130 research outputs found
Periodic orbit theory for realistic cluster potentials: The leptodermous expansion
The formation of supershells observed in large metal clusters can be
qualitatively understood from a periodic-orbit-expansion for a spherical
cavity. To describe the changes in the supershell structure for different
materials, one has, however, to go beyond that simple model. We show how
periodic-orbit-expansions for realistic cluster potentials can be derived by
expanding only the classical radial action around the limiting case of a
spherical potential well. We give analytical results for the leptodermous
expansion of Woods-Saxon potentials and show that it describes the shift of the
supershells as the surface of a cluster potential gets softer. As a byproduct
of our work, we find that the electronic shell and supershell structure is not
affected by a lattice contraction, which might be present in small clusters.Comment: 15 pages RevTex, 11 eps figures, additional information at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/users/koch/Diss
Supershells in Metal Clusters: Self-Consistent Calculations and their Semiclassical Interpretation
To understand the electronic shell- and supershell-structure in large metal
clusters we have performed self-consistent calculations in the homogeneous,
spherical jellium model for a variety of different materials. A scaling
analysis of the results reveals a surprisingly simple dependence of the
supershells on the jellium density. It is shown how this can be understood in
the framework of a periodic-orbit-expansion by analytically extending the
well-known semiclassical treatment of a spherical cavity to more realistic
potentials.Comment: 4 pages, revtex, 3 eps figures included, for additional information
see http://radix2.mpi-stuttgart.mpg.de/koch/Diss
Mechanisms of resonant low frequency Raman scattering from metallic nanoparticle Lamb modes
International audienceThe low frequency Raman scattering from gold nanoparticle bimodal assemblies with controlled size distributions has been studied. Special care has been paid to determining the size dependence of the Raman intensity corresponding to the quadrupolar Lamb mode. Existing models based on a microscopic description of the scattering mechanism in small particles (bond polarizability, dipole induced dipole models) predict, for any Raman-active Lamb modes, an inelastic intensity scaling as the volume of the nanoparticle. Surprisingly experimental intensity ratios are found to be anomalously much greater than theoretical ones, calling into question this scaling law. To explain these discrepancies, a simple mechanism of Raman scattering, based on the density fluctuations in the nanoparticles induced by the Lamb modes, is introduced. This modeling, in which the nanoparticle is described as an elastic isotropic continuous medium-as in Lamb theory, successfully explains the major features exhibited by low frequency Raman modes. Moreover this model provides a unified picture for any material, suitable for handling both small and large size ranges, as well as non-resonant and resonant excitation conditions in the case of metallic species. Published by AIP Publishing
Oscillatory Size-Dependence of the Surface Plasmon Linewidth in Metallic Nanoparticles
We study the linewidth of the surface plasmon resonance in the optical
absorption spectrum of metallic nanoparticles, when the decay into
electron-hole pairs is the dominant channel. Within a semiclassical approach,
we find that the electron-hole density-density correlation oscillates as a
function of the size of the particles, leading to oscillations of the
linewidth. This result is confirmed numerically for alkali and noble metal
particles. While the linewidth can increase strongly, the oscillations persist
when the particles are embedded in a matrix.Comment: RevTeX4, 5 pages, 2 figures, final versio
Origin of Shifts in the Surface Plasmon Resonance Frequencies for Au and Ag Nanoparticles
Origin of shifts in the surface plasmon resonance (SPR) frequency for noble
metal (Au, Ag) nanoclusters are discussed in this book chapter. Spill out of
electron from the Fermi surface is considered as the origin of red shift. On
the other hand, both screening of electrons of the noble metal in porous media
and quantum effect of screen surface electron are considered for the observed
blue shift in the SPR peak position.Comment: 37 pages, 14 Figures in the submitted book chapter of The Annual
Reviews in Plasmonics, edited by Professor Chris D. Geddes. Springer Scinec
Resonant Raman Scattering by quadrupolar vibrations of Ni-Ag Core-shell Nanoparticles
Low-frequency Raman scattering experiments have been performed on thin films
consisting of nickel-silver composite nanoparticles embedded in alumina matrix.
It is observed that the Raman scattering by the quadrupolar modes, strongly
enhanced when the light excitation is resonant with the surface dipolar
excitation, is mainly governed by the silver electron contribution to the
plasmon excitation. The Raman results are in agreement with a core-shell
structure of the nanoparticles, the silver shell being loosely bonded to the
nickel core.Comment: 3 figures. To be published in Phys. Rev.
Deformed Harmonic Oscillators for Metal Clusters: Analytic Properties and Supershells
The analytic properties of Nilsson's Modified Oscillator (MO), which was
first introduced in nuclear structure, and of the recently introduced, based on
quantum algebraic techniques, 3-dimensional q-deformed harmonic oscillator
(3-dim q-HO) with Uq(3) > SOq(3) symmetry, which is known to reproduce
correctly in terms of only one parameter the magic numbers of alkali clusters
up to 1500 (the expected limit of validity for theories based on the filling of
electronic shells), are considered. Exact expressions for the total energy of
closed shells are determined and compared among them. Furthermore, the
systematics of the appearance of supershells in the spectra of the two
oscillators is considered, showing that the 3-dim q-HO correctly predicts the
first supershell closure in alkali clusters without use of any extra parameter.Comment: 25 pages LaTeX plus 21 postscript figure
Twist Mode in Spherical Alkali Metal Clusters
A remarkable orbital quadrupole magnetic resonance, so-called twist mode, is
predicted in alkali metal clusters where it is represented by
low-energy excitations of valence electrons with strong M2 transitions to the
ground state. We treat the twist by both macroscopic and microscopic ways. In
the latter case, the shell structure of clusters is fully exploited, which is
crucial for the considered size region (). The
energy-weighted sum rule is derived for the pseudo-Hamiltonian. In medium and
heavy spherical clusters the twist dominates over its spin-dipole counterpart
and becomes the most strong multipole magnetic mode.Comment: 8 pages, 4 figures, to be published in Phys. Rev. Lett., v.85, n.15,
200
On the 3n+l Quantum Number in the Cluster Problem
It has recently been suggested that an exactly solvable problem characterized
by a new quantum number may underlie the electronic shell structure observed in
the mass spectra of medium-sized sodium clusters. We investigate whether the
conjectured quantum number 3n+l bears a similarity to the quantum numbers n+l
and 2n+l, which characterize the hydrogen problem and the isotropic harmonic
oscillator in three dimensions.Comment: 8 pages, revtex, 4 eps figures included, to be published in
Phys.Rev.A, additional material available at
http://radix2.mpi-stuttgart.mpg.de/koch/Diss
Atomic-scale confinement of optical fields
In the presence of matter there is no fundamental limit preventing
confinement of visible light even down to atomic scales. Achieving such
confinement and the corresponding intensity enhancement inevitably requires
simultaneous control over atomic-scale details of material structures and over
the optical modes that such structures support. By means of self-assembly we
have obtained side-by-side aligned gold nanorod dimers with robust
atomically-defined gaps reaching below 0.5 nm. The existence of
atomically-confined light fields in these gaps is demonstrated by observing
extreme Coulomb splitting of corresponding symmetric and anti-symmetric dimer
eigenmodes of more than 800 meV in white-light scattering experiments. Our
results open new perspectives for atomically-resolved spectroscopic imaging,
deeply nonlinear optics, ultra-sensing, cavity optomechanics as well as for the
realization of novel quantum-optical devices
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