1,802 research outputs found
Directionality in van der Waals Interactions: the Case of 4-Acetylbiphenyl Adsorbed on Au(111)
We report on a theoretical study of adsorption of 4-Acetylbiphenyl molecule
and its diffusion properties in the main directions of the Au(111) surface.
Structural changes of the molecule, which are induced by adsorption lead to
stronger conjugation of the -system. The molecule is adsorbed in a flat
configuration on the surface with roughly the same binding energy along the
[110] and [112] directions, in good agreement with experiments. Furthermore,
the diffusion barriers imply an important directionality of the
molecule-surface interactions. This is somewhat surprising because our
calculations show that the prevailing interaction is the long-range
molecule-surface van der Waals interaction. Despite of its weakness, the van
der Waals interaction discriminates the preferential adsorption sites as well
as imposes a molecular geometry that needs to be considered when rationalizing
the diffusion barriers
Quenching of magnetic excitations in single adsorbates at surfaces: Mn on CuN/Cu(100)
The lifetimes of spin excitations of Mn adsorbates on CuN/Cu(100) are
computed from first-principles. The theory is based on a strong-coupling
T-matrix approach that evaluates the decay of a spin excitation due to
electron-hole pair creation. Using a previously developed theory [Phys. Rev.
Lett. {\bf 103}, 176601 (2009) and Phys. Rev. B {\bf 81}, 165423 (2010)], we
compute the excitation rates by a tunneling current for all the Mn spin states.
A rate equation approach permits us to simulate the experimental results by
Loth and co-workers [Nat. Phys. {\bf 6}, 340 (2010)] for large tunnelling
currents, taking into account the finite population of excited states. Our
simulations give us insight into the spin dynamics, in particular in the way
polarized electrons can reveal the existence of an excited state population. In
addition, it reveals that the excitation process occurs in a way very different
from the deexcitation one. Indeed, while excitation by tunnelling electrons
proceeds via the s and p electrons of the adsorbate, deexcitation mainly
involves the d electrons
Size effects in surface reconstructed and silicon nanowires
The geometrical and electronic structure properties of
silicon nanowires in the absence of surface passivation are studied by means of
density-functional calculations. As we have shown in a recent publication [R.
Rurali and N. Lorente, Phys. Rev. Lett. {\bf 94}, 026805 (2005)] the
reconstruction of facets can give rise to surface metallic states. In this
work, we analyze the dependence of geometric and electronic structure features
on the size of the wire and on the growth direction
Metallic and semi-metallic <100> silicon nanowires
Silicon nanowires grown along the -direction with a bulk Si core are
studied with density functional calculations. Two surface reconstructions
prevail after exploration of a large fraction of the phase space of nanowire
reconstructions. Despite their energetical equivalence, one of the
reconstructions is found to be strongly metallic while the other one is
semi-metallic. This electronic-structure behavior is dictated by the particular
surface states of each reconstruction. These results imply that doping is not
required in order to obtain good conducting Si nanowires.Comment: 13 pages, 4 figures; Phys. Rev. Lett., in pres
A cotunneling mechanism for all-electrical Electron Spin Resonance of single adsorbed atoms
The recent development of all-electrical electron spin resonance (ESR) in a
scanning tunneling microscope (STM) setup has opened the door to vast
applications. Despite the fast growing number of experimental works on STM-ESR,
the fundamental principles remains unclear. By using a cotunneling picture, we
show that the spin resonance signal can be explained as a time-dependent
variation of the tunnel barrier induced by the alternating electric driving
field. We demonstrate how this variation translates into the resonant frequency
response of the direct current. Our cotunneling theory explains the main
experimental findings. Namely, the linear dependence of the Rabi flop rate with
the alternating bias amplitude, the absence of resonant response for
spin-unpolarized currents, and the weak dependence on the actual atomic
species.Comment: 11 pages, 3 figure
Raising and lowering operators, factorization and differential/difference operators of hypergeometric type
Starting from Rodrigues formula we present a general construction of raising
and lowering operators for orthogonal polynomials of continuous and discrete
variable on uniform lattice. In order to have these operators mutually adjoint
we introduce orthonormal functions with respect to the scalar product of unit
weight. Using the Infeld-Hull factorization method, we generate from the
raising and lowering operators the second order self-adjoint
differential/difference operator of hypergeometric type.Comment: LaTeX, 24 pages, iopart style (late submission
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