13,662 research outputs found
Two-Dimensional Electrons in a Strong Magnetic Field with Disorder: Divergence of the Localization Length
Electrons on a square lattice with half a flux quantum per plaquette are
considered. An effective description for the current loops is given by a
two-dimensional Dirac theory with random mass. It is shown that the
conductivity and the localization length can be calculated from a product of
Dirac Green's functions with the {\it same} frequency. This implies that the
delocalization of electrons in a magnetic field is due to a critical point in a
phase with a spontaneously broken discrete symmetry. The estimation of the
localization length is performed for a generalized model with fermion
levels using a --expansion and the Schwarz inequality. An argument for the
existence of two Hall transition points is given in terms of percolation
theory.Comment: 10 pages, RevTeX, no figure
Lower Bound for the Fermi Level Density of States of a Disordered D-Wave Superconductor in Two Dimensions
We consider a disordered d--wave superconductor in two dimensions. Recently,
we have shown in an exact calculation that for a lattice model with a
Lorentzian distributed random chemical potential the quasiparticle density of
states at the Fermi level is nonzero. As the exact result holds only for the
special choice of the Lorentzian, we employ different methods to show that for
a large class of distributions, including the Gaussian distribution, one can
establish a nonzero lower bound for the Fermi level density of states. The fact
that the tails of the distributions are unimportant in deriving the lower bound
shows that the exact result obtained before is generic.Comment: 15 preprint pages, no figures, submitted to PR
Effect of reduction of strategic Columbium addition in 718 Alloy on the structure and properties
A series of alloys was developed having a base composition similar to Inconel 718, with reduced Cb levels of 3.00 and 1.10 wt% Cb. Substitutions of 3.0% W, 3.0W + 0.9V or Mo increased from 3.0% to 5.8% were made for the Cb in these alloys. Two additional alloys, one containing 3.49% Cb and 1.10% Ti and another containing 3.89% Cb and 1.29% Ti were also studied. Tensile properties at rooom and elevated temperatures, stress-rupture tests, and an analysis of extracted phases were carried out for each of the alloys. Additions of solid solution elements to a reduced Cb alloy had no significant effect on the properties of the alloys under either process condition. The solution and age alloys with substitutions of 1.27% i at 3.89% Cb had tensile properties similar top hose of the original alloy and stress-rupture properties superior to the original alloy. The improved stress-rupture properties were the result of significant precipitation of Ni3Ti-gamma prime in the alloy, which is more stable than gamma' at the elevated temperatures. At lower temperatures, the new alloy benefits from gamma' strengthening. With more precise control and proper processing, the reduced Cb direct-age alloy could substitute for Alloy 718 in high strength applications
BD-22 3467, a DAO-type star exciting the nebula Abell 35
Spectral analyses of hot, compact stars with NLTE (non-local thermodynamical
equilibrium) model-atmosphere techniques allow the precise determination of
photospheric parameters. The derived photospheric metal abundances are crucial
constraints for stellar evolutionary theory.
Previous spectral analyses of the exciting star of the nebula A 35, BD-22
3467, were based on He+C+N+O+Si+Fe models only. For our analysis, we use
state-of-the-art fully metal-line blanketed NLTE model atmospheres that
consider opacities of 23 elements from hydrogen to nickel. For the analysis of
high-resolution and high-S/N (signal-to-noise) FUV (far ultraviolet, FUSE) and
UV (HST/STIS) observations, we combined stellar-atmosphere models and
interstellar line-absorption models to fully reproduce the entire observed UV
spectrum.
The best agreement with the UV observation of BD-22 3467 is achieved at Teff
= 80 +/- 10 kK and log g =7.2 +/- 0.3. While Teff of previous analyses is
verified, log g is significantly lower. We re-analyzed lines of silicon and
iron (1/100 and about solar abundances, respectively) and for the first time in
this star identified argon, chromium, manganese, cobalt, and nickel and
determined abundances of 12, 70, 35, 150, and 5 times solar, respectively. Our
results partially agree with predictions of diffusion models for DA-type white
dwarfs. A combination of photospheric and interstellar line-absorption models
reproduces more than 90 % of the observed absorption features. The stellar mass
is M ~ 0.48 Msun.
BD-22 3467 may not have been massive enough to ascend the asymptotic giant
branch and may have evolved directly from the extended horizontal branch to the
white dwarf state. This would explain why it is not surrounded by a planetary
nebula. However, the star, ionizes the ambient interstellar matter, mimicking a
planetary nebula.Comment: 13 pages, 17 figure
Interacting bosons in an optical lattice: Bose-Einstein condensates and Mott insulator
A dense Bose gas with hard-core interaction is considered in an optical
lattice. We study the phase diagram in terms of a special mean-field theory
that describes a Bose-Einstein condensate and a Mott insulator with a single
particle per lattice site for zero as well as for non-zero temperatures. We
calculate the densities, the excitation spectrum and the static structure
factor for each of these phases.Comment: 17 pages, 5 figures; 1 figure added, typos remove
Integer Quantum Hall Effect for Lattice Fermions
A two-dimensional lattice model for non-interacting fermions in a magnetic
field with half a flux quantum per plaquette and levels per site is
considered. This is a model which exhibits the Integer Quantum Hall Effect
(IQHE) in the presence of disorder. It presents an alternative to the
continuous picture for the IQHE with Landau levels. The large limit can be
solved: two Hall transitions appear and there is an interpolating behavior
between the two Hall plateaux. Although this approach to the IQHE is different
from the traditional one with Landau levels because of different symmetries
(continuous for Landau levels and discrete here), some characteristic features
are reproduced. For instance, the slope of the Hall conductivity is infinite at
the transition points and the electronic states are delocalized only at the
transitions.Comment: 9 pages, Plain-Te
Density of states "width parity" effect in d-wave superconducting quantum wires
We calculate the density of states (DOS) in a clean mesoscopic d-wave
superconducting quantum wire, i.e. a sample of infinite length but finite width
. For open boundary conditions, the DOS at zero energy is found to be zero
if is even, and nonzero if is odd. At finite chemical potential, all
chains are gapped but the qualtitative differences between even and odd
remain.Comment: 7 pages, 8 figures, new figures and extended discussio
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