26,675 research outputs found
Heitler-London model for acceptor-acceptor interactions in doped semiconductors
The interactions between acceptors in semiconductors are often treated in
qualitatively the same manner as those between donors. Acceptor wave functions
are taken to be approximately hydrogenic and the standard hydrogen molecule
Heitler-London model is used to describe acceptor-acceptor interactions. But
due to valence band degeneracy and spin-orbit coupling, acceptor states can be
far more complex than those of hydrogen atoms, which brings into question the
validity of this approximation. To address this issue, we develop an
acceptor-acceptor Heitler-London model using single-acceptor wave functions of
the form proposed by Baldereschi and Lipari, which more accurately capture the
physics of the acceptor states. We calculate the resulting acceptor-pair energy
levels and find, in contrast to the two-level singlet-triplet splitting of the
hydrogen molecule, a rich ten-level energy spectrum. Our results, computed as a
function of inter-acceptor distance and spin-orbit coupling strength, suggest
that acceptor-acceptor interactions can be qualitatively different from
donor-donor interactions, and should therefore be relevant to the control of
two-qubit interactions in acceptor-based qubit implementations, as well as the
magnetic properties of a variety of p-doped semiconductor systems. Further
insight is drawn by fitting numerical results to closed-form energy-level
expressions obtained via an acceptor-acceptor Hubbard model.Comment: 19 pages, 10 figures, text revised, figure quality improved,
additional references adde
The Pure Spinor Formulation of Superstrings
In this lectures we outline the construction of pure spinor superstrings. We
consider both the open and closed pure spinor superstrings in critical and
noncritical dimensions and on flat and curved target spaces with RR flux. We
exhibit the integrability properties of pure spinor superstrings on curved
backgrounds with RR fluxes.Comment: These lectures have been given in the RTN Winter School on Strings,
Supergravity and Gauge Theories, CERN (2008). 32 pages, a typo correcte
The 2MASS Wide-Field T Dwarf Search. II. Discovery of Three T Dwarfs in the Southern Hemisphere
We present the discovery of three new Southern Hemisphere T dwarfs identified
in the Two Micron All Sky Survey. These objects, 2MASS 0348-6022, 2MASS
0516-0445, and 2MASS 2228-4310, have classifications T7, T5.5, and T6.5,
respectively. Using linear absolute magnitude/spectral type relations derived
from T dwarfs with measured parallaxes, we estimate spectrophotometric
distances for these discoveries; the closest, 2MASS 0348-6022, is likely within
10 pc of the Sun. Proper motions and estimated tangential velocities are
consistent with membership in the Galactic disk population. We also list
Southern Hemisphere T dwarf candidates that were either not found in subsequent
near-infrared imaging observations and are most likely uncatalogued minor
planets, or have near-infrared spectra consistent with background stars.Comment: 12 pages including 4 figures (one as jpeg), accepted to A
Weak-Field Thermal Hall Conductivity in the Mixed State of d-Wave Superconductors
Thermal transport in the mixed state of a d-wave superconductor is considered
within the weak-field regime. We express the thermal conductivity,
, and the thermal Hall conductivity, , in terms of
the cross section for quasiparticle scattering from a single vortex. Solving
for the cross section (neglecting the Berry phase contribution and the
anisotropy of the gap nodes), we obtain and
in surprisingly good agreement with the qualitative features
of the experimental results for YBaCuO. In particular, we
show that the simple, yet previously unexpected, weak-field behavior,
, is that of thermally-excited nodal
quasiparticles, scattering primarily from impurities, with a small skew
component provided by vortex scattering.Comment: 5 pages, 2 figures; final version as published in Phys Rev Let
Beyond the T Dwarfs: Theoretical Spectra, Colors, and Detectability of the Coolest Brown Dwarfs
We explore the spectral and atmospheric properties of brown dwarfs cooler
than the latest known T dwarfs. Our focus is on the yet-to-be-discovered
free-floating brown dwarfs in the \teff range from 800 K to 130 K
and with masses from 25 to 1 \mj. This study is in anticipation of the new
characterization capabilities enabled by the launch of SIRTF and the eventual
launch of JWST. We provide spectra from 0.4 \mic to 30 \mic, highlight
the evolution and mass dependence of the dominant HO, CH, and NH
molecular bands, consider the formation and effects of water-ice clouds, and
compare our theoretical flux densities with the sensitivities of the
instruments on board SIRTF and JWST. The latter can be used to determine the
detection ranges from space of cool brown dwarfs. In the process, we determine
the reversal point of the blueward trend in the near-infrared colors with
decreasing \teff, the \teffs at which water and ammonia clouds appear, the
strengths of gas-phase ammonia and methane bands, the masses and ages of the
objects for which the neutral alkali metal lines are muted, and the increasing
role as \teff decreases of the mid-infrared fluxes longward of 4 \mic. These
changes suggest physical reasons to expect the emergence of at least one new
stellar class beyond the T dwarfs. Our spectral models populate, with cooler
brown dwarfs having progressively more planet-like features, the theoretical
gap between the known T dwarfs and the known giant planets. Such objects likely
inhabit the galaxy, but their numbers are as yet unknown.Comment: Includes 14 figures, most in color; accepted to the Astrophysical
Journa
SALT Long-slit Spectroscopy of Luminous Obscured Quasars: An Upper Limit on the Size of the Narrow-Line Region?
We present spatially resolved long-slit spectroscopy from the Southern
African Large Telescope (SALT) to examine the spatial extent of the narrow-line
regions (NLRs) of a sample of 8 luminous obscured quasars at 0.10 < z < 0.43.
Our results are consistent with an observed shallow slope in the relationship
between NLR size and L_[OIII], which has been interpreted to indicate that NLR
size is limited by the density and ionization state of the NLR gas rather than
the availability of ionizing photons. We also explore how the NLR size scales
with a more direct measure of instantaneous AGN power using mid-IR photometry
from WISE, which probes warm to hot dust near the central black hole and so,
unlike [OIII], does not depend on the properties of the NLR. Using our results
as well as samples from the literature, we obtain a power-law relationship
between NLR size and L_8micron that is significantly steeper than that observed
for NLR size and L_[OIII]. We find that the size of the NLR goes approximately
as L^(1/2)_8micron, as expected from the simple scenario of constant-density
clouds illuminated by a central ionizing source. We further see tentative
evidence for a flattening of the relationship between NLR size and L_8micron at
the high luminosity end, and propose that we are seeing a limiting NLR size of
10 - 20 kpc, beyond which the availability of gas to ionize becomes too low. We
find that L_[OIII] ~ L_8micron^(1.4), consistent with a picture in which the
L_[OIII] is dependent on the volume of the NLR. These results indicate that
high-luminosity quasars have a strong effect in ionizing the available gas in a
galaxy.Comment: 9 Pages, 5 figures, accepted to Ap
Gut microbiota dictates the metabolic response of Drosophila to diet
Animal nutrition is profoundly influenced by the gut microbiota, but knowledge of the scope and core mechanisms of the underlying animal-microbiota interactions is fragmentary. To investigate the nutritional traits shaped by the gut microbiota of Drosophila, we determined the microbiota-dependent response of multiple metabolic and performance indices to systematically varied diet composition. Diet-dependent differences between Drosophila bearing its unmanipulated microbiota (conventional flies) and experimentally deprived of its microbiota (axenic flies) revealed evidence for: microbial sparing of dietary B vitamins, especially riboflavin, on low-yeast diets; microbial promotion of protein nutrition, particularly in females; and microbiota-mediated suppression of lipid/carbohydrate storage, especially on high sugar diets. The microbiota also sets the relationship between energy storage and body mass, indicative of microbial modulation of the host signaling networks that coordinate metabolism with body size. This analysis identifies the multiple impacts of the microbiota on the metabolism of Drosophila, and demonstrates that the significance of these different interactions varies with diet composition and host sex
Memory, space and time: Researching children's lives
This article discusses the research approach in 'Pathways through Childhood', a small qualitative study drawing on memories of childhood. The research explores how wider social arrangements and social change influence children's everyday lives.The article discusses the way that the concepts of social memory, space and time have been drawn on to access and analyse children's experiences, arguing that attention to the temporal and spatial complexity of childhood reveals less visible yet formative influences and connections. Children's everyday engagements involve connections between past and present time, between children, families, communities and nations, and between different places. Children carve out space and time for themselves from these complex relations. © The Author(s) 2010
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