The physics of twisted magnetic tubes rising in a stratified medium: two dimensional results

The physics of a twisted magnetic flux tube rising in a stratified medium is studied using a numerical MHD code. The problem considered is fully compressible (no Boussinesq approximation), includes ohmic resistivity, and is two dimensional, i.e., there is no variation of the variables in the direction of the tube axis. We study a high plasma beta case with small ratio of radius to external pressure scaleheight. The results obtained can therefore be of relevance to understand the transport of magnetic flux across the solar convection zone.Comment: To be published in ApJ, Vol. 492, Jan 10th, 1998; 25 pages, 16 figures. NEW VERSION: THE PREVIOUS ONE DIDN'T PRINT CORRECTLY. The style file overrulehere.sty is include

Dynamics of Impurity and Valence Bands in GaMnAs within the Dynamical Mean Field Approximation

We calculate the density-of-states and the spectral function of GaMnAs within the dynamical mean-field approximation. Our model includes the competing effects of the strong spin-orbit coupling on the J=3/2 GaAs hole bands and the exchange interaction between the magnetic ions and the itinerant holes. We study the quasi-particle and impurity bands in the paramagnetic and ferromagnetic phases for different values of impurity-hole coupling at the Mn doping of x=0.05. By analyzing the anisotropic angular distribution of the impurity band carriers at T=0, we conclude that the carrier polarization is optimal when the carriers move along the direction parallel to the average magnetization.Comment: 6 pages, 4 figure

Lifshitz Transition in the Two Dimensional Hubbard Model

Using large-scale dynamical cluster quantum Monte Carlo simulations, we study the Lifshitz transition of the two dimensional Hubbard model with next-nearest-neighbor hopping ($t'$), chemical potential and temperature as control parameters. At $t'\le0$, we identify a line of Lifshitz transition points associated with a change of the Fermi surface topology at zero temperature. In the overdoped region, the Fermi surface is complete and electron-like; across the Lifshitz transition, the Fermi surface becomes hole-like and develops a pseudogap. At (or very close to) the Lifshitz transition points, a van Hove singularity in the density of states crosses the Fermi level. The van Hove singularity occurs at finite doping due to correlation effects, and becomes more singular when $t'$ becomes more negative. The resulting temperature dependence on the bare d-wave pairing susceptibility close to the Lifshitz points is significantly different from that found in the traditional van Hove scenarios. Such unambiguous numerical observation of the Lifshitz transition at $t'\le0$ extends our understanding of the quantum critical region in the phase diagram, and shines lights on future investigations of the nature of the quantum critical point in the two dimensional Hubbard model.Comment: 9 pages, 8 figures, accepted for publication in Physics Review

Dual-fermion approach to the Anderson-Hubbard model

We apply the recently developed dual fermion algorithm for disordered interacting systems to the Anderson-Hubbard model. This algorithm is compared with dynamical cluster approximation calculations for a one-dimensional system to establish the quality of the approximation in comparison with an established cluster method. We continue with a three-dimensional (3d) system and look at the antiferromagnetic, Mott and Anderson localization transitions. The dual fermion approach leads to quantitative as well as qualitative improvement of the dynamical mean-field results and it allows one to calculate the hysteresis in the double occupancy in 3d taking into account nonlocal correlations

Electron microscopy study of CeOx–Pd/α-Al2O3 catalysts for methane dry reforming

We have investigated the interaction between Pd and Ce in a 0.47 wt % CeOx–Pd 1 wt %/ -Al2O3 catalyst that is used in the reforming reaction of CH4 with CO2. The freshly reduced catalyst was characterized by various electron microscopy techniques, such as elemental mapping, Z-contrast imaging, and electron energy-loss spectroscopy to understand the role of Ce on a microscopic scale. The high spatial resolution elemental mapping indicates that CeOx is located in close proximity of the palladium nanoparticles. High-resolution lattice images and energy-loss spectra obtained in the vicinity of the Pd particles show an anisotropic distribution of CeOx crystallites limited to the interface region between Pd and the substrate but not covering the surface of the Pd nanoparticles. Energy-loss near edge fine structure of Pd M edges reveals that the Pd nanoparticles are not oxidized.Fil: Moreno, M. S.. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Wang, Feiya. National Institute for Nanotechnology; CanadáFil: Malac, M.. National Institute for Nanotechnology; CanadáFil: Kasama, T.. University of Cambridge; Reino UnidoFil: Gigola, Carlos Eugenio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Planta Piloto de Ingeniería Química. Universidad Nacional del Sur. Planta Piloto de Ingeniería Química; ArgentinaFil: Costilla, Ignacio Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Planta Piloto de Ingeniería Química. Universidad Nacional del Sur. Planta Piloto de Ingeniería Química; ArgentinaFil: Sánchez, M. D.. Universidad Nacional del Sur. Departamento de Física; Argentin