Germanene – a two-dimensional, graphene-like form of the element germanium – can carry electricity along its edges with no resistance. This unusual behaviour is characteristic of materials known as topological insulators, and the researchers who observed it say the phenomenon could be used to make faster and more energy-efficient electronic devices.
Like graphene, germanene is an atomically thin material with a honeycomb structure. Like graphene, germanene’s electronic band structure contains a point at which the valence and conduction bands meet. At this meeting point, spin-orbit coupling creates a narrow gap between the bands within the material’s bulk, causing it to act as an insulator. Along the material’s edges, however, special topological states arise that bridge this gap and allow electrons to flow unhindered.
Materials with this property – conducting electricity along their edges, while acting as insulators in their bulk – are called topological insulators. Since the edge-state electric current induces a transverse spin current, they are also known as quantum spin Hall systems by analogy with the better-known quantum Hall effect, in which strong magnetic fields induce electric current to flow along the edge of a semiconductor.
A new topological insulator emerges
In graphene, the quantum spin Hall effect is too weak to observe, but researchers led by Pantelis Bampoulis of the University of Twente in the Netherlands have now spotted it in germanene. To do this, they employed a variety of experimental and theoretical techniques, including low-temperature scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) as well as density functional theory and tight-binding calculations.
“With STM and STS, we could directly measure the electronic band structure of germanene and showed that it has a band gap in its interior and conductive states at its edges,” Bampoulis explains. “This means that it doesn’t conduct electricity in the middle, but does along its edges.”
Switching between states
The fact that germanene is slightly buckled, rather than completely flat like graphene, introduces a potentially useful property, Bampoulis adds. “In our study, we were also able to apply an electric field to our sample using the STM to change the topological state of germanene,” he tells Physics World. “When a critical field is reached, the topological band gap closes and the material becomes a topological semimetal. Beyond this field, a conventional band gap opens up and the topological edge states disappear – in other words, the germanene becomes a normal insulator.”
A periodic table for topological materials
Because germanene transitions so readily between a perfect conductor and an insulator, the researchers say it could be used to make a novel type of field-effect transistor. In the “on” state of such a device, current would flow without energy loss along the topological edge states. The team also suggests that such edge states could be useful in quantum computing, where their robustness could make quantum devices more stable and resistant to errors.
The University of Twente researchers say they are now busy trying to increase the number of conductive channels and further tune the quantum state of germanene. “These efforts will involve fabricating germanene nanoribbons (thin, elongated strips) and implementing a twist in stacks of two germanene layers,” Bampoulis reveals.
The present work is detailed in Physical Review Letters.
