Scientists from the University of Manchester, Nottingham and Loughborough have discovered a quantum phenomenon for understanding the fundamental limits of graphene electronics. It describes the way electrons in one atomic-thin sheet of graphene scatter the vibrating carbon atoms that make the hexagonal crystal lattice. The study has been published in Nature Communications.
Applying magnetic field perpendicular to graphene plane, current-carrying electrons are forced to move in the “cyclotron orbits” which are closed and circular. In pure graphene, electrons escape from the orbit by bouncing off “phonon” while scattering. Phonons are particle-like bundles of momentum and energy and are “quanta” of sound waves that are associated with vibrating carbon atom. When graphene crystal is warmed from very low temperature, they are generated in large numbers.
The team passed a small electric current through graphene sheet for precisely measuring the amount of momentum and energy which is transferred between electron and phonon while scattering. It revealed that two kinds of phonons scatter electrons, transverse acoustic (TA) phonons where carbon atoms vibrate perpendicular to the direction in which phonon propagates and wave motion and longitudinal acoustic phonons (LA) where carbon atoms vibrate in the direction of phonon and wave motion. These accurately measure the speeds of two kinds of phonons which is difficult to make in a single atomic layer. It also shows that TA phonon scattering dominates LA phonon scattering.
This phenomenon is termed as magnetophonon oscillation and it was measured in many semiconductors several years before graphene was discovered. It has been known longer than the quantum Hall effect and is one of the oldest quantum transport phenomena.
Roshan Krishna Kumar and Laurence Eaves, co-authors in the work said that they were surprised to discover such magnetophonon oscillations in graphene and at the same time confused why it had not been discovered before in graphene. It had two key requirements. Scientists had to fabricate high-quality transistors of graphene having large areas at National Graphene Institute. It had not been discovered if the device dimensions were smaller than a few micrometers.
Piranavan Kumaravadivel, University of Manchester and lead author of the paper said that macroscopic, millimeter-sized crystals were studied at the beginning of quantum transport experiments. The studied devices in most work on quantum transport on graphene are normally a few micrometers in size. Larger graphene devices are important for both applications and fundamental studies.
The next ingredient is temperature. Graphene quantum transport experiments are carried out at ultra-cold temperatures for slowing the carbon atoms which are vibrating and “freeze-out” the phonons which break quantum coherence. So graphene is warmed so that phonons are active to cause the effect.
Mark Greenaway, Loughborough University who also worked on the quantum theory of this effect said that the result is quite exciting as it opens a new route for understanding the phonon properties in two-dimensional crystals and heterostructures. It will also help to understand electron-phonon interactions in promising materials which is vital for new devices and applications.
Research Paper: https://www.nature.com/articles/s41467-019-11379-3