20,627 research outputs found
Finite temperature behavior of impurity doped Lithium cluster {\em viz} LiSn
We have carried out extensive isokinetic {\it ab initio} molecular dynamic
simulations to investigate the finite temperature properties of the impurity
doped cluster LiSn along with the host cluster Li. The data obtained
from about 20 temperatures and total simulation time of at least 3 ns is used
to extract thermodynamical quantities like canonical specific heat. We observe
a substantial charge transfer from all Li atoms to Sn which inturn weakens the
Li-Li bonds in LiSn compared to the bonds in Li. This weakening of
bonds changes the finite temperature behavior of LiSn significantly.
Firstly, LiSn becomes liquid-like around 250 K, a much lower temperature
than that of Li (~425 K). Secondly, an additional quasirotational
motion of lithium atoms appears at lower temperatures giving rise to a shoulder
around 50 K in the specific heat curve of LiSn. The peak in the specific
heat of Li is very broad and the specific heat does not show any premelting
features.Comment: 16 pages, 10 figures Submitted to J. Chem. Phy
Magic Melters' Have Geometrical Origin
Recent experimental reports bring out extreme size sensitivity in the heat
capacities of Gallium and Aluminum clusters. In the present work we report
results of our extensive {\it ab initio} molecular dynamical simulations on
Ga and Ga, the pair which has shown rather dramatic size
sensitivity. We trace the origin of this size sensitive heat capacities to the
relative order in their respective ground state geometries. Such an effect of
nature of the ground state on the characteristics of heat capacities is also
seen in case of small Gallium and Sodium clusters indicating that the observed
size sensitivity is a generic feature of small clusters.Comment: 4 pages, 6 figure
Finite Temperature Behavior of Small Silicon and Tin Clusters: An Ab Initio Molecular Dynamics Study
The finite temperature behavior of small Silicon (Si, Si, and
Si) and Tin (Sn and Sn) clusters is studied using
isokinetic Born-Oppenheimer molecular dynamics. The lowest equilibrium
structures of all the clusters are built upon a highly stable tricapped
trigonal prism unit which is seen to play a crucial role in the finite
temperature behavior of these clusters. Thermodynamics of small tin clusters
(Sn and Sn) is revisited in light of the recent experiments on
tin clusters of sizes 18-21 [G. A. Breaux et. al. Phys. Rev. B {\bf 71} 073410
(2005)]. We have calculated heat capacities using multiple histogram technique
for Si, Sn and Si clusters. Our calculated specific heat
curves have a main peak around 2300 K and 2200 K for Si and Sn
clusters respectively. However, various other melting indicators such as root
mean square bond length fluctuations, mean square displacements show that
diffusive motion of atoms within the cluster begins around 650 K. The finite
temperature behavior of Si and Sn is dominated by isomerization
and it is rather difficult to discern the temperature range for transition
region. On the other hand, Si does show a liquid like behavior over a
short temperature range followed by the fragmentation observed around 1800 K.
Finite temperature behavior of Si and Sn show that these clusters
do not melt but fragment around 1200 K and 650 K respectively.Comment: 9 figure
Dopant Induced Stabilization of Silicon Cluster at Finite Temperature
With the advances in miniaturization, understanding and controlling
properties of significant technological systems like silicon in nano regime
assumes considerable importance. It turns out that small silicon clusters in
the size range of 15-20 atoms are unstable upon heating and in fact fragment in
the temperature range of 1200 K to 1500 K. In the present work we demonstrate
that it is possible to stabilize such clusters by introducing appropriate
dopant (in this case Ti). Specifically, by using the first principle density
functional simulations we show that Ti doped Si, having the Frank-Kasper
geometry, remains stable till 2200 K and fragments only above 2600 K. The
observed melting transition is a two step process. The first step is initiated
by the surface melting around 600 K. The second step is the destruction of the
cage which occurs around 2250 K giving rise to a peak in the heat capacity
curve.Comment: 6 pages, 8 Figs. Submitted to PR
A complete radio study of SNR G15.4+0.1 from new GMRT observations
The supernova remnant G15.4+0.1 is considered to be the possible counterpart
of the gamma-ray source HESSJ1818-154. With the goal of getting a complete view
of this remnant and understanding the nature of the gamma-ray flux, we
conducted a detailed radio study that includes the search for pulsations and a
model of the broadband emission for the G15.4+0.1/HESSJ1818-154 system.
Low-frequency imaging at 624 MHz and pulsar observations at 624 and 1404 MHz
towards G15.4+0.1 were carried out with the Giant Metrewave Radio Telescope
(GMRT). We correlated the new radio data with observations of the source at
X-ray and infrared wavelengths from XMM-Newton and Herschel observatories,
respectively. To characterize the neutral hydrogen medium (HI) towards
G15.4+0.1, we used data from the Southern Galactic Plane Survey. We modelled
the spectral energy distribution using both hadronic and leptonic scenarios.
From the combination of the new GMRT observations with existing data, we
derived a continuum spectral index alpha=-0.62+-0.03 for the whole remnant. The
local synchrotron spectra of G15.4+0.1, calculated from the combination of the
GMRT data with 330 MHz observations from the VLA, tends to be flatter in the
central part of the remnant, accompanying the region where the blast wave is
impinging molecular gas. No spectral index trace was found indicating the radio
counterpart to the pulsar wind nebula proposed from X-ray observations. In
addition, the search for radio pulsations yielded negative results. Emission at
far-infrared wavelengths is observed in the region where the SNR shock is
interacting with dense molecular clumps. We also identified HI features forming
a shell that wraps most of the outer border of G15.4+0.1. Characteristic
parameters were estimated for the shocked HI gas. We found that either a purely
hadronic or leptonic model is compatible with the broadband emission known so
far.Comment: 11 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Mappings preserving locations of movable poles: a new extension of the truncation method to ordinary differential equations
The truncation method is a collective name for techniques that arise from
truncating a Laurent series expansion (with leading term) of generic solutions
of nonlinear partial differential equations (PDEs). Despite its utility in
finding Backlund transformations and other remarkable properties of integrable
PDEs, it has not been generally extended to ordinary differential equations
(ODEs). Here we give a new general method that provides such an extension and
show how to apply it to the classical nonlinear ODEs called the Painleve
equations. Our main new idea is to consider mappings that preserve the
locations of a natural subset of the movable poles admitted by the equation. In
this way we are able to recover all known fundamental Backlund transformations
for the equations considered. We are also able to derive Backlund
transformations onto other ODEs in the Painleve classification.Comment: To appear in Nonlinearity (22 pages
Why do gallium clusters have a higher melting point than the bulk?
Density functional molecular dynamical simulations have been performed on
Ga and Ga clusters to understand the recently observed
higher-than-bulk melting temperatures in small gallium clusters [Breaux {\em et
al.}, Phys. Rev. Lett. {\bf 91}, 215508 (2003)]. The specific-heat curve,
calculated with the multiple-histogram technique, shows the melting temperature
to be well above the bulk melting point of 303 K, viz. around 650 K and 1400 K
for Ga and Ga, respectively. The higher-than-bulk melting
temperatures are attributed mainly to the covalent bonding in these clusters,
in contrast with the covalent-metallic bonding in the bulk.Comment: 4 pages, including 6 figures. accepted for publication in Phys. Rev.
Let
Gravitational collapse of an isentropic perfect fluid with a linear equation of state
We investigate here the gravitational collapse end states for a spherically
symmetric perfect fluid with an equation of state . It is shown that
given a regular initial data in terms of the density and pressure profiles at
the initial epoch from which the collapse develops, the black hole or naked
singularity outcomes depend on the choice of rest of the free functions
available, such as the velocities of the collapsing shells, and the dynamical
evolutions as allowed by Einstein equations. This clarifies the role that
equation of state and initial data play towards determining the final fate of
gravitational collapse.Comment: 7 Pages, Revtex4, To appear in Classical and Quantum Gravit
Instability of black hole formation under small pressure perturbations
We investigate here the spectrum of gravitational collapse endstates when
arbitrarily small perfect fluid pressures are introduced in the classic black
hole formation scenario as described by Oppenheimer, Snyder and Datt (OSD) [1].
This extends a previous result on tangential pressures [2] to the more
physically realistic scenario of perfect fluid collapse. The existence of
classes of pressure perturbations is shown explicitly, which has the property
that injecting any smallest pressure changes the final fate of the dynamical
collapse from a black hole to a naked singularity. It is therefore seen that
any smallest neighborhood of the OSD model, in the space of initial data,
contains collapse evolutions that go to a naked singularity outcome. This gives
an intriguing insight on the nature of naked singularity formation in
gravitational collapse.Comment: 7 pages, 1 figure, several modifications to match published version
on GR
- …