A quantum phenomenon highlights the limits of graphene electronics

Copyright: iStock/Andrey Prokhorov

A quantum phenomenon that tests the limits of graphene’s use in electricity has been discovered by a research team from The University of Manchester, The University of Nottingham and The University of Loughborough.

The research addressed how electrons in graphene scatter off the vibrating carbon atoms in the hexagonal crystal lattice. The researchers applied a magnetic field perpendicular to the atomically thin sheet of graphene. This magnetic field forced the current-carrying electrons to move in a closed circular orbit.

There is only one way for an electron from pure graphene to escape this orbit, this is by bouncing off a “phonon” in a scattering event. These phonons are particle-like bundles of energy and momentum. By warming graphene crystals for a very low temperature, researchers discovered they can generate these phonons.

Once the research team triggered the phonon scattering event, they passed a small electrical current through the sheet of graphene in order to measure the precise amount of energy and momentum that can be transferred between and electron and a phonon during the event.

What happens during these scatter events?

The researchers discovered that there are two types of phonon scatter. The first being named transverse acoustic (TA) phonons. TA phonons force the carbon atoms to vibrate perpendicular to the direction of phonon propagation and wave motions, such motion can be likened to the way waves flow on the surface of water.

The second type of phonon scatter is longitudinal acoustic (LA). LA phonons stimulate the carbon atoms to vibrate back and forth along the direction of the phonon and the wave motion, which motion is comparable to the motion sound waves make through the air.

By assessing these events, researchers have found a very accurate way to measure the speed of both types of phonons. Such measurements have indicated that the TA phonon scattering events dominate over LA phonon scattering.

Laurence Eaves and Roshan Krishna Kumar, co-authors of the work, said “We were pleasantly surprised to find such prominent magnetophonon oscillations appearing in graphene. We were also puzzled why people had not seen them before, considering the extensive amount of literature on quantum transport in graphene.”

Mark Greenaway, from Loughborough University, worked on the theory of this effect said: “This result is extremely exciting – it opens a new route to probe the properties of phonons in two-dimensional crystals and their heterostructures. This will allow us to better understand electron-phonon interactions in these promising materials, understanding which is vital to develop them for use in new devices and applications.”

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