Two-dimensional materials could bring about the next generation of ultra-low-power transistors, according to a new study that uses composite materials developed from monolayers of graphene and a semiconducting layered material. An international team of researchers has shown a way of producing the transistors based on such a composite material, the first to theoretically propose spin polarisation in non-magnetic 2D materials.
As reported in the journal Physical Review Letters [Offidani et al. Phys. Rev. Lett. (2017) DOI: 10.1103/PhysRevLett.119.196801], scientists from the University of York and Roma Tre University have identified a conductor that allows them to achieve a delicate electrical control over an electron's spin by combining 2D graphene with the transition metal dichalcogenide (TMDC), where the application of small voltages across the graphene layer induces a net polarization of conduction spins.
“Our calculations show how the application of small voltages across the graphene composite device enables a nearly optimal charge to spin conversion even at room temperature”Mirco Milletari
An electron's spin is similar to a small magnet that can only point either up or down. For materials where a major ratio of the electrons’ spin is aligned, this produces a magnetic response that can be used to encode information. Such spin currents have no net charge, and theoretically produce no heating, so if you can control spin information it could lead to ultra-energy-efficient computer chips as overheating continues to be a problem for transistors in semiconductor devices. As researcher Mirco Milletari said “Our calculations show how the application of small voltages across the graphene composite device enables a nearly optimal charge to spin conversion even at room temperature”.
The team found that when a small current is passed through the graphene layer, the electrons' spin polarize in plane due to “spin orbital” forces due to their proximity to the TMDC base, as well as demonstrating that the efficiency of charge-to-spin conversion can be quite high even at room temperature. It was also shown that the unique character of electronic states in graphene enable charge-to-spin conversion efficiency of up to 94%, opening up the possibility for a graphene-based composite material being the basis for ultra-compact and greener spin-logic devices.
A spintronics-based CPU could improve battery duration through a direct reduction of the dissipated energy, and also by eliminating the need for heat dissipation devices that further contribute to energy consumption. The researchers now hope to experimentally demonstrate the effect, have made early steps towards understanding quantum interference effects, and would also like to investigate the quantised version of the spin Hall effect, which could offer a novel route towards low dissipation spintronics.