Determinant Quantum Monte Carlo Study of the Orbitally Selective Mott Transition

We study the conductivity, density of states, and magnetic correlations of a two dimensional, two band fermion Hubbard model using determinant Quantum Monte Carlo (DQMC) simulations. We show that an orbitally selective Mott transition (OSMT) occurs in which the more weakly interacting band can be metallic despite complete localization of the strongly interacting band. The DQMC method allows us to test the validity of the use of a momentum independent self-energy which has been a central approximation in previous OSMT studies. In addition, we show that long range antiferromagnetic order (LRAFO) is established in the insulating phase, similar to the single band, square lattice Hubbard Hamiltonian. Because the critical interaction strengths for the onset of insulating behavior are much less than the bandwidth of the itinerant orbital, we suggest that the development of LRAFO plays a key role in the transitions.Comment: 4 pages, 5 figure

Cooper pair islanding model of insulating nanohoneycomb films

We first review evidence for the Cooper pair insulator (CPI) phase in amorphous nanohoneycomb (NHC) films. We then extend our analysis of superconducting islands induced by film thickness variations in NHC films to examine the evolution of island sizes through the magnetic field-driven SIT. Finally, using the islanding picture, we present a plausible model for the appearance and behavior of the CPI phase in amorphous NHC films.Comment: 7 pages, 3 figure

Single and two-particle energy gaps across the disorder-driven superconductor-insulator transition

The competition between superconductivity and localization raises profound questions in condensed matter physics. In spite of decades of research, the mechanism of the superconductor-insulator transition (SIT) and the nature of the insulator are not understood. We use quantum Monte Carlo simulations that treat, on an equal footing, inhomogeneous amplitude variations and phase fluctuations, a major advance over previous theories. We gain new microscopic insights and make testable predictions for local spectroscopic probes. The energy gap in the density of states survives across the transition, but coherence peaks exist only in the superconductor. A characteristic pseudogap persists above the critical disorder and critical temperature, in contrast to conventional theories. Surprisingly, the insulator has a two-particle gap scale that vanishes at the SIT, despite a robust single-particle gap.Comment: 7 pages, 5 figures (plus supplement with 4 pages, 5 figures

Phases métalliques de fermions corrélés

Cette thèse traite des fermions fortement corrélés. En particulier, nous étudions la possibilité d observer des transitions de phase quantiques de l état isolant vers l état métallique ou vers la phase supraconductrice dans des systèmes de fermions en interaction sur réseaux. Dans un premier chapitre, nous allons introduire le modèle de Hubbard qui permet de décrire le comportement des électrons dans des solides cristallins dans la limite des liaisons fortes. Nous prouverons dans un second chapitre la possibilité d obtenir des transitions de phase isolant-métal gouvernées par les interactions entre électrons. Ces transitions sont obtenues dans le cadre du modèle de Hubbard ionique où deux types de comportement isolant peuvent être mis en évidence ? L état métallique est obtenu lorsque les effets de localisation des deux isolants se compensent ? Puis, dans la continuité de cette étude, nous considérerons un système où les électrons peuvent occuper deux plans couplés. Il existe aussi une phase intermédiaire métallique dans ce modèle. Nous étudierons aussi la possibilité d obtenir une phase supraconductrice grâce à une symétrie particulière dans ce modèle. Finalement, étudiant u modèle de Hubbard dynamique, nous analysons l effet du couplage des électrons avec les degrés de liberté internes de l atome. Ce modèle a l avantage de présenter une dissymétrique particule-trou qui pourrait donner lieu à un diagramme de phase dissymétrique comme observé dans les matériaux supraconducteurs à haute température de transition.We study the possibility of interaction driven insulator to metal transitions and superconductivity in fermionic systems on lattices. We first introduce the Hubbard model which describes fermions in a crystal in the tight-binding limit.Then we show evidence for interaction-driven insulator-metal transitions in the ionic Hubbard model. At half-filling, when the interaction strength or the staggered potential dominates we find Mott and band insulator, respectively. When these two energies are of the same order we find a metallic phase. Then we study a bilayer Hubbard model which exhibits such insulator to metal transitions and shows an interesting superconducting signal. Finally, we study a bilayer Hubaard model which describes interacting fermions on a lattice whose on-site repulsion is modulated by a coupling to fluctuating bosonic field.NICE-BU Sciences (060882101) / SudocSudocFranceF