17,129 research outputs found
A molecular dynamics simulation of water confined in a cylindrical SiO2 pore
A molecular dynamics simulation of water confined in a silica pore is
performed in order to compare it with recent experimental results on water
confined in porous Vycor glass at room temperature. A cylindrical pore of 40 A
is created inside a vitreous SiO2 cell, obtained by computer simulation. The
resulting cavity offers to water a rough hydrophilic surface and its geometry
and size are similar to those of a typical pore in porous Vycor glass. The
site-site distribution functions of water inside the pore are evaluated and
compared with bulk water results. We find that the modifications of the
site-site distribution functions, induced by confinement, are in qualitative
agreement with the recent neutron diffraction experiment, confirming that the
disturbance to the microscopic structure of water mainly concerns orientational
arrangement of neighbouring molecules. A layer analysis of MD results indicates
that, while the geometrical constraint gives an almost constant density profile
up to the layers closest to the interface, with an uniform average number of
hydrogen bonds (HB), the hydrophilic interaction produces the wetting of the
pore surface at the expenses of the adjacent water layers. Moreover the
orientational disorder togheter with a reduction of the average number of HB
persists in the layers close to the interface, while water molecules cluster in
the middle of the pore at a density and with a coordination similar to bulk
water.Comment: RevTex, 11 pages, 12 figures; to appear in June 15 issue of J. Chem.
Phy
A model for liquid-striped liquid phase separation in liquids of anisotropic polarons
The phase separation between a striped polaron liquid at the particular
density and a high density polaron liquid is described by a modified Van der
Waals scheme. The striped polaron liquid represents the pseudo gap matter or
Wigner-like polaron phase at 1/8 doping in cuprate superconductors. The model
includes the tendency of pseudo- Jahn-Teller polarons to form anisotropic
directional bonds at a preferential volume with the formation of different
liquid phases. The model gives the coexistence of a first low density polaron
striped liquid and a second high density liquid that appears in cuprate
superconductors for doping larger than 1/8. We discuss how the strength of
anisotropic bonds controls the variation the phase separation scenarios for
complex systems in the presence of a quantum critical point where the phase
separation vanishes.Comment: 10 pages, 3 figure
Influence of the Lower Hybrid Drift Instability on the onset of Magnetic Reconnection
Two-dimensional and three-dimensional kinetic simulation results reveal the
importance of the Lower-Hybrid Drift Instability LHDI to the onset of magnetic
reconnection. Both explicit and implicit kinetic simulations show that the LHDI
heats electrons anisotropically and increases the peak current density. Linear
theory predicts these modifications can increase the growth rate of the tearing
instability by almost two orders of magnitude and shift the fastest growing
modes to significantly shorter wavelengths. These predictions are confirmed by
nonlinear kinetic simulations in which the growth and coalescence of small
scale magnetic islands leads to a rapid onset of large scale reconnection
BeppoSAX LECS background subtraction techniques
We present 3 methods for the subtraction of non-cosmic and unresolved cosmic
backgrounds observed by the Low-Energy Concentrator Spectrometer (LECS)
on-board BeppoSAX. Removal of these backgrounds allows a more accurate modeling
of the spectral data from point and small-scale extended sources. At high
(>|25| degree) galactic latitudes, subtraction using a standard background
spectrum works well. At low galactic latitudes, or in complex regions of the
X-ray sky, two alternative methods are presented. The first uses counts
obtained from two semi-annuli near the outside of the LECS field of view to
estimate the background at the source location. The second method uses ROSAT
Position Sensitive Proportional Counter (PSPC) all-sky survey data to estimate
the LECS background spectrum for a given pointing position. A comparison of the
results from these methods provides an estimate of the systematic
uncertainties. For high galactic latitude fields, all 3 methods give 3 sigma
confidence uncertainties of <0.9 10^-3 count/s (0.1-10 keV), or <1.5 10^-3
count/s (0.1-2 keV). These correspond to 0.1-2.0 keV fluxes of 0.7-1.8 and
0.5-1.1 10^-13 erg/cm2/s for a power-law spectrum with a photon index of 2 and
photoelectric absorption of 3 10^20 and 3 10^21 atom/cm2, respectively. At low
galactic latitudes, or in complex regions of the X-ray sky, the uncertainties
are a factor ~2.5 higher.Comment: 13 pages. Accepted for publication in A&A
Programming Characteristics of Electrochemical Random Access Memory (ECRAM)—Part I: Experimental Study
Hiding solutions in random satisfiability problems: A statistical mechanics approach
A major problem in evaluating stochastic local search algorithms for
NP-complete problems is the need for a systematic generation of hard test
instances having previously known properties of the optimal solutions. On the
basis of statistical mechanics results, we propose random generators of hard
and satisfiable instances for the 3-satisfiability problem (3SAT). The design
of the hardest problem instances is based on the existence of a first order
ferromagnetic phase transition and the glassy nature of excited states. The
analytical predictions are corroborated by numerical results obtained from
complete as well as stochastic local algorithms.Comment: 5 pages, 4 figures, revised version to app. in PR
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