Abstract
We use a combination of experimental techniques to demonstrate a general occurrence of spin-orbit interaction (SOI) in graphene on transition metal dichalcogenide (TMD) substrates. Our measurements indicate that SOI is ultrastrong and extremely robust, despite it being merely interfacially induced, with neither graphene nor the TMD substrates changing their structure. This is found to be the case irrespective of the TMD material used, of the transport regime, of the carrier type in the graphene band, or of the thickness of the graphene multilayer. Specifically, we perform weak antilocalization (WAL) measurements as the simplest and most general diagnostic of SOI, and we show that the spin relaxation time is very short (approximately 0.2 ps or less) in all cases regardless of the elastic scattering time, whose value varies over nearly 2 orders of magnitude. Such a short spin-relaxation time strongly suggests that the SOI originates from a modification of graphene band structure. We confirmed this expectation by measuring a gate-dependent beating, and a corresponding frequency splitting, in the low-field Shubnikov–de Haas magnetoresistance oscillations in high-quality bilayer graphene devices on . These measurements provide an unambiguous diagnostic of a SOI-induced splitting in the electronic band structure, and their analysis allows us to determine the SOI coupling constants for the Rashba term and the so-called spin-valley coupling term, i.e., the terms that were recently predicted theoretically for interface-induced SOI in graphene. The magnitude of the SOI splitting is found to be on the order of 10 meV, more than 100 times greater than the SOI intrinsic to graphene. Both the band character of the interfacially induced SOI and its robustness and large magnitude make graphene-on-TMD a promising system to realize and explore a variety of spin-dependent transport phenomena, such as, in particular, spin-Hall and valley-Hall topological insulating states.
2 More- Received 30 May 2016
DOI:https://doi.org/10.1103/PhysRevX.6.041020
Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
In recent years, researchers have successfully produced an array of crystalline materials only one or a few atoms thick with nearly perfect structure. It has also become possible to manipulate these crystals and select where they are placed. A question that is attracting increasing attention pertains to how electrons move when two such atomically thin crystals are placed in direct contact (i.e., on top of each other) to form an interface. Here, we show that the interaction at the interface can be so strong that it affects not only how the electrons move in space but also their internal quantum degree of freedom, known as spin.
At interfaces formed between graphene and materials known as transition metal dichalcogenides, we find that electrons experience an extremely strong interaction between their orbital motion and spin degree of freedom. Such a phenomenon is called a spin-orbit interaction, and it is at the basis of many new counterintuitive physical effects that are the subject of current research. One example is the occurrence of so-called two-dimensional topological insulators, systems that are conducting at their edges and insulating in their interiors. These systems have been predicted to exist by theoretically studying how spin-orbit interactions influence the properties of graphene. In bare graphene, however, the spin-orbit interaction is too weak to observe the effect experimentally. We recover unexpected findings when interfaces are formed with transition metal dichalcogenides (e.g., , , and ), and we find that spin-orbit interactions are extremely strong and independent of the thickness of the graphene multilayer. Furthermore, the presence of large spin-orbit interactions does not damage the electronic properties of graphene.
We expect that our results will pave the way for future studies of spin-dependent transport phenomena such as topological insulating states.