6,391 research outputs found
Study of Unmanned Systems to Evaluate the Martian Environment. Volume IV - Summary
Unmanned space mission to determine Martian environmental factors influencing design of systems for manned space mission to Mars - summary repor
Study of Unmanned Systems to Evaluate the Martian Environment. Volume II - Experiments
Unmanned space mission to determine Martian environmental factors influencing design of systems for manned space mission to Mars - mission experiment requirement
DMRG evaluation of the Kubo formula -- Conductance of strongly interacting quantum systems
In this paper we present a novel approach combining linear response theory
(Kubo) for the conductance and the Density Matrix Renormalization Group (DMRG).
The system considered is one-dimensional and consists of non-interacting tight
binding leads coupled to an interacting nanostructure via weak links. Electrons
are treated as spinless fermions and two different correlation functions are
used to evaluate the conductance.
Exact diagonalization calculations in the non-interacting limit serve as a
benchmark for our combined Kubo and DMRG approach in this limit. Including both
weak and strong interaction we present DMRG results for an extended
nanostructure consisting of seven sites. For the strongly interacting structure
a simple explanation of the position of the resonances is given in terms of
hard-core particles moving freely on a lattice of reduced size.Comment: 7 pages, 2 figures. Minor typos correcte
The M31 Velocity Vector. I. Hubble Space Telescope Proper Motion Measurements
We present the first proper motion measurements for the galaxy M31. We
obtained new V-band imaging data with the HST ACS/WFC and WFC3/UVIS of a
spheroid field near the minor axis, an outer disk field along the major axis,
and a field on the Giant Southern Stream. The data provide 5-7 year time
baselines with respect to pre-existing deep first-epoch observations. We
measure the positions of thousands of M31 stars and hundreds of compact
background galaxies in each field. High accuracy and robustness is achieved by
building and fitting a unique template for each individual object. The average
proper motion for each field is obtained from the average motion of the M31
stars between the epochs with respect to the background galaxies. For the three
fields, the observed proper motions (mu_W,mu_N) are (-0.0458, -0.0376),
(-0.0533, -0.0104), and (-0.0179,-0.0357) mas/yr, respectively. The ability to
average over large numbers of objects and over the three fields yields a final
accuracy of 0.012 mas/yr. The robustness of the proper-motion measurements and
uncertainties are supported by the fact that data from different instruments,
taken at different times and with different telescope orientations, as well as
measurements of different fields, all yield statistically consistent results.
Papers II and III explore the implications for our understanding of the
history, future, and mass of the Local Group. (Abridged)Comment: 42 pages, 13 figures, to be published in ApJ. Version with high
resolution figures and N-body movies available at
http://www.stsci.edu/~marel/M31 . Press materials, graphics, and
visualizations available at
http://hubblesite.org/newscenter/archive/releases/2012/2
Low temperature terahertz spectroscopy of n-InSb through a magnetic field driven metal-insulator transition
We use fiber-coupled photoconductive emitters and detectors to perform
terahertz (THz) spectroscopy of lightly-doped n-InSb directly in the cryogenic
(1.5 K) bore of a high-field superconducting magnet. We measure transmission
spectra from 0.1-1.1 THz as the sample is driven through a metal-insulator
transition (MIT) by applied magnetic field. In the low-field metallic state,
the data directly reveal the plasma edge and magneto-plasmon modes. With
increasing field, a surprisingly broad band (0.3-0.8 THz) of low transmission
appears at the onset of the MIT. This band subsequently collapses and evolves
into the sharp 1s -> 2p- transition of electrons `frozen' onto isolated donors
in the insulating state.Comment: 4 pages, 3 figure
The M31 Velocity Vector. III. Future Milky Way-M31-M33 Orbital Evolution, Merging, and Fate of the Sun
We study the future orbital evolution and merging of the MW-M31-M33 system,
using a combination of collisionless N-body simulations and semi-analytic orbit
integrations. Monte-Carlo simulations are used to explore the consequences of
varying the initial phase-space and mass parameters within their observational
uncertainties. The observed M31 transverse velocity implies that the MW and M31
will merge t = 5.86 (+1.61-0.72) Gyr from now, after a first pericenter at t =
3.87 (+0.42-0.32) Gyr. M31 may (probability p=41%) make a direct hit with the
MW (defined here as a first-pericenter distance less than 25 kpc). Most likely,
the MW and M31 will merge first, with M33 settling onto an orbit around them.
Alternatively, M33 may make a direct hit with the MW first (p=9%), or M33 may
get ejected from the Local Group (p=7%). The MW-M31 merger remnant will
resemble an elliptical galaxy. The Sun will most likely (p=85%) end up at
larger radius from the center of the MW-M31 merger remnant than its current
distance from the MW center, possibly further than 50 kpc (p=10%). The Sun may
(p=20%) at some time in the next 10 Gyr find itself moving through M33 (within
10 kpc), but while dynamically still bound to the MW-M31 merger remnant. The
arrival and possible collision of M31 (and possibly M33) with the MW is the
next major cosmic event affecting the environment of our Sun and solar system
that can be predicted with some certainty. (Abridged)Comment: 58 pages, 16 figures, to be published in ApJ. Version with high
resolution figures and N-body movies available at
http://www.stsci.edu/~marel/M31 . Press materials, graphics, and
visualizations available at
http://hubblesite.org/newscenter/archive/releases/2012/2
Critical Currents of Josephson-Coupled Wire Arrays
We calculate the current-voltage characteristics and critical current
I_c^{array} of an array of Josephson-coupled superconducting wires. The array
has two layers, each consisting of a set of parallel wires, arranged at right
angles, such that an overdamped resistively-shunted junction forms wherever two
wires cross. A uniform magnetic field equal to f flux quanta per plaquette is
applied perpendicular to the layers. If f = p/q, where p and q are mutually
prime integers, I_c^{array}(f) is found to have sharp peaks when q is a small
integer. To an excellent approximation, it is found in a square array of n^2
plaquettes, that I_c^{array}(f) \propto (n/q)^{1/2} for sufficiently large n.
This result is interpreted in terms of the commensurability between the array
and the assumed q \times q unit cell of the ground state vortex lattice.Comment: 4 pages, 4 figure
Surface resonance of the (2×1) reconstructed lanthanum hexaboride (001)-cleavage plane : a combined STM and DFT study
We performed a combined study of the (001)-cleavage plane of lanthanum hexaboride (LaB6) using scanning tunneling microscopy and density-functional theory (DFT). Experimentally, we found a (2×1) reconstructed surface on a local scale. The reconstruction is only short-range ordered and tends to order perpendicularly to step edges. At larger distances from surface steps, the reconstruction evolves to a labyrinthlike pattern. These findings are supported by low-energy electron diffraction experiments. Slab calculations within the framework of DFT show that the atomic structure consists of parallel lanthanum chains on top of boron octahedra. Scanning tunneling spectroscopy shows a prominent spectral feature at −0.6eV. Using DFT, we identify this structure as a surface resonance of the (2×1) reconstructed LaB6 (100) surface which is dominated by boron dangling bond states and lanthanum d states
A hierarchical Dirichlet process mixture model for haplotype reconstruction from multi-population data
The perennial problem of "how many clusters?" remains an issue of substantial
interest in data mining and machine learning communities, and becomes
particularly salient in large data sets such as populational genomic data where
the number of clusters needs to be relatively large and open-ended. This
problem gets further complicated in a co-clustering scenario in which one needs
to solve multiple clustering problems simultaneously because of the presence of
common centroids (e.g., ancestors) shared by clusters (e.g., possible descents
from a certain ancestor) from different multiple-cluster samples (e.g.,
different human subpopulations). In this paper we present a hierarchical
nonparametric Bayesian model to address this problem in the context of
multi-population haplotype inference. Uncovering the haplotypes of single
nucleotide polymorphisms is essential for many biological and medical
applications. While it is uncommon for the genotype data to be pooled from
multiple ethnically distinct populations, few existing programs have explicitly
leveraged the individual ethnic information for haplotype inference. In this
paper we present a new haplotype inference program, Haploi, which makes use of
such information and is readily applicable to genotype sequences with thousands
of SNPs from heterogeneous populations, with competent and sometimes superior
speed and accuracy comparing to the state-of-the-art programs. Underlying
Haploi is a new haplotype distribution model based on a nonparametric Bayesian
formalism known as the hierarchical Dirichlet process, which represents a
tractable surrogate to the coalescent process. The proposed model is
exchangeable, unbounded, and capable of coupling demographic information of
different populations.Comment: Published in at http://dx.doi.org/10.1214/08-AOAS225 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
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