272 research outputs found
Free Energy Approach to the Formation of an Icosahedral Structure during the Freezing of Gold Nanoclusters
The freezing of metal nanoclusters such as gold, silver, and copper exhibits
a novel structural evolution. The formation of the icosahedral (Ih) structure
is dominant despite its energetic metastability. This important phenomenon,
hitherto not understood, is studied by calculating free energies of gold
nanoclusters. The structural transition barriers have been determined by using
the umbrella sampling technique combined with molecular dynamics simulations.
Our calculations show that the formation of Ih gold nanoclusters is attributed
to the lower free energy barrier from the liquid to the Ih phases compared to
the barrier from the liquid to the face-centered-cubic crystal phases
Molecular dynamics simulations of the dipolar-induced formation of magnetic nanochains and nanorings
Iron, cobalt and nickel nanoparticles, grown in the gas phase, are known to
arrange in chains and bracelet-like rings due to the long-range dipolar
interaction between the ferromagnetic (or super-paramagnetic) particles. We
investigate the dynamics and thermodynamics of such magnetic dipolar
nanoparticles for low densities using molecular dynamics simulations and
analyze the influence of temperature and external magnetic fields on two- and
three-dimensional systems. The obtained phase diagrams can be understood by
using simple energetic arguments.Comment: 6 pages, 6 figure
Particle-size dependence of orbital order-disorder transition in LaMnO3
The latent heat (L) associated with the orbital order-disorder transition at
T_JT is found to depend significantly on the average particle size (d) of
LaMnO3. It rises slowly with the decrease in d down to ~100 nm and then jumps
by more than an order of magnitude in between d ~ 100 nm and ~30 nm. Finally, L
falls sharply to zero at a critical particle size d_c ~ 19 nm. The transition
temperature T_JT also exhibits an almost similar trend of variation with the
particle size, near d ~ 30 nm and below, even though the extent of variation is
relatively small. The zero-field-cooled (ZFC) and field-cooled (FC)
magnetization versus temperature study over a temperature range 10-300 K
reveals that the antiferromagnetic transition temperature decreases with d
while the temperature range, over which the ZFC and FC data diverge, increases
with the drop in d. The FC magnetization also is found to increase sharply with
the drop in particle size. A conjecture of nonmonotonic variation in orbital
domain structure with decrease in particle size - from smaller domains with
large number of boundaries to larger domains with small number of boundaries
due to lesser lattice defects and, finally, down to even finer domain
structures with higher degree of metastability - along with increase in surface
area in core-shell structure, could possibly rationalize the observed L versus
d and T_JT versus d patterns. Transmission electron microscopy data provide
evidence for presence of core-shell structure as well as for increase in
lattice defects in finer particles.Comment: 26 pages including 5 figures; pdf only; accepted for publication in
Phys. Rev.
Premelting of Thin Wires
Recent work has raised considerable interest on the nature of thin metallic
wires. We have investigated the melting behavior of thin cylindrical Pb wires
with the axis along a (110) direction, using molecular dynamics and a
well-tested many-body potential. We find that---in analogy with cluster
melting---the melting temperature of a wire with radius is lower
than that of a bulk solid, , by . Surface melting
effects, with formation of a thin skin of highly diffusive atoms at the wire
surface, is observed. The diffusivity is lower where the wire surface has a
flat, local (111) orientation, and higher at (110) and (100) rounded areas. The
possible relevance to recent results on non-rupturing thin necks between an STM
tip and a warm surface is addressed.Comment: 10 pages, 4 postscript figures are appended, RevTeX, SISSA Ref.
131/94/CM/S
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
Finite size melting of spherical solid-liquid aluminium interfaces
We have investigated the melting of nano-sized cone shaped aluminium needles
coated with amorphous carbon using transmission electron microscopy. The
interface between solid and liquid aluminium was found to have spherical
topology. For needles with fixed apex angle, the depressed melting temperature
of this spherical interface, with radius , was found to scale linearly with
the inverse radius . However, by varying the apex angle of the needles we
show that the proportionality constant between the depressed melting
temperature and the inverse radius changes significantly. This lead us to the
conclusion that the depressed melting temperature is not controlled solely by
the inverse radius . Instead we found a direct relation between the
depressed melting temperature and the ratio between the solid-liquid interface
area and the molten volume.Comment: to appear in Philosophical Magazine (2009
Size--sensitive melting characteristics of gallium clusters: Comparison of Experiment and Theory for Ga and Ga
Experiments and simulations have been performed to examine the
finite-temperature behavior of Ga and Ga clusters.
Specific heats and average collision cross sections have been measured as a
function of temperature, and the results compared to simulations performed
using first principles Density--Functional Molecular--Dynamics. The
experimental results show that while Ga apparently undergoes a
solid--liquid transition without a significant peak in the specific--heat,
Ga melts with a relatively sharp peak. Our analysis of the
computational results indicate a strong correlation between the ground--state
geometry and the finite--temperature behavior of the cluster. If the
ground--state geometry is symmetric and "ordered" the cluster is found to have
a distinct peak in the specific--heat. However, if the ground--state geometry
is amorphous or "disordered" the cluster melts without a peak in the
specific--heat.Comment: 6 figure
Impurity effects on the melting of Ni clusters
We demonstrate that the addition of a single carbon impurity leads to
significant changes in the thermodynamic properties of Ni clusters consisting
of more than a hundred atoms. The magnitude of the change induced is dependent
upon the parameters of the Ni-C interaction. Hence, thermodynamic properties of
Ni clusters can be effectively tuned by the addition of an impurity of a
particular type. We also show that the presence of a carbon impurity
considerably changes the mobility and diffusion of atoms in the Ni cluster at
temperatures close to its melting point. The calculated diffusion coefficients
of the carbon impurity in the Ni cluster can be used for a reliable estimate of
the growth rate of carbon nanotubes.Comment: 27 pages, 13 figure
Substituting a copper atom modifies the melting of aluminum clusters
ProducciĂłn CientĂficaHeat capacities have been measured for Al(nâ1)Cuâ clusters (n = 49â62) and compared with results for pure Aln+ clusters. Al(nâ1)Cuâ and Aln+ have the same number of atoms and the same number of valence electrons (excluding the copper d electrons). Both clusters show peaks in their heat capacities that can be attributed to melting transitions; however, substitution of an aluminum atom by a copper atom causes significant changes in the melting behavior. The sharp drop in the melting temperature that occurs between n = 55 and 56 for pure aluminum clusters does not occur for the Al(nâ1)Cuâ analogs. First-principles density-functional theory has been used to locate the global minimum energy structures of the doped clusters. The results show that the copper atom substitutes for an interior aluminum atom, preferably one with a local face-centered-cubic environment. Substitution does not substantially change the electronic or geometric structures of the host cluster unless there are several Aln+ isomers close to the ground state. The main structural effect is a contraction of the bond lengths around the copper impurity, which induces both a contraction of the whole cluster and a stress redistribution between the AlâAl bonds. The size dependence of the substitution energy is correlated with the change in the latent heat of melting on substitution
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