July 9, 2007 Scientists have proved the existence of a new type of electron wave present on metal surfaces that could assist in the development of nano-optics and high-temperature superconductors. Known as the “acoustic surface plasmon”, the phenomenon has previously been predicted in theory but has been difficult to prove because of the incredible accuracy required to make measurements on such a minute scale. Spreading only a few nanometers (millionths of a millimetre) and lasting only millionths of a billionth of a second, researchers led by University of New Hampshire detected the waves by shooting electrons at a specially prepared surface of a beryllium crystal in an ultra-high vacuum chamber.
Standing electron waves are created when a small charge is placed close to a metal surface. Plasmons, which have been considered as a means of transmitting information on computer chips, occur when the charge is agitated and require high energy levels to generate, differing from the newly proven “acoustic” type of plasmon which can be excited with any energy (wavelength). According to the leaders of the research team, Bogdan Diaconescu and Karsten Pohl, the acoustic surface plasmon can be compared to water waves on the surface of a lake.
“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.”
Detection of the acoustic surface plasmon – which as recently as a year ago were said not to exist - involved measuring the energy loss of electrons fired at a specially prepared surface of a beryllium crystal. As the electrons were reflected back a loss of energy was detected and this tiny loss corresponds to the creation of an acoustic plasmon wave predicted in theory.
Along with the improvement of our understanding of chemical reactions on surfaces, the research will be significant in a range of areas including the development of new catalysts for cleaning exhaust systems, the undistorted transmission of optical signals to enable processing on a nano-scale and to enhance the understanding of high temperature superconductivity.
The UNH led research is published in the July 5 issue of the journal “Nature” and the paper - "Low-energy acoustic plasmons at metal surfaces” - can be downloaded in PDF format here.