Physicists from the University of New Hampshire (UNH) have proven the existence of a new type of electron wave on metal surfaces: the acoustic surface plasmon. The work, published in Nature, will impact future developments in nano-optics, high-temperature superconductors, and our understanding of chemical reactions on surfaces. Acoustic surface plasmons have long been predicted but their existence has been extraordinarily difficult to prove experimentally. "Just one year ago, another group of scientists concluded that these waves do not exist," said UNH's Karsten Pohl. "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."
To identify the wave, the UNH researchers 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 was measured with a detector that was placed in an ultra-high vacuum chamber together with the beryllium sample. The energy loss was small but corresponded exactly to the theoretical prediction.
"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 co-researcher Bogdan Diaconescu. "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."
As the new plasmons are 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, and die out after a few femtoseconds after they have been created.
One of the most interesting, but still very speculative, applications of the plasmons relates to high temperature superconductivity. It is known that superconductivity happens in two-dimensional "sheets" which give rise to the special electron pairs (Cooper Pairs) which can move without resistance through the conductor. How this happens precisely is unclear but acoustic plasmons could be part of the explanation.
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Source: University of New Hampshire
