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UNH Researchers Prove
Existence Of New Type Of Electron Wave
Durham, NH, 5 July 2007: New research
led by University of
New Hampshire 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 of UNH, is 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 at
UNH. “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 at UNH. “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 lose 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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