ANNUAL REPORT - MTA SzFKI

the α2 reconstruction increases on the surface, but the β2 phase remains within the whole
temperature range, in agreement with previous experimental findings. Our results show
that the combination of the experimental and theoretical results is a prerequisite for the
accurate analysis of the SCLSs of the complex reconstructed surfaces.
The surface core-level binding-energy shift of Pd at the Ag c Pd 1−c (111) surface is
calculated as a function of bulk concentration of the alloy. The equilibrium volume and the
surface concentration profile used in the calculations refer to the 0 K case. The SCLSs are
evaluated within the Z + 1 approximation. The results are analysed using the mixing
enthalpy of the alloy and the bulk and surface chemical potentials. A relation of the SCLS
to the bulk concentration is considered. This relation is shown to be mediated by the
surface concentration profile which induces the observed nonlinear behaviour. The results
are interpreted using a simple model for the alloy electronic structure.
We have performed first principles calculations with the density functional theory based
VASP code to study the vibrational properties of various carbon nanostructures. In
detail, the following topics were investigated:
We calculated the Raman active longitudinal optical (LO) frequencies for carbon
oligoynes and polyyne. We introduced a linear/exponential scaling scheme based on the
exponential behavior of the carbon-carbon bond stretching force constant couplings in
quasi-one-dimensional conjugated chains. This novel scaling scheme was found to yield
frequencies that agree well with experimental results on long linear carbon chains
encapsulated inside multiwalled or double walled carbon nanotubes. Vibrational modes of
13 C isotope enriched single walled carbon nanotubes are inhomogeneously broadened due
to the random distribution of isotopes. We studied this effect on the radial breathing mode
theoretically and compared the result with experiments on inner tubes in double walled
carbon nanotubes grown from 13 C-enriched fullerenes. We have found evidence of the
absence of carbon diffusion along the tube axis during inner tube growth, supporting the
theory of inner tube growth by Stone-Wales transformations from interconnected
fullerenes. The energy dispersion of the D* (G') band shows a strong diameter dependence
according to Raman measurements on double walled carbon nanotubes, with the Raman
shift of the small diameter inner tubes showing an average softening. We have shown that
the experimental observation can be reproduced by simple model calculations if the
curvature effects are taken into account. The phonon softening with increasing curvature
was proven by first principles calculations.
We performed ab-initio calculations of the magnetic moments and magnetic anisotropy
energies of small FeCo clusters of varying composition on top of a Cu(100) substrate.
Three different cluster layouts have been considered, namely 2X2, 3X3 and cross-like
pentamer clusters. The ratio of Co atoms with respect to the total number in a chosen
cluster (”concentration”) was varied and all possible arrangements of the atomic species
were taken into account. Calculations have been performed fully relativistic using the
embedded-cluster technique in conjunction with the screened Korringa-Kohn-Rostoker
method and the magnetocrystalline anisotropy energy (MAE) has been evaluated by means
of the magnetic force theorem. A central result of the investigations is that the size of the
magnetic moments of the individual Fe and Co atoms and their contributions to the
anisotropy energy depend on the position they occupy in a particular cluster and on the
type and the number of nearest-neighbors. The MAE for the 2X2 and 3X3 clusters varies
with respect to the „concentration” of Co atoms in the same manner as the corresponding
monolayer case, whereas the pentamer clusters show a slightly different behavior.
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