3D Electrons Viewed At High Speed
September 18, 2012

Scientists Working To Unlock The Mysteries Of Promising Electronic Material

Lee Rannals for redOrbit.com - Your Universe Online

A team of researchers at MIT are trying to unlock the mysteries of a promising material for future electronic devices.

Scientists have managed to create three-dimensional "movies" of electron behavior in a topological insulator (TI), which were first discovered a few years ago.

TIs are considered to hold great promise for new kinds of electronic devices, and having a better understanding of the unusual behavior within the material could help scientists unlock the future benefits.

The movies the team captured show the motions of electrons as they scatter in response to a very short pulse of light. Electrons normally have mass, but when moving along the surface of TIs, they move as if they were massless.

The bulk of TI acts as an insulator, completely blocking any flow of electrons. However, the surface of the material behaves as a very good conductor, allowing electrons to flow freely and at speeds near the speed of light.

TIs are considered to be a promising material for electronic circuits and data-storage devices, but before using the material in these new devices, scientists must gather more information about the material and its behavior with electrons. In order to do, new techniques have to be developed.

Writing in the journal Physical Review Letters, the team said they have developed a new technique called the pump-probe technique to help better understand TIs.

When using the technique, the scientists shoot off a short pulse of laser light to energize the material, causing electrons to scatter. Then, they apply a second, slightly delayed pulse to illuminate it and produce an image.

“The first pulse does something to the electrons, and the second pulse captures what happened,” said assistant professor of physics Nuh Gedik.

Afterwards, the process is repeated, with the second laser pulse delayed by every-increasing increments of just a few femtoseconds, or millionths of a billionth of a second.

Each image taken shows the response of the electrons to the beam after a corresponding interval. These images can then be assembled into a movie that shows how the response changes with time.

Gedik says the fastest imaging systems now available are able to produce exposure lasting hundreds of trillionths of a second, or picoseconds, but the electron motions they were trying to capture happen so fast that "in about five picoseconds, everything is done already."

The new technique has allowed the team to discover interactions between a TI's surface and bulk electrons that had never been seen before.

“With this 3-D movie, in real time we can visualize how one population of electrons [those on the surface] scatters into the other population [inside the material],” Gedik said. “This is very important to understand.”

The researchers found that the interaction between the two is mediated by sound waves, and this interaction will take place even more intensely at higher temperatures.

“We can detect that the way they are exchanging energy is through sound waves,” he added.

Ultimately, the team's research may help open up doors to use TIs full potential, maybe even putting magnetic storage devices like hard drives in a museum.

Gedik said that understanding how the surface electrons interact will guide the future work.