158 research outputs found
Infrared electron modes in light deformed clusters
Infrared quadrupole modes (IRQM) of the valence electrons in light deformed
sodium clusters are studied by means of the time-dependent local-density
approximation (TDLDA). IRQM are classified by angular momentum components
20, 21 and 22 whose branches are separated by cluster
deformation. In light clusters with a low spectral density, IRQM are
unambiguously related to specific electron-hole excitations, thus giving access
to the single-electron spectrum near the Fermi surface (HOMO-LUMO region). Most
of IRQM are determined by cluster deformation and so can serve as a sensitive
probe of the deformation effects in the mean field. The IRQM branch 21 is coupled with the magnetic scissors mode, which gives a chance to detect
the latter. We discuss two-photon processes, Raman scattering (RS), stimulated
emission pumping (SEP), and stimulated adiabatic Raman passage (STIRAP), as the
relevant tools to observe IRQM. A new method to detect the IRQM population in
clusters is proposed.Comment: 22 pages, 6 figure
Ionic structure and photoabsorption in medium sized sodium clusters
We present ground-state configurations and photoabsorption spectra of Na-7+,
Na-27+ and Na-41+. Both the ionic structure and the photoabsorption spectra of
medium-size sodium clusters beyond Na-20 have been calculated self-consistently
with a nonspherical treatment of the valence electrons in density functional
theory. We use a local pseudopotential that has been adjusted to experimental
bulk properties and the atomic 3s level of sodium. Our studies have shown that
both the ionic structure of the ground state and the positions of the plasmon
resonances depend sensitively on the pseudopotential used in the calculation,
which stresses the importance of its consistent use in both steps.Comment: 4 pages, 3 figures. Accepted for publication in PRB, tentatively July
15th, 1998 some typos corrected, brought to nicer forma
Ionic and electronic structure of sodium clusters up to N=59
We determined the ionic and electronic structure of sodium clusters with even
electron numbers and 2 to 59 atoms in axially averaged and three-dimensional
density functional calculations. A local, phenomenological pseudopotential that
reproduces important bulk and atomic properties and facilitates structure
calculations has been developed. Photoabsorption spectra have been calculated
for , , and to
. The consistent inclusion of ionic structure considerably
improves agreement with experiment. An icosahedral growth pattern is observed
for to . This finding is supported by
photoabsorption data.Comment: To appear in Phys. Rev. B 62. Version with figures in better quality
can be requested from the author
Thermal expansion in small metal clusters and its impact on the electric polarizability
The thermal expansion coefficients of clusters with and , and
are obtained from {\it ab initio} Born-Oppenheimer LDA molecular dynamics.
Thermal expansion of small metal clusters is considerably larger than that in
the bulk and size-dependent. We demonstrate that the average static electric
dipole polarizability of Na clusters depends linearly on the mean interatomic
distance and only to a minor extent on the detailed ionic configuration when
the overall shape of the electron density is enforced by electronic shell
effects. The polarizability is thus a sensitive indicator for thermal
expansion. We show that taking this effect into account brings theoretical and
experimental polarizabilities into quantitative agreement.Comment: 4 pages, 2 figures, one table. Accepted for publication in Physical
Review Letters. References 10 and 23 update
Electric multipole plasmons in deformed sodium clusters
The random-phase-approximation (RPA) method with separable residual forces
(SRPA) is proposed for the description of multipole electric oscillations of
valence electrons in deformed alkali metal clusters. Both the deformed mean
field and residual interaction are derived self-consistently from the Kohn-Sham
functional. SRPA drastically simplifies the computational effort which is
urgent if not decisive for deformed systems. The method is applied to the
description of dipole, quadrupole and octupole plasmons in deformed sodium
clusters of a moderate size. We demonstrate that, in clusters with the size
N>50, Landau damping successfully competes with deformation splitting and even
becomes decisive in forming the width and gross structure of the dipole
plasmon. Besides, the plasmon is generated by excitations from both ground
state and shape isomers. In such clusters familiar experimental estimates for
deformation splitting of dipole plasmon are useless.Comment: 27 pages, 10 figure
Optical response of small silver clusters
The time-dependent local density approximation is applied to the optical
response of the silver clusters, Ag_2, Ag_3, Ag_8 and Ag_9^+. The calculation
includes all the electrons beyond the closed-shell Ag^{+11} ionic core, thus
including for the first time explicitly the filled d-shell in the response. The
excitation energy of the strong surface plasmon near 4 eV agrees well with
experiment. The theoretical transition strength is quenched by a factor of 4
with respect to the pure s-electron sum rule in Ag_8 due to the d-electrons. A
comparable amount of strength lies in complex states below 6 eV excitation. The
total below 6 eV, about 50% of the s sum rule, is consistent with published
experiments.Comment: 13 pages RevTex and 9 Postscript figure
Thermodynamics of Na_8 and Na_{20} clusters studied with ab-initio electronic structure methods
We study the thermodynamics of Na_8 and Na_{20} clusters using
multiple-histogram methods and an ab initio treatment of the valence electrons
within density functional theory. We consider the influence of various electron
kinetic-energy functionals and pseudopotentials on the canonical ionic specific
heats. The results for all models we consider show qualitative similarities,
but also significant temperature shifts from model to model of peaks and other
features in the specific-heat curves. The use of phenomenological
pseudopotentials shifts the melting peak substantially (~ 50--100 K) when
compared to ab-initio results. It is argued that the choice of a good
pseudopotential and use of better electronic kinetic-energy functionals has the
potential for performing large time scale and large sized thermodynamical
simulations on clusters.Comment: LaTeX file and EPS figures. 24 pages, 13 figures. Submitted to Phys.
Rev.
An orbital-free molecular dynamics study of melting in K_20, K_55, K_92, K_142, Rb_55 and Cs_55 clusters
The melting-like transition in potasium clusters K_N, with N=20, 55, 92 and
142, is studied by using an orbital-free density-functional constant-energy
molecular dynamics simulation method, and compared to previous theoretical
results on the melting-like transition in sodium clusters of the same sizes.
Melting in potasium and sodium clusters proceeds in a similar way: a surface
melting stage develops upon heating before the homogeneous melting temperature
is reached. Premelting effects are nevertheless more important and more easily
established in potasium clusters, and the transition regions spread over
temperature intervals which are wider than in the case of sodium. For all the
sizes considered, the percentage melting temperature reduction when passing
from Na to K clusters is substantially larger than in the bulk. Once those two
materials have been compared for a number of different cluster sizes, we study
the melting-like transition in Rb_55 and Cs_55 clusters and make a comparison
with the melting behavior of Na_55 and K_55. As the atomic number increases,
the height of the specific heat peaks decreases, their width increases, and the
melting temperature decreases as in bulk melting, but in a more pronounced way.Comment: LaTeX file. 6 pages with 17 pictures. Final version with minor
change
Evolution of electronic and ionic structure of Mg-clusters with the growth cluster size
The optimized structure and electronic properties of neutral and singly
charged magnesium clusters have been investigated using ab initio theoretical
methods based on density-functional theory and systematic post-Hartree-Fock
many-body perturbation theory accounting for all electrons in the system. We
have systematically calculated the optimized geometries of neutral and singly
charged magnesium clusters consisting of up to 21 atoms, electronic shell
closures, binding energies per atom, ionization potentials and the gap between
the highest occupied and the lowest unoccupied molecular orbitals. We have
investigated the transition to the hcp structure and metallic evolution of the
magnesium clusters, as well as the stability of linear chains and rings of
magnesium atoms. The results obtained are compared with the available
experimental data and the results of other theoretical works.Comment: 30 pages, 10 figures, 3 table
Structure and properties of small sodium clusters
We have investigated structure and properties of small metal clusters using
all-electron ab initio theoretical methods based on the Hartree-Fock
approximation and density functional theory, perturbation theory and compared
results of our calculations with the available experimental data and the
results of other theoretical works. We have systematically calculated the
optimized geometries of neutral and singly charged sodium clusters having up to
20 atoms, their multipole moments (dipole and quadrupole), static
polarizabilities, binding energies per atom, ionization potentials and
frequencies of normal vibration modes. Our calculations demonstrate the great
role of many-electron correlations in the formation of electronic and ionic
structure of small metal clusters and form a good basis for further detailed
study of their dynamic properties, as well as structure and properties of other
atomic cluster systems.Comment: 47 pages, 16 figure
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