ICREA Prof. Stephan Roche and ICREA Prof. Sergio Valenzuela, Group Leaders at ICN2, in collaboration with researchers from Delaware University and CEA have recently published an article in Physical Review Letters. They question the interpretation of large values from the Spin Hall effect (SHE) signals, reported experimentally in chemically decorated graphene. Further, they propose a new device geometry to suppress background contributions on the non-local resistance to access the upper limit of SHE in two-dimensional materials.

Although graphene has attractive properties, it is inactive for the Spin Hall effect (SHE), a spin transport phenomenon mediated by strong spin-orbit coupling, in which opposite spins are deviated in contrary directions while propagating inside a channel. Large values of SHE have been recently reported in graphene decorated with adatoms. Members of two ICN2 Groups, in collaboration with researchers in the USA and France, have produced a fully quantum simulation of this phenomenon to analyse these experimental results. Further, they have found multiple background contributions to the non-local resistance, which was argued to be the smoking gun of SHE. A novel device configuration is proposed to suppress these contributions and quantify the upper limit for the SHE in two-dimensional materials.

The results have been recently published in Physical Review Letters. The first author of the article is Dr Dinh Van Tuan, a researcher that got his PhD at ICN2. ICREA Research Professor Stephan Roche, Group Leader of the ICN2 Theoretical and Computational Nanoscience Group, and ICREA Research Professor Sergio Valenzuela, Group Leader of the ICN2 Physics and Engineering Nanodevices Group, have collaborated on this study to clarify the experimental validity of prior claims and advance in the understanding of SHE in disordered graphene.

The spin, an intrinsic property of quantum mechanics from elementary particles, is one of the key concepts needed to understand the present work. Another one is the already mentioned Spin Hall effect, a transport phenomenon based on the spin accumulation on the lateral surfaces of a sample. It appears under the influence of both a spin-orbit coupling effect, an interaction of a particle’s spin with its motion, and the external action of an electric current. The discovery of graphene has ignited a considerable amount of activity, owing to its unique electronic properties and versatility for practical applications. Nevertheless, the intrinsically small spin-orbit coupling makes graphene inactive for the SHE. However, recent measurements on graphene decorated with heavy adatoms (atoms on a crystal surface, like copper, gold and silver) have extracted exceptionally large values of the Spin Hall angle. This angle, defined as the ratio of the generated spin current and injected charge current, enables the researchers to gauge the Spin Hall effect on materials.

The experiments reporting an unexpectedly large SHE in graphene decorated with adatoms have raised a fierce controversy. To date, measured values for the Spin Hall angle range from 0.0001 in semiconductors up to 0.3 in some metals, which are finding important applications in the magnetic memory market. The measurements on decorated graphene indicate a spin Hall angle of about 0.2, which would make modified graphene technologically relevant. In the context of an international collaboration, Prof. Sergio Valenzuela’s Group and Prof. Stephan Roche’s Group, have theoretically analysed if such large Hall angle observed experimentally is plausible.

The authors found multiple background contributions to the non-local resistance, as measured experimentally, that question the spin origin of the measurements. That finding motivated them to design a device which suppresses the background contributions and allowed them to quantify the upper limit for spin current generation. The ICN2 researchers are opening new directions for the experiments on this field and making possible the engineering of the Spin Hall effect.