December 10, 2016 at 11:13 am

Ulloa, Alsharari Article Shows How Physical Properties Are ‘Contagious’

Abdulrhman Alsharari and Sergio Uloa, standing outside Clippinger labs

Abdulrhman Alsharari and Sergio Uloa

Gold is shiny, glass is transparent, and so on. But researchers at Ohio University and Louisiana State University have proven theoretically that it is possible to impart properties of one material onto another, by placing them in close proximity.

Physical properties of materials can be contagious, according to work by Ohio University Physics & Astronomy graduate student Abdulrhman Alsharari, who co-authored an article in Physical Review B on “Mass inversion in graphene by proximity to dichalcogenide monolayer.” His co-authors are Dr. Sergio Ulloa, Professor of Physics & Astronomy at Ohio University, and Ohio University alum Mahmoud M. Asmar ’15Ph.D. of Louisiana State University. Asmar studied with Ulloa at OHIO.

In their research, the two materials in question are in the large family of layered materials, graphene and molybdenum disulfide (MoS2). Both exist at a monoatomic layer (graphene) or triatomic layer (MoS2) and represent examples of the ultimately thin layers that different experimental groups all over the world are now routinely preparing.

By placing a layer of graphene on top of one of MoS2, the peculiarly strong spin-orbit interaction in the latter is transferred over to graphene. This example of “proximity effect” makes the electrons in graphene experience new forces that align their spin according to their direction of motion within the graphene layer. Researchers in many labs have explored before other ways of imparting such spin-orbit effects on graphene, but none as simple as adhering graphene to a single layer of MoS2.

The research, carried out at Ohio by Alsharari and his doctoral advisor Ulloa, in collaboration with Asmar at LSU, predicts that different interesting regimes of such proximity behavior are possible when graphene is attached onto different members of the dichalcogenide family (MoS2, WS2, WSe2, etc.), and when applying additional pressure or electrical fields.

The resulting close connection between the motion and spin direction for electrons in graphene in such combined system gives rise to fundamentally different properties from each of their individual components. In particular, they exhibit interesting topological edge states under suitable conditions, with highly conductive states along the edges of such structures. This hybrid material reflects the fundamental quantum mechanical properties deeply embedded in the twist acquired by the wave functions as they circulate around the material. This twist in the (spinor) wave function leads to an unusual insulating state in the middle of the system, coexisting with metallic edge states, in what is known as a two-dimensional topological insulator.  Other regimes are also interesting and reveal unique insights into the intrinsic structure of the electronic states in these hybrid materials.

Abstract: Proximity effects resulting from depositing a graphene layer on a TMD substrate layer change the dynamics of the electronic states in graphene, inducing spin orbit coupling (SOC) and staggered potential effects. An effective Hamiltonian that describes different symmetry breaking terms in graphene, while preserving time reversal invariance, shows that an inverted mass band gap regime is possible. The competition of different perturbation terms causes a transition from an inverted mass phase to a staggered gap in the bilayer heterostructure, as seen in its phase diagram. A tight-binding calculation of the bilayer validates the effective model parameters. A relative gate voltage between the layers may produce such phase transition in experimentally accessible systems. The phases are characterized in terms of Berry curvature and valley Chern numbers, demonstrating that the system may exhibit quantum spin Hall and valley Hall effects.

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