223 research outputs found
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
Superconducting Gap Nodal Surface and Fermi Surface: their partial overlap in cuprates
Electron correlation in cuprates leads to a global constraint on the gap function resulting in a gap
nodal surface. We give physical arguments supported by numerical results and
discuss some experimental results to argue that correlations also lead to a
local constraint on charge fluctuations in -space close to the Fermi
surface, which may result in a substantial overlap of the Fermi surface with
the gap nodal surface.Comment: RevTeX 3.0, 4 Pages, 6 PostScript Figures
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
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
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.
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