June 19, 2018 at 12:40 pm

Ulloa, Alumni Probe Topology of Bilayer Graphene

Dr Abdulrhman Alsharari

Dr. Abdulrhman Alsharari in Clippinger Laboratory

By Amanda Biederman
NQPI Editorial Intern

A team of researchers at Ohio University have discovered a way to alter the electronic behavior of bilayer graphene, a finding that may be applied to the control of heat dissipation in electronic devices.

To implement this feat, OHIO Ph.D. alumni Dr. Abdulrhman Alsharari and Dr. Mahmoud M. Asmar ’15Ph.D. worked with Dr. Sergio Ulloa, Professor of Physics & Astronomy, to construct a theoretical model, allowing them to visualize bilayer graphene over a single layer of a transition metal dichalcogenide with semiconducting properties. The team’s work, “Proximity-induced topological phases in bilayer graphene,” was recently published as a Rapid Communications article in Physical Review B.

Many materials researchers are interested in graphene because of its unique electrical and structural properties. Graphene has a high electrical current density, allowing for the conduction of electricity within a miniscule space and low resistance. For this reason, graphene may be useful in the development of small electronics.

In the proposed approach, Ulloa’s team characterized the topological phases of the bilayer graphene-dichalcogenide structures. These phases can be controlled and driven through an applied gate voltage to the system. By understanding this phenomenon, researchers can control the structural topology of this system and fine-tune its conductive properties. Ulloa’s group has identified a unique insulating phase, which had been identified previously only in other materials, that is likely to be useful in electronics applications.

Ulloa said different types of insulators are distinguished by the information contained in the electronic wave function band topology. He said these phases are mathematically comparable to a sphere with no holes and a torus (donut) with one hole; they cannot be smoothly deformed into each other without closing the insulating bandgap.

In other words, the sphere and the torus cannot be deformed into each other without closing or opening a hole in the structure. In a similar way, the information contained in the electronic wave function cannot be deformed away without changing the character of the insulator.

“Our theoretical investigation shows that the new phase is topologically non-trivial,” Alsharari said.  “(Once this phase is achieved), new edge states appear around the borders of the system, along conducting channels that are impervious to defects and impurities.”

Abstract for Proximity-induced topological phases in bilayer graphene: We study the band structure of phases induced by depositing bilayer graphene on a transition-metal dichalcogenide monolayer. Tight-binding and low-energy effective Hamiltonian calculations show that it is possible to induce topologically nontrivial phases that should exhibit the quantum spin Hall effect in these systems. We classify bulk insulating phases through calculation of the Z2 invariant, which unequivocally identifies the topology of the structure. The study of these and similar hybrid systems under an applied gate voltage opens the possibility for tunable topological structures in real experimental systems.


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