Nav: Home

Physics at the edge

June 12, 2019

In 2005, condensed matter physicists Charles Kane and Eugene Mele considered the fate of graphene at low temperatures. Their work led to the discovery of a new state of matter dubbed a "topological insulator," which would usher in a new era of materials science.

"A topological insulator is a material that is an insulator in its interior but is highly conducting on its surface," said UC Santa Barbara assistant physics professor Andrea Young. In two-dimensions, an ideal topological insulator would have "ballistic" conductance at its edges, Young explained, meaning that electrons traveling through the region would encounter zero resistance.

Ironically, while Kane and Mele's work would lead to the discovery of topological insulating behavior in a wide variety of materials, their original prediction -- of a topological insulator in graphene -- has remained unrealized.

At the heart of the trouble is spin-orbit coupling -- a weak effect in which the spin of the electron interacts with its orbital motion aroun the nucleus. Critical to all topological insulators, spin-orbit coupling is exceptionally weak in graphene, so that any topological insulating behavior is drowned out by other effects arising from the surface on which the graphene is supported.

"The weak spin-orbit coupling in graphene is a great pity," said postdoctoral researcher Joshua Island, because in practice things haven't really worked out that well for topological insulators in two dimensions. "The two dimensional topological insulators known to date are disordered and not very easy to work with," Island said. The conductance at the edges tends to diminish rapidly with the distance the electrons travel, suggesting it is far from ballistic. Realizing a topological insulator in graphene, an otherwise highly perfect two dimensional material, could provide a basis for low-dissipation ballistic electrical circuits or form the material substrate for topologically protected quantum bits.

Now, in work published in the journal Nature, Island, Young and their collaborators have found a way to turn graphene into a topological insulator (TI). "The goal of the project was to increase or enhance the spin-orbit coupling in graphene," lead author Island said, adding that attempts have been made over the years with limited success. "A way to do this is to put a material that has a very large spin-orbit coupling in close proximity with the graphene. The hope was that by doing that your graphene electrons will take on this property of the underlying material," he explained.

The material of choice? After studying several possibilities, the researchers settled on a transition metal dichalcogenide (TMD), consisting of the transition metal tungsten and the chalcogen selenium. Similar to graphene, tungsten diselenide comes in two-dimensional monolayers, bound together by van der Waals forces, which are relatively weak and distance-dependent interactions between atoms or molecules. Unlike graphene, however, the heavier atoms of the TMD lead to stronger spin-orbit coupling. The resulting device feature's graphene's ballistic electron conductance imbued with the strong spin-orbit coupling from the nearby TMD layer.

"We did see a very clear enhancement of that spin-orbit coupling," Island said.

"By adding spin-orbit coupling of just the right type, Joshua was able to find that this in fact leads to a new phase which is almost topologically insulating," Young said. In the original idea, he explained, the topological insulator consisted of a monolayer of graphene with a strong spin-orbit coupling.

"We had to use a trick only available in graphene multilayers to create the right type of spin-orbit coupling," Young explained about their experiment, which used a graphene bilayer. "And so you get something that approximates two topological insulators stacked on top of each other." Functionally, however, Island's device performs as well as other known 2D topological insulators -- the all-important edge states propagate for at least several microns, much longer than in other known TI materials.

Furthermore, according to Young, this work is one step closer to building an actual topological insulator with graphene. "Theoretical work has since shown that a graphene trilayer, fabricated in the same way, would lead to a true topological insulator."

Most importantly, the devices realized by Island and Young can be easily tuned between a topological insulating phase and a regular insulator, which does not have conducting edge states.

"You can route these perfect conductors around wherever you want," he said, "And that's something nobody's been able to do with other materials."
-end-
Research in this study was conducted also by Xingshan Cui, Eric Spanton and Haoxin Zhou at UCSB; Cyprian Lewandowski, Jun Yong Khoo and Leonid Levitov at Massachusetts Institute of Technology; Daniel Rhodes and James Hone at Columbia University; Takashi Taniguchi and Kenji Watanabe at National Institute for Materials Science, Japan; and Michael Zaletel at UC Berkeley

University of California - Santa Barbara

Related Graphene Articles:

How do you know it's perfect graphene?
Scientists at the US Department of Energy's Ames Laboratory have discovered an indicator that reliably demonstrates a sample's high quality, and it was one that was hiding in plain sight for decades.
Graphene is 3D as well as 2D
Graphene is actually a 3D material as well as a 2D material, according to a new study from Queen Mary University of London.
Conductivity at the edges of graphene bilayers
For nanoribbons of bilayer graphene, whose edge atoms are arranged in zigzag patterns, the bands of electron energies which are allowed and forbidden are significantly different to those found in monolayer graphene.
How to purify water with graphene
Scientists from the National University of Science and Technology 'MISIS' together with their colleagues from Derzhavin Tambov State University and Saratov Chernyshevsky State University have figured out that graphene is capable of purifying water, making it drinkable, without further chlorination.
Decoupled graphene thanks to potassium bromide
The use of potassium bromide in the production of graphene on a copper surface can lead to better results.
1 + 1 does not equal 2 for graphene-like 2D materials
Physicists from the University of Sheffield have discovered that when two atomically thin graphene-like materials are placed on top of each other their properties change, and a material with novel hybrid properties emerges, paving the way for design of new materials and nano-devices.
Graphene's magic is in the defects
A team of researchers at the New York University Tandon School of Engineering and NYU Center for Neural Science has solved a longstanding puzzle of how to build ultra-sensitive, ultra-small electrochemical sensors with homogenous and predictable properties by discovering how to engineer graphene structure on an atomic level.
Graphene on the way to superconductivity
Scientists at HZB have found evidence that double layers of graphene have a property that may let them conduct current completely without resistance.
A human enzyme can biodegrade graphene
Graphene Flagship partners discovered that a natural human enzyme can biodegrade graphene.
Sculpting with graphene foam
Rice University scientists have developed a simple way to produce conductive, three-dimensional objects made of graphene foam.
More Graphene News and Graphene Current Events

Top Science Podcasts

We have hand picked the top science podcasts of 2019.
Now Playing: TED Radio Hour

Risk
Why do we revere risk-takers, even when their actions terrify us? Why are some better at taking risks than others? This hour, TED speakers explore the alluring, dangerous, and calculated sides of risk. Guests include professional rock climber Alex Honnold, economist Mariana Mazzucato, psychology researcher Kashfia Rahman, structural engineer and bridge designer Ian Firth, and risk intelligence expert Dylan Evans.
Now Playing: Science for the People

#540 Specialize? Or Generalize?
Ever been called a "jack of all trades, master of none"? The world loves to elevate specialists, people who drill deep into a single topic. Those people are great. But there's a place for generalists too, argues David Epstein. Jacks of all trades are often more successful than specialists. And he's got science to back it up. We talk with Epstein about his latest book, "Range: Why Generalists Triumph in a Specialized World".
Now Playing: Radiolab

Dolly Parton's America: Neon Moss
Today on Radiolab, we're bringing you the fourth episode of Jad's special series, Dolly Parton's America. In this episode, Jad goes back up the mountain to visit Dolly's actual Tennessee mountain home, where she tells stories about her first trips out of the holler. Back on the mountaintop, standing under the rain by the Little Pigeon River, the trip triggers memories of Jad's first visit to his father's childhood home, and opens the gateway to dizzying stories of music and migration. Support Radiolab today at Radiolab.org/donate.