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Transport properties

Transport properties

Chirality affects current flow in graphene transistors

23 Oct 2015 Isabelle Dumé
Chiral subways: electrons tunnel between the two wavefunctions

The handedness or “chirality” of electrons affects how current flows in graphene transistors, according to new work done by researchers in the UK and Russia. The team’s findings could help to make better graphene-based electronic devices and could even lead to a new technology, dubbed “chiraltronics”.

Graphene is a sheet of carbon atoms just one atom thick, arranged in a honeycomb lattice. The material is unique in that each electron moves along the sheet relativistically, as if it had no mass, with a speed of 1000 km/s. These electrons are also “chiral” in that they are either “right-handed” or “left-handed” – they are mirror images of each other. The electronic states that they can occupy are also chiral.

The UK–Russia team has now studied these electrons in detail by looking at the way current flows in a simple structure made up of a four-atom-thick layer of boron nitride (BN), sandwiched between two layers of graphene. When a voltage is applied, more electrons can be added to one of the graphene layers, so that it becomes negatively charged, and electrons are removed from the other layer so that it becomes positively charged. The BN barrier layer is thin enough so that electrons can pass between the graphene layers by quantum tunnelling, giving rise to an electrical current.

Quantum ‘selection rule’

Team-member Laurence Eaves at the universities of Nottingham and Manchester explains that in the tunnelling process, electrons obey a quantum “selection rule” – right-handed electrons prefer to enter right-handed states while left-handed electrons prefer to enter left-handed states. These processes determine how strong the tunnel current is in these devices. Processes in which a right-handed electron tunnels into a left-handed state (and vice versa) are rare, and do not contribute significantly to the current.

“The chirality or handedness of our tunnelling electrons shows up clearly when we measure how the current flowing through the graphene transistor changes with applied bias voltage,” explains Eaves. “However, we can more precisely study the effect by applying a strong magnetic field perpendicular to the plane of the graphene layer. This field acts to quantize the electron motion, giving rise to a ‘ladder’ of unequally spaced energy levels,” he adds. The high magnetic-field measurements allowed the researchers to demonstrate that the energy, momentum and spin of the electrons are conserved in the tunnelling process, along with their chirality.

“Electronics is a technology that processes information by controlling the free motion of electrons, while spintronics exploits the spin of an electron as well as its charge,” says Eaves. “It will be interesting to see if the chirality of electrons in graphene-based electronics devices could be exploited in the future to develop a new technology – chiraltronics,” he adds.

The research is published in Nature Physics doi:10.1038/nphys3507.

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