of our model, and we give alternative proofs to some results of Chapter 2. The second part occupies chapters 4 and 5. Chapter 4 essentially reproduces our article "Some abstract Wegner estimates with applications" [98]. In Chapter 5 we continue the study of Wegner estimates by giving more abstract theorems in Section 5. 2 and yet more applications in Section 5. 3. We conclude with two appendices A and B. In the first one we explain the theory of generalized eigenfunction expansions in great detail. In Appendix B, we prove some classical resutls usedin the text
Anfray, Valentin. "Étude numérique du point critique de systèmes quantiques de spin désordonnés en dimensions élevées." Electronic Thesis or Diss., Université de Lorraine, 2022. http://www.theses.fr/2022LORR0127.
Several random quantum spin models have been numerically studied in dimension D>1 by Strong Disorder Renormalisation Group (SDRG). We have implemented an efficient algorithm to be able to consider a system with up to a billion spins independently of its spatial dimension. Critical properties of the 2D and 3D random quantum Potts model with q=2,3,5,10,20 and 50 states are shown to be governed by an infinite disorder fixed point. We have computed the correlation-length exponent u, the magnetization exponent d_f and the energy gap exponent psi. Using finite-size scaling and taking into account finite-size corrections, critical properties of the Potts model are shown to be q-independent. Random quantum Clock models with q=2,3,5,8 and 10 states have been also studied in 2D and 3D. A minimum amount of initial disorder strength is required to flow to an infinite disorder fixed point. Despite large error bars on psi exponent, our estimates for the critical exponents u and psi for all q are compatible with those of the random transverse-field Ising model. Our estimates for the critical exponent d_f are incompatible within error bar but very close. Lastly, the tricritical point of the random quantum Ashkin-Teller model has been studied in dimension two and three. We have shown that the correlation-length exponent associated with one of the two unstable directions does not belong to the university class of the random transverse-field Ising model
Bocquet, Marc. "Chaînes de Spins, Fermions de Dirac, et Systèmes Désordonnés." Phd thesis, Ecole Polytechnique X, 2000. http://tel.archives-ouvertes.fr/tel-00001560.
Sabri, Mostafa. "Étude de la localisation pour des systèmes désordonnés sur un graphe quantique." Phd thesis, Université Paris-Diderot - Paris VII, 2014. http://tel.archives-ouvertes.fr/tel-01001715.
Bayo, Djénabou. "Détermination de phase par Deep Learning pour les systèmes désordonnés." Electronic Thesis or Diss., CY Cergy Paris Université, 2024. http://www.theses.fr/2024CYUN1280.
Our first model is the two-dimensional site percolation. In this paradigmatic model, sites are randomly occupied with probability «p»; a second-order phase transition from a non-percolating to a fully percolating phase appears at occupation density «p_c», called percolation threshold. Through supervised deep learning approaches like classification and regression, we explore the ability of convolutional neural networks (CNNs) to predict the density of occupation «p» of percolation states, the correlation length «xi», as well as the presence of a spanning cluster. We find that image recognition tools such as CNN, which are not naturally tailored for physics, successfully identify «p». However, when dealing with parameters like «xi» or the presence of a spanning cluster, these same techniques fail to provide quantitative results. The second model is the three-dimensional Anderson model of localisation. This model is characterised by a localisation of the wavefunctions above a critical disorder «W_c». We begin by reproducing previous work done on phase classification, and perform several new studies with classification and regression methods, to identify individual disorders in both phases. Throughout our investigation, multiple parameters such as the size of the system or the nature of the input are studied to observe their influence on the performance of the model. Via the study of these two models and the use of several ML methods, we will display the successes and limitations that one might be confronted with when using ML for phase recognition