123 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
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
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
Clinical profile and prognosis of patients with posterior circulation stroke
Background: Posterior circulation stroke is less common than stroke involving the anterior circulation. An understanding of the Posterior circulation stroke phenomenology and mechanisms requires knowledge of neurovascular anatomy and of the structure-function relationships of this region of the brain. Identifying mechanisms of stroke is essential so that appropriate preventive therapies may be instituted.Methods: A prospective study was conducted over a period of 1 year after obtaining permission from institutional ethics committee. Recently diagnosed cases of posterior circulation stroke from medicine and neurology wards were enrolled, who fulfilled inclusion criteria.Results: Most common age for posterior circulation stroke in this study was 41-70 years of age with median age of 56.5 years. It is more common in males (55%) than females (45%). Ischemic posterior circulation stroke is more common (63.15%) than haemorrhagic posterior circulation stroke (36.85%). Most common presenting feature of posterior circulation was ataxia (77.3%). Most common risk factor for posterior circulation stroke was hypertension (60.5%) followed by dyslipidemia (55.26%). Most common site of involvement in ischemic posterior circulation stroke was cerebellum (37.5%) followed by occipital lobe (24%). Commonest site of bleed found was cerebellum (64.4%) followed by pons (35.6%). Mortality in posterior circulation stroke in present study was 15.78%.Conclusions: Posterior circulation stroke present with a wide variety of symptoms. Episodes are often staggering and more protracted than those of anterior circulation stroke. Further studies are needed to determine the safest and most effective treatment modalities for the various types of posterior circulation stroke
- …