Researchers Prove Existence Of New Type Of Electron Wave
By Beth Potier, Media Relations
July 11, 2007
New research led by UNH physicists has proved the existence of a new type
of electron wave on metal surfaces: the acoustic surface plasmon, which
will have implications for developments in nano-optics, high-temperature
superconductors, and the fundamental understanding of chemical reactions
on surfaces. The research, led by Bogdan Diaconescu and Karsten Pohl, was
published in the July 5 issue of the journal “Nature.”
“The existence of this wave means that the electrons on the surfaces
of copper, iron, beryllium and other metals behave like water on a lake’s
surface,” says Pohl, associate professor of physics. “When
a stone is thrown into a lake, waves spread radially in all directions.
A similar wave can be created by the electrons on a metal surface when
they are disturbed, for instance, by light.”
Acoustic surface plasmons have long been predicted on merely theoretical
grounds, their existence has been extraordinarily difficult to prove experimentally. “Just
one year ago, another group of scientists concluded that these waves do
not exist,” says Diaconescu, a postdoctoral research associate in
the Condensed Matter Group of the physics department. “These researchers
have probably not been able to find the acoustic plasmon because the experiments
require extreme precision and great patience. One attempt after the other
did not show anything if, for example, the surface was not prepared well
enough or the detectors were not adjusted precisely enough.”
The new experiment that found the acoustic surface plasmon used an extremely
precise electron gun, which shoots slow electrons on a specially prepared
surface of a beryllium crystal. When the electrons are reflected back from
the electron lake on the surface of the metal, some of them loose an amount
of energy that corresponds to the excitation of an acoustic plasmon wave.
This energy loss could be measured with a detector that was placed in an
ultra-high vacuum chamber, together with the beryllium sample. The energy
loss is small but corresponds exactly to the theoretical prediction.
Research on metal surfaces is important for the development of new industrial
catalysts and for the cleaning the exhaust of factories and cars. As the
new plasmons are very likely to play a role in chemical reactions on metal
surfaces, theoretical and experimental research will have to take them
into account as a new phenomenon in the future. In addition, there are
several promising perspectives in nano-microscopy and optical signal processing
when the new plasmons are excited directly with light diffracted off very
small nano-features.
The researchers estimate that, depending on their energy, the waves spread
down to a few nanometers (one millionth of a millimeter), and die out after
a few femtoseconds (one millionth of a billionth of a second) after they
have been created, thus witnessing very fast chemical processes at the
atomic scale.
Another potential application is using the waves to carry optical signals
along nanometer-wide channels for up to few micrometers and as such allowing
the integration of optical signal propagation and processing devices on
nanometer-length scales. And one of the most interesting but still very
speculative applications of the plasmons relates to high temperature superconductivity.
It is known today that the superconductivity happens in two-dimensional
sheets in the material, which give rise to the special electron pairs which
can move without resistance through the conductor. How this happens precisely
is unclear but acoustic plasmons could be part of the explanation. If this
is the case, it is a great advantage that it is now possible to study the
new acoustic plasmons on surfaces, where they is much easier to probe them
than inside the material.
Diaconescu and Pohl received funding for this research from the National
Science Foundation.
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