Wave lake on metal surfaces

Physicists at the University of New Hampshire have proved the existence of a new type of electron wave on metal surfaces, known as the acoustic surface plasmon.

The new electron could be used in the development of nano-optics, high-temperature superconductors as well as the understanding of chemical reactions on surfaces.

‘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,’ said Bogdan Diaconescu, a postdoctoral research associate in the UNH’s physics department and one of the project leaders.

‘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 been difficult to prove in practice, and a year ago, some scientists decided that the waves did not exist.

‘These researchers have probably not been able to find the acoustic plasmon because the experiments require extreme precision and great patience,’ said Karsten Pohl, an associate professor of physics at UNH.

‘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 UNH researchers found that the acoustic surface plasmon used an extremely precise electron gun that shot slow electrons on a specially prepared surface of a beryllium crystal.

When the electrons were reflected back from the electron ‘lake’ on the surface of the metal, some of them lost an amount of energy that corresponded to the excitation of an acoustic plasmon wave. This energy loss was measured with a detector placed in an ultra-high vacuum chamber with the beryllium sample.

The researchers estimated that, depending on their energy, the waves spread down to a few nanometres and died out after a few femtoseconds (one millionth of a billionth of a second) after they have been created.

Research on metal surfaces is useful for the development of new industrial catalysts and for cleaning the exhaust of factories and cars, but it also could be applied to nano-sized engineering processes.

It could be applied, for example, to nano-microscopy and optical signal processing, as the new electron waves could be used to carry optical signals along nanometre-wide channels for up to a few micrometres.