1,321 research outputs found
Glass Dynamics at High Strain Rates
We present a shear-transformation-zone (STZ) theoretical analysis of
molecular-dynamics simulations of a rapidly sheared metallic glass. These
simulations are especially revealing because, although they are limited to high
strain rates, they span temperatures ranging from well below to well above the
glass transition. With one important discrepancy, the STZ theory reproduces the
simulation data, including the way in which those data can be made to collapse
onto simple curves by a scaling transformation. The STZ analysis implies that
the system's behavior at high strain rates is controlled primarily by
effective-temperature thermodynamics, as opposed to system-specific details of
the molecular interactions. The discrepancy between theory and simulations
occurs at the lower strain rates for temperatures near the glass transition. We
argue that this discrepancy can be resolved by the same multi-species
generalization of STZ theory that has been proposed recently for understanding
frequency-dependent viscoelastic responses, Stokes-Einstein violations, and
stretched-exponential relaxation in equilibrated glassy materials.Comment: 9 pages, 6 figure
Direct observation of the formation of polar nanoregions in Pb(MgNb)O using neutron pair distribution function analysis
Using neutron pair distribution function (PDF) analysis over the temperature
range from 1000 K to 15 K, we demonstrate the existence of local polarization
and the formation of medium-range, polar nanoregions (PNRs) with local
rhombohedral order in a prototypical relaxor ferroelectric
Pb(MgNb)O. We estimate the volume fraction of the PNRs as a
function of temperature and show that this fraction steadily increases from 0 %
to a maximum of 30% as the temperature decreases from 650 K to 15 K.
Below T200 K the PNRs start to overlap as their volume fraction reaches
the percolation threshold. We propose that percolating PNRs and their
concomitant overlap play a significant role in the relaxor behavior of
Pb(MgNb)O.Comment: 4 pages, 3 figure
Orbital correlations in the pseudo-cubic \emph{O} and rhombohedral -phases of LaMnO
The local and intermediate structure of stoichiometric LaMnO has been
studied in the pseudocubic and rhombohedral phases at high temperatures (300 to
1150 K). Neutron powder diffraction data were collected and a combined Rietveld
and high real space resolution atomic pair distribution function analysis
carried out. The nature of the Jahn-Teller (JT) transition around 750 K is
confirmed to be orbital order to disorder. In the high temperature orthorhombic
() and rhombohedral () phases the MnO octahedra are still fully
distorted locally. The data suggest the presence of local orbitally ordered
clusters of diameter \AA (four MnO octahedra) implying
strong nearest neighbor JT anti-ferrodistortive coupling.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Le
The Nature of the Compact/Symmetric Near-IR Continuum Source in 4C 40.36
Using NICMOS on HST, we have imaged the emission-line nebulae and the
line-free continuum in 4C 40.36, a ultra-steep spectrum FR II radio galaxy at
z=2.269. The line-free continuum was found to be extremely compact and
symmetric while the emission-line nebulae seen in H-alpha+[N II] show very
clumpy structures spreading almost linearly over 16 kpc. However, this linear
structure is clearly misaligned from the radio axis. The SED of the line-free
continuum is very flat, suggesting that if the continuum emission is produced
by a single source, it is likely to be a young bursting stellar population or
scattered AGN light. However, because of the lack of a line-free optical image
with a comparable spatial resolution, we cannot exclude the possibility that
the observed SED is a composite of a young blue population and an old red
population.Comment: 4 pages, 2 figures; to appear in the proceedings of "The Hy-Redshift
Universe: Galaxy Formation and Evolution at High Redshift", eds. A.J.Bunker
and W. J. M. van Breuge
Nanoscale clusters in the high performance thermoelectric AgPbmSbTem+2
The local structure of the AgPbmSbTem+2 series of thermoelectric materials
has been studied using the atomic pair distribution function (PDF) method.
Three candidate-models were attempted for the structure of this class of
materials using either a one-phase or a two-phase modeling procedure. Combining
modeling the PDF with HRTEM data we show that AgPbmSbTem+2 contains nanoscale
inclusions with composition close to AgPb3SbTe5 randomly embedded in a PbTe
matrix.Comment: 7 pages, 5 figures, 2 tables, submitted to PR
Local Fluctuations and Ordering in Liquid and Amorphous Metals
A molecular-dynamics study of the structure and dynamics of monatomic liquids and glasses is presented. The local atomic structure and its development during the quenching process are analyzed in terms of fluctuations of atomic-level stresses and their correlations. This approach extends the basis for the description of the local structure from the usually employed scalar quantity, the local density fluctuation, to a tensorial quantity, the local stress fluctuation. It is shown here that the local stress fluctuations and their spatial and temporal correlations provide a detailed picture of the dynamics of the liquid and of the transition from an ideal fluid to a viscous liquid, and then to a glass. In particular, it is demonstrated that the shear stresses which are spatially uncorrelated at high temperatures become correlated below a temperature, Ts, which is about twice the glass transition temperature. At the same time the dynamic behavior of the liquid, characterized by the diffusivity, viscosity, and phonon states, changes sharply at this temperature. Implications of this apparent structural transition and its origin are then discussed
Deep Infrared Imaging of the Microquasars 1E1740-2942 and GRS 1758-258
We present deep infrared (m) imaging of the Galactic microquasars
1E1740-2942 and GRS 1758-258 using the Keck-I 10-meter telescope in June 1998.
The observations were taken under excellent seeing conditions (\sim 0.45
\arcsec full-width half-maximum), making them exceptionally deep for these
crowded fields. We used the USNO-A2.0 catalog to astrometrically calibrate the
infrared images (along with an optical CCD image in the case of GRS 1758-258),
providing independent frame ties to the known radio positions of the objects.
For 1E1740-2942, we confirm potential candidates for the microquasar previously
identified by Marti et al., and show that none of the objects near the
microquasar have varied significantly from 1998 to 1999. For GRS 1758-258, our
astrometry indicates a position shifted from previous reports of candidates for
the microquasar. We find no candidates inside our 90% confidence radius to a limiting magnitude of mag. We discuss the implications of
these results for the nature of the microquasar binary systems.Comment: To appear in the Astrophysical Journal; 15 pages, including 4 figure
First-principles transport calculation method based on real-space finite-difference nonequilibrium Green's function scheme
We demonstrate an efficient nonequilibrium Green's function transport
calculation procedure based on the real-space finite-difference method. The
direct inversion of matrices for obtaining the self-energy terms of electrodes
is computationally demanding in the real-space method because the matrix
dimension corresponds to the number of grid points in the unit cell of
electrodes, which is much larger than that of sites in the tight-binding
approach. The procedure using the ratio matrices of the overbridging
boundary-matching technique [Phys. Rev. B {\bf 67}, 195315 (2003)], which is
related to the wave functions of a couple of grid planes in the matching
regions, greatly reduces the computational effort to calculate self-energy
terms without losing mathematical strictness. In addition, the present
procedure saves computational time to obtain Green's function of the
semi-infinite system required in the Landauer-B\"uttiker formula. Moreover, the
compact expression to relate Green's functions and scattering wave functions,
which provide a real-space picture of the scattering process, is introduced. An
example of the calculated results is given for the transport property of the BN
ring connected to (9,0) carbon nanotubes. The wave function matching at the
interface reveals that the rotational symmetry of wave functions with respect
to the tube axis plays an important role in electron transport. Since the
states coming from and going to electrodes show threefold rotational symmetry,
the states in the vicinity of the Fermi level, whose wave function exhibits
fivefold symmetry, do not contribute to the electron transport through the BN
ring.Comment: 34 page
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