Complete report - Donostia International Physics Center - Euskal ...

2011 DIPC Highlight
Time-dependent electron
phenomena at surfaces
R. Díez Muiño, D. Sánchez-Portal, V.M. Silkin, E.V. Chulkov, and P.M. Echenique
Proceedings of the National Academy of Science PNAS 108, 971 (2011)
The research work summarized in the article provides new insight into the physical phenomena that
appear in very small systems, in the nanometer scale, and very short times, in the attosecond scale.
In the last decade and due to the development of sophisticated experimental techniques, mostly based
on laser sources, electron dynamics in nanometer-scale regions has become accessible and reasonably
well understood. However, these techniques were unable to obtain time resolutions below the femtosecond
scale until very recently. In practice, this limitation meant that electron dynamics could not be studied
in the time scale in which chemical reactions actually happen. Ultrashort laser pulses have dramatically
change the situation and attosecond physics is currently becoming a reality. Nowadays, it is indeed possible
to reach the time scale in which a chemical reaction is produced or an electron is photoemitted
from a surface or a nanostructure. The understanding and control of charge transfer times and electronic
excitation survival times is a necessary step to understand, control, and design physical and chemical
processes at surfaces. This is the focus of the scientific article published by DIPC researchers in PNAS.
In the research work summarized in the article, the authors use state-of-the-art theoretical methods to
analyze very refined features of femtosecond and attosecond electron dynamics. In particular, it is shown
that the screening of localized charges at metal surfaces is created locally in the attosecond time scale,
while collective excitations transfer the perturbation to larger distances in longer time scales. The time
necessary for an electron to jump from an adsorbed atom to a magnetic surface is also analyzed. It is predicted
that the elastic width of the resonance in excited alkali adsorbates on ferromagnetic surfaces can
depend on spin orientation in a counterintuitive way. Active manipulation and control of magnetic ordering
down to a single atomic magnet, such as a single atomic spin on a surface, is of great importance for potential
applications in spintronics. Finally, the electron–electron and electron–phonon contributions to the
electronic excited states widths in ultrathin metal layers are quantitatively evaluated. The time that an electron
spends traveling inside an ultrathin Pb film without suffering a collision with other electrons is found
to depend on the film thickness. Increasing the thickness of an ultrathin Pb film by just one atomic layer
can reduce the travelling time of electrons before collision. However, increasing the thickness of the film
by two atomic layers can considerably increase this travelling time.
Time evolution of the change in electronic density induced by an external point charge at a Cu surface.
Confinement and spin effects are
key factors in the behavior of electron
dynamics at metal surfaces.
In summary, the study provides new insight into the physical phenomena that appear in very small systems,
in the nanometer scale, and very short times, in the attosecond scale. It shows that some of the properties
found in ultrafast processes at the nanoscale are far from intuitive. An additional conclusion that can be
extracted from this research work is that confinement and spin effects are key factors in the behavior of
electron dynamics at metal surfaces.
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