33,929 research outputs found
Chemisorption on a model bcc metal
The system considered here is that of a single atom with one energy level chemisorbed on the (001) surface of a model bcc metal. We present the change in the density of electronic states Δn (E) due to chemisorption for two cases: one when the adatom is bound to a single substrate atom in the "on‐site" configuration and the other when it is bound to four substrate atoms in the "centered fourfold site." In principle, this change in the density of states Δn can be related to the results of photoemission measurements
Calibrated Langevin dynamics simulations of intrinsically disordered proteins
We perform extensive coarse-grained (CG) Langevin dynamics simulations of
intrinsically disordered proteins (IDPs), which possess fluctuating
conformational statistics between that for excluded volume random walks and
collapsed globules. Our CG model includes repulsive steric, attractive
hydrophobic, and electrostatic interactions between residues and is calibrated
to a large collection of single-molecule fluorescence resonance energy transfer
data on the inter-residue separations for 36 pairs of residues in five IDPs:
-, -, and -synuclein, the microtubule-associated protein
, and prothymosin . We find that our CG model is able to
recapitulate the average inter-residue separations regardless of the choice of
the hydrophobicity scale, which shows that our calibrated model can robustly
capture the conformational dynamics of IDPs. We then employ our model to study
the scaling of the radius of gyration with chemical distance in 11 known IDPs.
We identify a strong correlation between the distance to the dividing line
between folded proteins and IDPs in the mean charge and hydrophobicity space
and the scaling exponent of the radius of gyration with chemical distance along
the protein.Comment: 16 pages, 10 figure
Inflatonless Inflation
We consider a 4+N dimensional Einstein gravity coupled to a non-linear sigma
model. This theory admits a solution in which the N extra dimensions contract
exponentially while the ordinary space expand exponentially. Physically, the
non-linear sigma fields induce the dynamical compactification of the extra
dimensions, which in turn drives inflation. No inflatons are required.Comment: 12 pages, version to appear in IJMP
Will the U.S. productivity resurgence continue?
U.S. productivity growth has accelerated in recent years, despite a series of negative economic shocks. An analysis of the sources of this growth over the 1995-2003 period suggests that the production and use of information technology account for a large share of the gains. The authors project that during the next decade, private sector productivity growth will continue at a rate of 2.6 percent per year, a significant increase from their 2002 projection of 2.2 percent growth.Productivity ; Information technology
Spinor Bose Condensates in Optical Traps
In an optical trap, the ground state of spin-1 Bosons such as Na,
K, and Rb can be either a ferromagnetic or a "polar" state,
depending on the scattering lengths in different angular momentum channel. The
collective modes of these states have very different spin character and spatial
distributions. While ordinary vortices are stable in the polar state, only
those with unit circulation are stable in the ferromagnetic state. The
ferromagnetic state also has coreless (or Skyrmion) vortices like those of
superfluid He-A. Current estimates of scattering lengths suggest that the
ground states of Na and Rb condensate are a polar state and a
ferromagnetic state respectively.Comment: 11 pages, no figures. email : [email protected]
Projecting productivity growth: lessons from the U.S. growth resurgence
Following the 1995-2000 period of more rapid output growth and lower inflation in the United States, economists have strenuously debated whether improvements in economic performance can be sustained. The recession that began in March 2001 intensified the debate, and the economic impacts of the events of September 11 have yet to be fully understood. Both factors add to the considerable uncertainties about future growth that currently face decision makers in both the public and private sectors. ; In this article, the authors analyze the sources of U.S. labor productivity growth in the post-1995 period and present projections for both output and labor productivity growth for the next decade. Despite the 2001 downward revisions to U.S. gross domestic product and software investment, the authors show that information technology (IT) played a substantial role in the U.S. productivity revival. The article then outlines a methodology for projecting trend output and productivity growth. The base-case projection puts the rate of trend productivity growth at 2.21 percent per year over the next decade with a range of 1.33 to 2.92 percent, reflecting fundamental uncertainties about the rate of technological progress in IT-production and investment patterns. The central projection is only slightly below the average growth rate of 2.36 percent during the 1995-2000 period.Productivity ; Technology ; Economic development
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SLS Processing Studies of Nylon 11 Nanocomposites
Selective Laser Sintering (SLS) is widely used for rapid prototyping/manufacturing of
nylon 11 and nylon 12 parts. This processing technique has not been explored for
nylon nanocomposites. This study investigates the technicalities of processing nylon
11-clay and nylon-carbon nanofiber nanocomposites with SLS. Microstructural
analyses of the SLS powders and parts were conducted under SEM. Results suggest
that SLS processing is possible with the new nylon 11 nanocomposites. Yet the SLS
parts built have inferior properties relative to those of injection molding, suggesting
that more fine tuning for the processing is required.Mechanical Engineerin
Radio Emission from the Intermediate-mass Black Hole in the Globular Cluster G1
We have used the Very Large Array (VLA) to search for radio emission from the
globular cluster G1 (Mayall-II) in M31. G1 has been reported by Gebhardt et al.
to contain an intermediate-mass black hole (IMBH) with a mass of ~2 x 10^4
solar masses. Radio emission was detected within an arcsecond of the cluster
center with an 8.4 GHz power of 2 x 10^{15} W/Hz. The radio/X-ray ratio of G1
is a few hundred times higher than that expected for a high-mass X-ray binary
in the cluster center, but is consistent with the expected value for accretion
onto an IMBH with the reported mass. A pulsar wind nebula is also a possible
candidate for the radio and X-ray emission from G1; future high-sensitivity
VLBI observations might distinguish between this possibility and an IMBH. If
the radio source is an IMBH, and similar accretion and outflow processes occur
for hypothesized ~ 1000-solar-mass black holes in Milky Way globular clusters,
they are within reach of the current VLA and should be detectable easily by the
Expanded VLA when it comes on line in 2010.Comment: ApJ Letters, accepted, 11 pages, 1 figur
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