Graphene-based composite puts ultra-low-power transistors in a spin

It is now possible to place 100 million transistors in each square millimetre of a computer chip, but such advances come at a cost in relation to devices overheating.

Researchers from York University and Roma Tre University in Italy believe the solution lies in composite materials built from monolayers of graphene and the transition metal dichalcogenide (TMDC). They found these materials could be used to achieve a fine electrical control over the electron’s spin.

The new research, published in the journal Physical Review Letters, could lead to low-energy consumer electronics.

Lead researcher Dr Aires Ferreira, from York University’s Department of Physics, said: “For many years, we have been searching for good conductors allowing efficient electrical control over the electron’s spin.

“We found this can be achieved with little effort when two-dimensional graphene is paired with certain semiconducting layered materials. Our calculations show that the application of small voltages across the graphene layer induces a net polarisation of conduction spins.

“We believe that our predictions will attract substantial interest from the spintronics community. The flexible, atomically thin nature of the graphene-based structure is a major advantage for applications. Also, the presence of a semiconducting component opens up the possibility for integration with optical communication networks.”

An electron’s spin can only point up or down. In materials where a major fraction of electrons’ spins is aligned, a magnetic response is produced, which can be used to encode information.

According to York University, ‘spin currents’ - built from ‘up’ and ‘down’ spins flowing in opposite directions - carry no net charge, and in theory produce no heating. The control of spin information could potentially lead to ultra-energy-efficient computer chips.

The researchers showed that when a small current is passed through the graphene layer, the electrons’ spin polarise in plane due to so-called spin-orbital forces brought about by the proximity to the TMDC base. They also showed that the efficiency of charge-to-spin conversion can be quite high, even at room temperature.

Manuel Offidani, a PhD student with York’s Department of Physics, said: “The current-induced polarisation of the electron’s spin is an elegant relativistic phenomenon that arises at the interface between different materials.

“We chose graphene mainly because of its superb structural and electronic properties. In order to enhance the relativistic effects experienced by charge carriers in graphene, we investigated the possibility of matching it with recently discovered layered semiconductors.”

Current-induced spin polarisation in non-magnetic media was first demonstrated in 2001 in semiconductors and, more recently, in metallic hetero-interfaces. Now the researchers predict that a similar effect occurs in graphene on TMDC monolayer.

Surprisingly they found that the unique character of electronic states in graphene enable charge-to-spin conversion efficiency of up to 94 per cent, opening up the possibility of a graphene-based composite material becoming the basis for ultra-compact and greener spin-logic devices.