A new quantum phenomenon helps to understand the fundamental limits of graphene electronics

A team of researchers from the universities of Manchester, Nottingham and Loughborough has discovered a quantum phenomenon that helps to understand the fundamental limits of graphene electronics.

Posted in Nature Communications, the book describes how electrons of a single sheet of atomically thin graphene disperse out of the vibrating carbon atoms that make up the hexagonal crystal lattice.

By applying a magnetic field perpendicular to the graphene plane, the current-carrying electrons are forced to move into closed "cyclotron" circular orbits. In pure graphene, the only way for an electron to escape from this orbit is to bounce off a "phonon" during a broadcast event. These phonons are energy beams and momentum resembling particles and are the "quanta" sound waves badociated with the vibrating carbon atom. Phonons are generated in increasing numbers when the graphene crystal is warmed from very low temperatures.

By pbading a small electrical current through the graphene sheet, the team was able to accurately measure the amount of energy and momentum transferred between an electron and a phonon during an event. of diffusion.

Their experience revealed that two types of phonons scatter electrons: transverse acoustic phonons (TAs) in which the carbon atoms vibrate perpendicularly to the propagation direction of the phonons and wave motion (somewhat similar to the surface waves on the surface). Water) and longitudinal acoustic phonons (LA). in which the carbon atoms vibrate in both directions in the direction of the phonon and the motion of the wave; (This movement is somewhat badogous to the movement of sound waves in the air).

The measurements provide a very accurate measurement of the speed of both types of phonons, a measurement that is otherwise difficult to perform for the case of a single atomic layer. An important result of the experiments is the discovery that TA phonon scattering dominates LA phonon scattering.

The observed phenomenon, commonly called magnetophonon oscillation, has been measured in many semiconductors years before the discovery of graphene. It is one of the oldest quantum transport phenomena known for more than 50 years, prior to the quantum Hall effect. While graphene has a number of new exotic electronic properties, this rather basic phenomenon has remained hidden.

Laurence Eaves and Roshan Krishna Kumar, co-authors of the book, said: "We were pleasantly surprised to see such prominent magnetophonon oscillations appearing in graphene, transport in graphene."

Their appearance requires two key ingredients. First and foremost, the team had to produce high quality graphene transistors with large surfaces at the National Graphene Institute. If the dimensions of the device are less than a few micrometers, the phenomenon can not be observed.

Piranavan Kumaravadivel of the University of Manchester, the lead author of the paper, said: "At the beginning of experiments on quantum transport, macroscopic crystals of millimeter size were studied.In most work on quantum transport on graphene, devices studied a few micrometers It seems that the manufacture of larger devices with graphene is not only important for the applications, but also for the fundamental studies. "

The second ingredient is temperature. Most graphene quantum transport experiments are performed at ultra-cold temperatures to slow the carbon atoms into vibration and to "freeze" the phonons that usually break quantum coherence. Therefore, graphene is heated because the phonons must be active to cause the effect.

Mark Greenaway, of Loughborough University, who has worked on the quantum theory of this effect, said: "This result is extremely exciting: it opens a new way to probe the properties of phonons in two-dimensional crystals and their heterostructures, to better understand the electron-phonon interactions in these promising materials, which is essential to develop them for use in new devices and applications. "