Physicists use oscillations of atoms to control a phase transition


Date:
July 8, 2020
Source:
University of Go"ttingen
Summary:
The goal of ''Femtochemistry'' is to film and control chemical
reactions with short flashes of light. Using consecutive
laser pulses, atomic bonds can be excited precisely and broken
as desired. So far, this has been demonstrated for selected
molecules. Researchers have now succeeded in transferring this
principle to a solid, controlling its crystal structure on the
surface.



FULL STORY ==========================================================================
The goal of "Femtochemistry" is to film and control chemical reactions
with short flashes of light. Using consecutive laser pulses, atomic bonds
can be excited precisely and broken as desired. So far, this has been demonstrated for selected molecules. Researchers at the University of Go"ttingen and the Max Planck Institute for Biophysical Chemistry have
now succeeded in transferring this principle to a solid, controlling
its crystal structure on the surface.

The results have been published in the journal Nature.


==========================================================================
The team, led by Jan Gerrit Horstmann and Professor Claus Ropers,
evaporated an extremely thin layer of indium onto a silicon crystal and
then cooled the crystal down to -220 degrees Celsius. While the indium
atoms form conductive metal chains on the surface at room temperature,
they spontaneously rearrange themselves into electrically insulating
hexagons at such low temperatures. This process is known as the transition between two phases -- the metallic and the insulating -- and can be
switched by laser pulses. In their experiments, the researchers then illuminated the cold surface with two short laser pulses and immediately afterwards observed the arrangement of the indium atoms using an electron
beam. They found that the rhythm of the laser pulses has a considerable influence on how efficiently the surface can be switched to the metallic
state.

This effect can be explained by oscillations of the atoms on the surface,
as first author Jan Gerrit Horstmann explains: "In order to get from
one state to the other, the atoms have to move in different directions
and in doing so overcome a sort of hill, similar to a roller coaster
ride. A single laser pulse is not enough for this, however, and the
atoms merely swing back and forth. But like a rocking motion, a second
pulse at the right time can give just enough energy to the system to make
the transition possible." In their experiments the physicists observed
several oscillations of the atoms, which influence the conversion in
very different ways.

Their findings not only contribute to the fundamental understanding of
rapid structural changes, but also open up new perspectives for surface physics. "Our results show new strategies to control the conversion
of light energy at the atomic scale," says Ropers from the Faculty of
Physics at the University of Go"ttingen, who is also a Director at the
Max Planck Institute for Biophysical Chemistry. "The targeted control
of the movements of atoms in solids using laser pulse sequences could
also make it possible to create previously unobtainable structures with completely new physical and chemical properties."

========================================================================== Story Source: Materials provided by University_of_Go"ttingen. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Jan Gerrit Horstmann, Hannes Bo"ckmann, Bareld Wit, Felix Kurtz,
Gero
Storeck, Claus Ropers. Coherent control of a surface
structural phase transition. Nature, 2020; 583 (7815): 232 DOI:
10.1038/s41586-020-2440-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200708125355.htm

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