Picture of research.
Electronic detection of the spin-galvanic effect in a graphene-topological insulator van der Waals heterostructure based Hall-bar device. Spin-polarized carriers injected into the heterostructure occupy the spin-split proximitized graphene bands and shift their Fermi circles in the k-space. Consequently, the carriers acquire a transverse momentum and produce a net charge current perpendicular to their spin direction, which is known as the spin-galvanic effect. Credit: Dmitrii Khokhriakov et al., Nature Communications 11, 3657 (2020).

Spin-galvanic effect in graphene with topological topping demonstrated

Researchers at Chalmers University of Technology, Sweden, have demonstrated the spin-galvanic effect, which allows for the conversion of non-equilibrium spin density into a charge current. Here, by combining graphene with a topological insulator, the authors realize a gate-tunable spin-galvanic effect at room temperature. The findings were published in the scientific journal Nature Communications.
“We believe that this experimental realization will attract a lot of scientific attention and put topological insulators and graphene on the map for applications in spintronic and quantum technologies,” says Associate Professor Saroj Prasad Dash, who leads the research group at the Quantum Device Physics Laboratory (QDP), the Department of Microtechnology and Nanoscience – MC2.

Graphene, a single layer of carbon atoms, has extraordinary electronic and spin transport properties. However, electrons in this material experience low interaction of their spin and orbital angular moments, called spin-orbit coupling, which does not allow to achieve tunable spintronic functionality in pristine graphene. On the other hand, unique electronic spin textures and the spin-momentum locking phenomenon in topological insulators are promising for emerging spin-orbit driven spintronics and quantum technologies. Picture of Dmitrii Khokriakov.
However, the utilization of topological insulators poses several challenges related to their lack of electrical gate-tunability, interference from trivial bulk states, and destruction of topological properties at heterostructure interfaces.
“Here, we address some of these challenges by integrating two-dimensional graphene with a three-dimensional topological insulator in van der Waals heterostructures to take advantage of their remarkable spintronic properties and engineer a proximity-induced spin-galvanic effect at room temperature,” says Dmitrii Khokhriakov (to the right), PhD Student at QDP, and first author of the article.

Since graphene is atomically thin, its properties can be drastically changed when other functional materials are brought in contact with it, which is known as the proximity effect. Therefore, graphene-based heterostructures are an exciting device concept since they exhibit strong gate-tunability of proximity effects arising from its hybridization with other functional materials. Previously, combining graphene with topological insulators in van der Waals heterostructures, the researchers have shown that a strong proximity-induced spin-orbit coupling could be induced, which is expected to produce a Rashba spin-splitting in the graphene bands. As a consequence, the proximitized graphene is expected to host the spin-galvanic effect, with the anticipated gate-tunability of its magnitude and sign. However, this phenomenon has not been observed in these heterostructures previously.
“To realize this spin-galvanic effect, we developed a special Hall-bar-like device of graphene-topological insulator heterostructures,” says Dmitrii Khokhriakov.

Picture of Saroj Dash.The devices were nanofabricated in the state-of-the-art cleanroom at MC2 and measured at the Quantum Device Physics Laboratory. The novel device concept allowed the researchers to perform complementary measurements in various configurations via spin switch and Hanle spin precession experiments, giving an unambiguous evidence of the spin-galvanic effect at room temperature.
“Moreover, we were able to demonstrate a strong tunability and a sign change of the spin galvanic effect by the gate electric field, which makes such heterostructures promising for the realization of all-electrical and gate-tunable spintronic devices,” concludes Saroj Prasad Dash (to the left).

The researchers acknowledge financial support from the European Union Graphene Flagship, Swedish Research Council, VINNOVA 2D Tech Center, FlagEra, and AoA Materials and EI Nano program at Chalmers University of Technology.

Illustration: Dmitrii Khokhriakov
Photo of Saroj Prasad Dash: Oscar Mattsson
Photo of Dmitrii Khokhriakov: Private

Read the full paper in Nature Communications >>>

References
1. D. Khokhriakov, A.M. Hoque, B. Karpiak, & S.P. Dash, Gate-tunable spin-galvanic effect in graphene-topological insulator van der Waals heterostructures at room temperature, Nature Communications. 11, 3657 (2020).
2. A. Dankert, P. Bhaskar, D. Khokhriakov, I. H. Rodrigues, B. Karpiak, M. V. Kamalakar, S. Charpentier, I. Garate, S.P. Dash. Origin and evolution of surface spin current in topological insulators. Phys. Rev. B 97, 125414 (2018).
3. A. Dankert, J. Geurs, M. V. Kamalakar, S. Charpentier, & S.P. Dash, Room Temperature Electrical Detection of Spin Polarized Currents in Topological Insulators. Nano Letters 15, 7976–7981 (2015).
4. D. Khokhriakov, A. W. Cummings, K. Song, M. Vila, B. Karpiak, A. Dankert, S. Roche, S. P. Dash, Tailoring emergent spin phenomena in Dirac material heterostructures. Science Advances. 4, eaat9349 (2018).

Published: Mon 31 Aug 2020.