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Physics on the Edge

Electrons get moving along the surfaces of a new class of materials

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For the hottest thing in condensed-matter physics, check out the local liquor store. Hidden inside a device for chilling wine is the unusual compound called bismuth telluride.

For physicists, bismuth telluride does more than keep champagne frosty. Under the right laboratory conditions, this crystal can start behaving in weird and wonderful ways. Over the past couple of years, researchers have made several toast-worthy new discoveries involving bismuth telluride and other related materials, known as topological insulators.

These materials exhibit a split personality when it comes to conducting electrons. The bulk of the material is an insulator — in other words, it blocks the flow of electric current. But sometimes the surface can act as a conductor, shuttling electrons merrily along their way.

Just a few years ago, no one thought that materials could both insulate and conduct at the same time in this way. “This is a new state of matter — in condensed-matter physics this is the highest goal,” says Shoucheng Zhang, a theoretical physicist at Stanford University. “It is such a beautiful thing.”

Beyond their theoretical beauty, topological insulators might also one day prove practical in the electronics industry: Already researchers at Prince-ton University have made a topological insulator behave as a superconductor, transporting electrons without any resistance. And topological insulators might serve as a laboratory for creating and studying new types of particles never before seen in nature.

Topological insulators are “a new way of thinking of the states of matter,” says Zahid Hasan, an experimental physicist at Princeton. “They’re teaching us totally new physics.”

From the top

The story of topological insulators begins with another one of physics’s trendiest materials: graphene, a sheet of carbon atoms in a honeycomb pattern just one layer thick. Researchers have been keen to discover what unusual properties result from this atomic arrangement — a search that led to the discovery of a behavior called the quantum spin Hall effect.

Electrons moving through a material come in two flavors, called “spin up” and “spin down.” These flavors refer to the electrons spinning like tiny tops in opposite directions, meaning that they have opposite angular momentum. In materials that exhibit the quantum spin Hall effect, electrons don’t move in a crystal’s interior but instead flow along its edges; electrons with spin up move in one direction, while electrons with spin down flow the other way. Such an orderly flow of electrons excited physicists, who thought they might be able to take advantage of the quantum spin Hall effect to build new kinds of electronic devices.

By 2005, theoretical physicist Charles Kane of the University of Pennsylvania in Philadelphia and a colleague had proposed that graphene could exhibit the quantum spin Hall effect, and they began pondering what other materials might do so. One crucial issue in this theoretical scenario was that graphene’s spin-up and spin-down electrons were traveling right on top of one another — meaning the electrons might smash into each other.

Kane realized, however, that a mysterious feature of quantum physics can prevent such crashes by making the spin-up and spin-down electrons behave as if they move in separate, well-defined traffic lanes. “The lightbulb went off in my head,” he says. “That’s when I realized there was something really new about this.”

His calculations showed that certain crystals would display these unusual edge effects with divided highways of flowing electrons. The electronic structures of these materials were closely related, but not identical, to graphene. Coaxing graphene to behave in this way required difficult-to-reach low temperatures and crystal purity. But new materials — those now known as topological insulators — provided a more feasible alternative.

Kane’s initial paper, published with his Pennsylvania colleague Eugene Mele in Physical Review Letters in 2005, was the first recognition that topological insulators could exist. By 2007, a team led by Laurens Molenkamp of the University of Würzburg in Germany had observed mercury telluride behaving as a topological insulator in two dimensions in the laboratory. And the following year, a group led by Hasan at Princeton found crystals of bismuth antimony operating as topological insulators in three dimensions. All of the materials behaved as insulators in the bulk, but as metals — conducting electricity — at their surfaces.

Soon papers were following fast and furious, laying out predictions for what materials might act as topological insulators and then reporting observations of that behavior. “Most of us were shocked by the rapidity of the experimental developments,” says Joel Moore, a physicist at the University of California, Berkeley who helped coin the term topological insulators.

Into the lab

In part, scientists say, the field took off because topological insulators don’t require complicated laboratory setups. Unlike other areas of quantum physics, which demand specialized cooling devices, powerful lasers or other expensive equipment, the study of topological insulators can be conducted at room temperature using materials found in many labs around the world.

Like bismuth telluride. That material is used in wine chillers for its thermoelectric properties, meaning it can convert a temperature difference to an electric current, and vice versa. Because of these properties, scientists had already been studying it in the lab. Along with its relatives bismuth selenide and bismuth antimony, bismuth telluride also turns out to be easy to work with. “Here is a bulk crystal you can hold in your hand, and it has this exotic state,” says Robert Cava, a chemist at Princeton.

But researchers need to find just the right kind of crystal to observe topological insulator behavior. “Any old crystal will not do,” Cava says. “We have to make them free from chemical and physical defects to make them work.”

Any defects could interfere with measurements of the topological insulator properties — for instance, by causing electrons to move around within the crystal’s interior, where it is supposed to be an insulator. That, in turn, makes it harder to detect the electrons moving on the surface.

Cava and his Princeton colleagues have continued to work with bismuth-related topological insulators, and in February of this year reported a new feat with them in Physical Review Letters. By doping bismuth selenide with a bit of copper, the researchers were able to make it superconduct — a first for a topological insulator. Making topological insulators into superconductors could have big implications for various fields of physics, such as in efforts to build a quantum computer.

Beyond the bismuth compounds, other groups are hunting among various materials for signs of topological insulator behavior. In Germany, Molenkamp’s group continues to work with mercury telluride, which is often used in devices that detect far-infrared radiation. Mercury telluride was the first topological insulator demonstrated in two dimensions and now, in preliminary work, Molenkamp thinks he sees a three-dimensional chunk of the material behaving the same way in the lab.

Theoretical papers have proposed many other classes of topological insulators, and at a recent meeting of the American Physical Society in Portland, Ore., Molenkamp and other researchers ran a tutorial for an overflow crowd on how researchers could enter the field and start finding topological insulators of their own.

As some researchers look for new kinds of topological insulators, others are playing around with slicing and dicing the existing ones to see how that changes their electrical behavior. At Stanford, for instance, Yi Cui and colleagues have made tiny “nanoribbons” out of bismuth selenide. The long thin strands have a much higher surface-to-bulk ratio than a chunk of bismuth selenide, so Cui and his colleagues have a lot more of the conducting edge states to watch than they otherwise would.

At the physics meeting in Portland, the researchers reported that they can observe electrons zipping around and around the nanoribbons. Similarly, a team led by Qi-Kun Xue of Tsinghua University in Beijing has been laying down thin sheets of topological insulators atop one another to see how that changes their behavior.

Where the wild things happen

Within the next few years, physicists expect a raft of new discoveries — not only toward making practical devices, but also toward probing some fundamental and exotic physics.

For instance, putting a topological insulator right next to a superconductor could create an unusual state at the interface in which unseen particles could pop into existence and be observed. These include the long-sought Majorana fermion, a particle that can act as its own antiparticle and has never been observed directly.

The proximity of the topological insulator and the superconductor would set up a kind of vortex — a quick-and-dirty way to get a single Majorana fermion, researchers say. “It would be the RadioShack way to make Majorana fermions,” says Moore of UC Berkeley. Among other things, these fermions could potentially be used to store information in a quantum computer, bypassing some of the challenges that bedevil other proposals for storing quantum information.

Other researchers think that they might be able to use topological insulators to spot a different, never-before-seen particle called the axion. The axion is one of the candidate particles that could constitute dark matter, the unseen stuff that makes up 85 percent of matter in the universe. Astronomers can’t see dark matter but know it must exist because of the gravitational effect it exerts on other matter, and they have been hunting for more direct signs of its existence.

In a paper published online March 7 in Nature Physics, Zhang’s group at Stanford, with colleagues at Tsinghua University, argue that the math describing how topological insulators behave in an electromagnetic field is the same as the math that describes how axions move in that same field. So topological insulators could create conditions for elusive particles like axions to pop into existence, says Zhang. The insulators could serve as a sort of “baby universe” on a tabletop, he speculates, allowing the creation, manipulation and study of particles that physicists wouldn’t otherwise have access to.

As exotic as these scenarios sound, they may not be that far in the future, scientists say. And with the pace of research into topological insulators picking up dramatically, additional unexpected findings may be just over the horizon. No matter what the next few years in the field brings, says Cava, “an exciting period is ahead.”

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