Synopsis: A Good Match for Germanene

Germanene—the germanium analog of graphene—can remain a topological insulator when placed on a molybdenum disulfide substrate.
Synopsis figure
L. Zhang et al., Phys. Rev. Lett. (2016)

Researchers have recently synthesized a number of graphene “cousins”: one-atom-thick sheets of group-IV elements, such as silicene (made of silicon) and germanene (germanium). Such materials are predicted to be topological insulators—materials that are insulating in their interior but have conducting surface states. However, germanene and silicene have so far been synthesized on metallic substrates, whose conductivity makes it impossible to study the topological-insulator properties. Two studies now indicate that germanene can be stably grown on an insulating MoS2 substrate while remaining a topological insulator.

The difference between graphene and germanene lies in the spin-orbit coupling (SOC) needed to create the bandgap necessary for topological insulators. Germanene’s SOC is much stronger than graphene’s, but it can decrease when germanene’s electrons hybridize with those of a substrate.

Calculations by Taher Amlaki and Paul Kelly at the University of Twente, Netherlands, and colleagues suggest the SOC loss can be kept at bay when a MoS2 substrate is used. The authors found that germanene-MoS2 bilayers retain a small insulating bandgap, and germanene sandwiched between two MoS2 layers keeps an even larger bandgap. In both sets of calculations germanene behaved as a topological insulator.

In an independent study, Lijie Zhang, also at the University of Twente, and colleagues grew germanene on MoS2 and characterized its electronic behavior via scanning tunneling experiments. The measured density of electronic states of the material is suggestive of a “Dirac” material, in which electrons move in two dimensions as massless particles. According to Zhang et al., germanene retains its Dirac nature because its electrons couple only weakly to the MoS2 substrate through van der Waals interactions. Planned experiments at low temperatures will assess whether germanene-on-MoS2 is a topological insulator as predicted by Kelly’s team.

This research is published in Physical Review Letters.

–Matteo Rini

Matteo Rini is the Deputy Editor of Physics.


More Features »


More Announcements »

Subject Areas

Topological Insulators

Previous Synopsis

Related Articles

Synopsis: One-Way Sound Transport

Synopsis: One-Way Sound Transport

An array of air channels behaves as an “acoustic Chern insulator” in which sound waves travel only around the edges and only in one direction. Read More »

Synopsis: Interrupting Flow in a 2D Topological Insulator
Topological Insulators

Synopsis: Interrupting Flow in a 2D Topological Insulator

Theorists predict that backscattering of electrons by nonmagnetic impurities can disrupt current flow in a 2D topological insulator, in agreement with experiments. Read More »

Viewpoint: Non-Hermitian Topological Systems
Topological Insulators

Viewpoint: Non-Hermitian Topological Systems

A theoretical framework tries to sort out where topological phases may arise in non-Hermitian systems—which are systems with gain and loss. Read More »

More Articles