An international team of researchers has used a graphene-based composite material and found a new route to ultra-low-power transistors.

New research could lead the way to much needed low-energy consumption electronics. Credit: University of York

Since transistors are squeezed into more smaller areas inside computer chips, the semiconductor industry struggles to control overheating in devices.

Now, the University of York and Roma Tre University researchers believe that the solution to this problem lies in composite materials developed from monolayers of the transition metal dichalcogenide (TMDC) and graphene. They found that these materials could be employed to attain good electrical control over the electron’s spin, its tiny compass needle.

The new research, published in the Physical Review Letters journal on November 7th, 2017, could guide to much needed low-energy consumption electronics.

Applications

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 polarization of conduction spins. Dr Aires Ferreira, Lead Researcher, Department of Physics, The University of York

Ferreira added, “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.”

Spin

The electron’s spin, like a tiny, point-like magnet, can only point in two directions – up or down. In materials where a key fraction of electrons’ spins is arranged, a magnetic response is generated, which can be employed to encode information.

‘Spin currents’ - developed from ‘up’ and ‘down’ spins flowing in opposite directions – contain no net charge, and thus in theory, generate no heating. Therefore, the control of spin information would lead the way towards ultra-energy-efficient computer chips.

The team of researchers revealed that when a small current passed through the graphene layer, the electrons’ spin polarizes in plane because of ‘spin-orbital’ forces caused by the proximity to the TMDC base. The researchers also revealed that the efficiency of charge-to-spin conversion could be rather high even at room temperature.

Properties

Manuel Offidani, a PhD student with York’s Department of Physics, performed most of the complex calculations in this research. He said, “The current-induced polarization 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. Manuel Offidani, PhD student, Department of Physics, The University of York

Professor Roberto Raimondi, who heads the spintronics group at Roma Tre University, stated, “The possibility of orienting the electron spin with electrical currents is attracting a lot of attention in the spintronics community and arises generally as a consequence of specific symmetry conditions.

As such this phenomenon represents a perfect example where fundamental and applied research go happily together. In this respect, our calculations demonstrate that graphene combined with the transition metal dichalcogenides is an ideal platform where abstract theoretical principles may find immediate application in showing the way to experimental and technological development. Professor Roberto Raimondi, Spintronics group, Roma Tre University

Efficiency

Current-induced spin polarization in non-magnetic media was demonstrated at first in 2001 in semiconductors and, very recently, in metallic hetero-interfaces. Currently, the researchers predict that a similar effect takes place in graphene on TMDC monolayer.

Surprisingly, the researchers discovered that the unique character of electronic states in graphene allow charge-to-spin conversion efficiency of up to 94%. This opens up the chance of graphene-based composite material becoming the source for ultra-compact and greener spin-logic devices.

Dr Mirco Milletarì, a former member of the spintronics group at Roma Tre University, stated, “This work follows insights gained from understanding fundamental laws that enabled us to envisage systems where the efficiency of charge-to-spin conversion can be optimal for technological applications. In particular, the much needed low-energy consumption electronics that will improve durability and performances of future devices.”

The Royal Society and the Engineering and Physical Sciences Research Council (EPSRC) funded the research.