114 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
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.
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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