3,833 research outputs found
Effect of geometric and electronic structures on the finite temperature behavior of Na, Na, and Na clusters
An analysis of the evolutionary trends in the ground state geometries of
Na to Na reveals Na, an electronic closed--shell system,
shows namely an electronically driven spherical shape leading to a disordered
but compact structure. This structural change induces a strong {\it
connectivity} of short bonds among the surface atoms as well as between core
and surface atoms with inhomogeneous strength in the ground state geometry,
which affects its finite--temperature behavior. By employing {\it ab initio}
density--functional molecular dynamics, we show that this leads to two distinct
features in specific heat curve compared to that of Na: (1) The peak is
shifted by about 100 K higher in temperature. (2) The transition region becomes
much broader than Na. The inhomogeneous distribution of bond strengths
results in a broad melting transition and the strongly connected network of
short bonds leads to the highest melting temperature of 375 K reported among
the sodium clusters. Na, which has one electron less than Na,
also possesses stronger short--bond network compared with Na, resulting
in higher melting temperature (350 K) than observed in Na. Thus, we
conclude that when a cluster has nearly closed shell structure not only
geometrically but also electronically, it show a high melting temperature. Our
calculations clearly bring out the size--sensitive nature of the specific heat
curve in sodium clusters.Comment: 7 pages, 11 figure
MRI evaluation of the anti-adhesion molecule antibody Natalizumab and the blood-brain barrier in Multiple Sclerosis
As Blood-brain barrier (BBB) breakdown is central to inflammatory lesion formation, it
presents a potential target in the formulation of putative therapeutic agents in MS. The
action of natalizumab, a monoclonal antibody acting at the BBB, is investigated through
a phase III monotherapy trial (AFFIRM) and associated substudies.
Subtle BBB disruption from non-inflamed lesions, which could contribute to axonal
damage through leakage of inflammatory cells and associated mediators into
surrounding parenchyma, is also studied.
Introductory chapters (1-3) provide a brief overview of MS, clinical trials, magnetic
resonance imaging (MRI), the BBB and natalizumab.
Chapter four describes MRI results of AFFIRM- a 2 year multi-centre trial involving
942 patients. Compared with placebo, natalizumab reduced number of gadolinium (Gd)- enhancing lesions by 92%, new/enlarging T2-hyperintense lesions by 83%, and new T1-
hypointense lesions by 76%.
Chapter five describes a 57 patient AFFIRM trial substudy in which the influence of
natalizumab on segmental atrophy was investigated. Atrophy was predominant in grey
matter (GM) and was independent of lesion load. Fluctuations in white matter (WM)
volume followed changes in inflammatory lesion load. Atrophy was not influenced by
natalizumab. The effect of natalizumab on subtle BBB disruption (inferred by measuring the post-Gd
%change in T1 weighted signal intensity) is studied in chapter 6. This AFFIRM
substudy involved 40 patients (27 on natalizumab, 13 on placebo.) Although subtle
BBB leakage was consistently detected in non-visibly enhancing lesions, natalizumab
did not influence the degree of leakage.
Chapter 7 describes a cross-sectional study which utilised post-Gd change in R1 (1/T1)
as a marker BBB leakage. 19 patients (10 RRMS, 9 SPMS) were involved in this study.
The subtle leakage observed from non-visibly enhancing lesions was distinct from
leakage from visibly enhancing lesions. This was sustained over 60 minutes, greater in
smaller lesions and in size-adjusted T1 hypointense lesions
First principles investigation of finite-temperature behavior in small sodium clusters
A systematic and detailed investigation of the finite-temperature behavior of
small sodium clusters, Na_n, in the size range of n= 8 to 50 are carried out.
The simulations are performed using density-functional molecular-dynamics with
ultrasoft pseudopotentials. A number of thermodynamic indicators such as
specific heat, caloric curve, root-mean-square bond length fluctuation,
deviation energy, etc. are calculated for each of the clusters. Size dependence
of these indicators reveals several interesting features. The smallest clusters
with n= 8 and 10, do not show any signature of melting transition. With the
increase in size, broad peak in the specific heat is developed, which
alternately for larger clusters evolves into a sharper one, indicating a
solidlike to liquidlike transition. The melting temperatures show irregular
pattern similar to experimentally observed one for larger clusters [ M. Schmidt
et al., Nature (London) 393, 238 (1998) ]. The present calculations also reveal
a remarkable size-sensitive effect in the size range of n= 40 to 55. While
Na_40 and Na_55 show well developed peaks in the specific heat curve, Na_50
cluster exhibits a rather broad peak, indicating a poorly-defined melting
transition. Such a feature has been experimentally observed for gallium and
aluminum clusters [ G. A. Breaux et al., J. Am. Chem. Soc. 126, 8628 (2004); G.
A.Breaux et al., Phys. Rev. Lett. 94, 173401 (2005) ].Comment: 8 pages, 11 figure
Solar Flare Intermittency and the Earth's Temperature Anomalies
We argue that earth's short-term temperature anomalies and the solar flare
intermittency are linked. The analysis is based upon the study of the scaling
of both the spreading and the entropy of the diffusion generated by the
fluctuations of the temperature time series. The joint use of these two methods
evidences the presence of a L\'{e}vy component in the temporal persistence of
the temperature data sets that corresponds to the one that would be induced by
the solar flare intermittency. The mean monthly temperature datasets cover the
period from 1856 to 2002.Comment: 4 pages, 5 figure
Geometric, electronic properties and the thermodynamics of pure and Al--doped Li clusters
The first--principles density functional molecular dynamics simulations have
been carried out to investigate the geometric, the electronic, and the finite
temperature properties of pure Li clusters (Li, Li) and Al--doped
Li clusters (LiAl, LiAl). We find that addition of two Al
impurities in Li results in a substantial structural change, while the
addition of one Al impurity causes a rearrangement of atoms. Introduction of
Al--impurities in Li establishes a polar bond between Li and nearby Al
atom(s), leading to a multicentered bonding, which weakens the Li--Li metallic
bonds in the system. These weakened Li--Li bonds lead to a premelting feature
to occur at lower temperatures in Al--doped clusters. In LiAl, Al
atoms also form a weak covalent bond, resulting into their dimer like behavior.
This causes Al atoms not to `melt' till 800 K, in contrast to the Li atoms
which show a complete diffusive behavior above 400 K. Thus, although one Al
impurity in Li cluster does not change its melting characteristics
significantly, two impurities results in `surface melting' of Li atoms whose
motions are confined around Al dimer.Comment: 9 pages, 7 figure
Electronic structures, equilibrium geometries, and finite-temperature properties of Na<SUB>n</SUB> (n=39-55) from first principles
Density-functional theory has been applied to investigate systematics of sodium clusters Nan in the size range of n=39-55. A clear evolutionary trend in the growth of their ground-state geometries emerges. The clusters at the beginning of the series (n=39-43) are symmetric and have partial icosahedral (two-shell) structure. The growth then goes through a series of disordered clusters (n=44-52) where the icosahedral core is lost. However, for n≥53, a three-shell icosahedral structure emerges. This change in the nature of the geometry is abrupt. In addition, density-functional molecular dynamics has been used to calculate the specific heat curves for the representative sizes n=43, 45, 48, and 52. These results along with already available thermodynamic calculations for n=40, 50, and 55 enable us to carry out a detailed analysis of the heat capacity curves and their relationship with respective geometries for the entire series. Our results clearly bring out strong correlation between the evolution of the geometries and the nature of the shape of the heat capacities. The results also firmly establish the size-sensitive nature of the heat capacities in sodium clusters
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