708 research outputs found
Hubbard physics in the symmetric half-filled periodic Anderson-Hubbard model
Two very different methods -- exact diagonalization on finite chains and a
variational method -- are used to study the possibility of a metal-insulator
transition in the symmetric half-filled periodic Anderson-Hubbard model. With
this aim we calculate the density of doubly occupied sites as a function of
various parameters. In the absence of on-site Coulomb interaction ()
between electrons, the two methods yield similar results. The double
occupancy of levels remains always finite just as in the one-dimensional
Hubbard model. Exact diagonalization on finite chains gives the same result for
finite , while the Gutzwiller method leads to a Brinkman-Rice transition
at a critical value (), which depends on and .Comment: 10 pages, 5 figure
Measuring nonlinear stresses generated by defects in 3D colloidal crystals
The mechanical, structural and functional properties of crystals are
determined by their defects and the distribution of stresses surrounding these
defects has broad implications for the understanding of transport phenomena.
When the defect density rises to levels routinely found in real-world
materials, transport is governed by local stresses that are predominantly
nonlinear. Such stress fields however, cannot be measured using conventional
bulk and local measurement techniques. Here, we report direct and spatially
resolved experimental measurements of the nonlinear stresses surrounding
colloidal crystalline defect cores, and show that the stresses at vacancy cores
generate attractive interactions between them. We also directly visualize the
softening of crystalline regions surrounding dislocation cores, and find that
stress fluctuations in quiescent polycrystals are uniformly distributed rather
than localized at grain boundaries, as is the case in strained atomic
polycrystals. Nonlinear stress measurements have important implications for
strain hardening, yield, and fatigue.Comment: in Nature Materials (2016
DSCOVR-EPIC MAIAC AOD - A Proxy for Understanding Aerosol Diurnal Patterns from Space
The Deep Space Climate Observatory (DSCOVR) spacecraft was launched on 11 February 2015 and in June 2015 achieved its orbit at the first Lagrange point (L1), 1.5 million km from Earth toward the sun. The Earth Polychromatic Imaging Camera (EPIC) onboard DSCOVR views the entire sunlit Earth from sunrise to sunset, every 1-2 hours, at scattering angles between 168.5 and 175.5 with 10 narrowband filters in the range of 317-779 nm. NASA Multiangle Implementation of Atmospheric Correction (MAIAC) algorithm, originally developed for MODIS, has been applied to EPIC data with an Aerosol Optical Depth (AOD) product at 440nm with a 10km spatial resolution. This high temporal resolution product is a unique dataset for investigating diurnal patterns in aerosols from space. Our work analyzed the capability of the satellite-borne data to capture the aerosol diurnal variation by associating it with AERONET AOD at 440nm data over the contiguous US. We validated the DSCOVR MAIAC AOD data over 100 AERONET stations during 2015-2018, and examined the contribution of the surface reflectance and relevant acquisition angles, derived by the MAIAC algorithm, to the predicted error. We used over 180,000 hourly DSCOVR-EPIC MAIAC AOD observations with collocated with AERONET AOD observations averaged over +-30 minutes from the satellite overpass time. The AERONET and DSCOVR AOD temporal patterns show that the diurnal variation is different across US AERONET sites, with higher diurnal variation in the DSCOVR dataset in general
Topological mechanics of origami and kirigami
Origami and kirigami have emerged as potential tools for the design of
mechanical metamaterials whose properties such as curvature, Poisson ratio, and
existence of metastable states can be tuned using purely geometric criteria. A
major obstacle to exploiting this property is the scarcity of tools to identify
and program the flexibility of fold patterns. We exploit a recent connection
between spring networks and quantum topological states to design origami with
localized folding motions at boundaries and study them both experimentally and
theoretically. These folding motions exist due to an underlying topological
invariant rather than a local imbalance between constraints and degrees of
freedom. We give a simple example of a quasi-1D folding pattern that realizes
such topological states. We also demonstrate how to generalize these
topological design principles to two dimensions. A striking consequence is that
a domain wall between two topologically distinct, mechanically rigid structures
is deformable even when constraints locally match the degrees of freedom.Comment: 5 pages, 3 figures + ~5 pages S
Stellar Collisions in the Galactic Center: Massive Stars, Collision Remnants, and Missing Red Giants
Like most galaxies, the Milky Way harbors a supermassive black hole (SMBH) at
its center, surrounded by a nuclear star cluster. In this dense star cluster,
direct collisions can occur between stars before they evolve off the
main-sequence. Using a statistical approach, we characterize the outcomes of
these stellar collisions within the inner parsec of the Galactic Center (GC).
Close to the SMBH, where the velocity dispersion is larger than the escape
speed from a Sun-like star, collisions lead to mass loss. We find that the
stellar population within pc is halved within about a Gyr because of
destructive collisions. Additionally, we predict a diffuse population of
peculiar low-mass stars in the GC. These stars have been divested of their
outer layers in the inner pc before migrating to larger distances from
the SMBH. Between and pc from the SMBH, collisions can result in
mergers. Our results suggest that repeated collisions between lower mass stars
can produce massive ( M) stars, and there may be
of them residing in this region. We provide predictions on the number of G
objects, dust and gas enshrouded stellar objects, that may result from
main-sequence stellar collisions. Lastly, we comment on uncertainties in our
model and possible connections between stellar collisions and the missing red
giants in the GC.Comment: Submitted to ApJ. Comments welcome. 24 pages, 13 figure
The tryptophan pathway genes of the Sargasso Sea metagenome: new operon structures and the prevalence of non-operon organization
An analysis of the seven genes of the tryptophan pathway in the Sargasso Sea metagenome shows that the majority of contigs and scaffolds contain whole or split operons that are similar to previously analyzed trp gene organizations
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