Newly discovered materials could clear the way for blisteringly fast laptops and smartphones that don’t warm our laps or singe our ears
AS I write this, an uncomfortable warmth is starting to overcome me. But this is no mystery fever to send me running to the medicine cupboard. The source is all too obvious: the laptop cradled in my lap. Time to fetch not a cold compress, but a pillow to place beneath my computer.
Today’s microelectronic devices pump out a lot of heat. If only they wouldn’t, processors would be zippier, batteries would last longer, laptops could be used on laps and smartphones wouldn’t singe our ears. But there is little we can do. Heat is a natural by-product of what goes on in a computer chip, released when electrons careering around the processor pathways smash into each other and the surrounding furniture, and through that become deflected from their intended course.
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Help might be at hand. In the past five years, physicists have uncovered a new kind of material that can keep electrons on the straight and narrow, eliminating collisions and slashing the amount of heat produced. Called topological insulators, these materials conduct electricity by harnessing a quantum-mechanical property of electrons called spin. Unlike superconductors, those other low-heat-loss marvel materials, they can perform this feat at room temperature. Forget silicon: it could soon be time to fasten your seat belt for a drive on the spin superhighway.
“Unlike superconductors, these low-heat marvel materials perform their feats at room temperature”
“Topological insulator” is an odd name for a wonder conductor. After all, insulators are the very opposite of conductors: their electrons are tightly bound to atoms and the material resists the flow of electrical current. That is indeed the case in the heart of a topological insulator. On its surface, however, a rather different picture emerges – of skittish electrons only too ready to roam and be marshalled into a current.
The first hints of such odd behaviour at the surfaces and boundaries of solids came in 1980, when the German physicist took a thin slice of a silicon-based semiconducting structure and cooled it to within a few degrees of absolute zero. When he bombarded his sample from above with a strong magnetic field, electrons flowing through the slab began to skitter towards one edge, deflected by the magnetic field. This sudden flight created positively charged “holes” at the opposite edge of the material, which the electrons ran around the edge to fill.
The result was a flow of electrons around the edge of the material, like skaters circling the perimeter of an ice rink. The newly established conductivity depended on the number of paths the electrons found to skate along, and increased stepwise, in discrete quantum jumps, as the magnetic field increased ().
The discovery of this “quantum Hall effect”, which had been predicted some years earlier, earned von Klitzing the in 1985. While it was an important fundamental discovery, it was not one that had an immediate practical application. The effect required temperatures that were too low and magnetic fields that were too high to be used in everyday devices.
Orderly queue
In 2005, and of the University of Pennsylvania in Philadelphia did away with half of that objection. Some materials, they showed, would naturally create their own magnetic fields that marshal electrons into orderly lanes along their edges.
To envisage how this works, imagine taking a ride on an electron that is travelling through a solid material. As we pass an atomic nucleus, to us on the electron it appears as a large charged body moving in the opposite direction. Moving charges create magnetic fields, so our electron experiences a magnetic field from the passing nucleus.
That is not the only field in play: our electron has its own mini-field as a result of its quantum-mechanical spin. Spin is akin to the rotation of a spherical particle around its axis one way or the other, and it creates a magnetic field either from its north to its south pole (which can be denoted spin “down”) or from its south to its north pole (spin “up”).
The degree of interaction between the magnetic field caused by an electron’s spin and that caused by its motion, or “orbit”, depends on the material it is moving through. In materials where the spin-orbit interaction is strong, a spin-up electron will be deflected in one direction on encountering a nuclear magnetic field, while a spin-down nucleus will be deflected in the opposite direction – an effect dubbed the quantum spin Hall effect.
On the edge
In a slice of the right material one atom thick, Kane and Mele showed, this would have an odd consequence. In the bulk of the material, there would be no conduction, as electrons deflected in opposite ways around adjacent nuclei would cancel each other out. At the edge of the material, however, with no adjacent nucleus available on one side, the net result would be a flow of spin-up electrons in one direction around the edge, and a flow of spin-down electrons in the opposite direction ().
A full quantum-mechanical description of how this comes about requires a detailed examination of the topology of the electrons’ probabilistic wave functions, and so the materials in question acquired the name “topological insulator”. The effect has an important upside: because the electrons’ spin is locked into their direction of motion, they cannot change direction without flipping their spin, something that quantum mechanics forbids. So the scattering of electrons, whether off themselves or imperfections in the crystal, is suppressed, and heat loss consequently eliminated. “These surface states have a very special property that prevents the scattering process from occurring,” says Kane. “It is like a one-way street with no U-turns.”
“The surface states have a special property that prevents scattering from occurring. It’s like a one-way street with no U-turns”
It only remained to find a material that fitted the description of a topological insulator in reality. Kane and Mele originally had single-atom layers of carbon, aka graphene, down as a candidate, but it turns out that carbon is subject to thermal fluctuations that overwhelm the delicate spin-orbit coupling. Then and of Stanford University, California, showed that the nether regions of the periodic table might be the place to look. Heavy elements have nuclei with large positive charges, which makes it more likely that they have strong spin-orbit coupling. The duo predicted that the effect would be particularly strong in one alloy, mercury telluride ().
It took little more than a year for and his colleagues at the University of Würzburg in Germany to confirm that prediction, demonstrating in 2007 that electrons could flow without losing heat in a sandwich of mercury telluride and cadmium telluride. This was the first experimental evidence of the quantum spin Hall effect in a one-atom-thick topological insulator ().
It was a significant advance but not yet a breakthrough. Not only were frigid temperatures close to absolute zero needed for the effect to kick in, but the ingredients for the sandwich were hard to make. Topological insulators seemed destined at that point to remain a laboratory curiosity. But then Kane and his student Liang Fu came out with another prediction: that a similar behaviour should exist in chunks of bismuth and antimony alloys (). Unlike atom-thick slices, where the electrons travel around the edge, in this case layers of electrons should circle a non-conducting interior like a ribbon encircling a birthday present.
Bismuth-based materials are easy to make or buy. If you are a wine connoisseur, you might even have some already: crystals of bismuth telluride are used as “thermoelectric” cooling materials in some wine chillers. Word spread quickly, and physicist and his student David Hsieh at Princeton University started to look for topological behaviour in bismuth alloys prepared by their colleague Robert Cava. By blasting a bismuth antimonide crystal with ultraviolet light, and measuring the spin and momentum of the electrons that were ejected, they showed that the electrons flowed on the surface in “lanes” of corresponding spin and direction ().
Hotting up
A clinching result? Not quite. Although this was the first convincing demonstration of topological insulator behaviour in a three-dimensional crystal, the effect only happened at temperatures of around 15 kelvin, so it was still of little practical use.
The temperature hurdle was finally overcome last year, when Hasan and Hsieh uncovered another compound, bismuth selenide, that retains its topological properties at room temperature (). In July this year, of Princeton University and his colleagues completed the final part of the puzzle, confirming the lack of electron scattering and heat dissipation (). “In these new materials, the electrons don’t get stuck,” says Yazdani. “They are able to flow through even the imperfections.”
Now that the zippy electrons are ready to roll, when can we expect the spin superhighway to make it to the chips in our laptops? Exploiting this technology for consumer devices is still several years off, but when it happens, less heat loss won’t be the only benefit. Electronic circuits based on the manipulation of spin, rather than charge currents, are a goal in their own right. This “spintronic” approach promises smaller, faster, more powerful and cooler electronic devices, as it takes a lot less time and energy to flip an electron between spin states than it does to move charge through transistors on a chip.
Controlling spin is easier said than done. Applying a magnetic field is one way to do it, but directing a strong magnetic field onto a microprocessor chip is no easy feat. An art we have already mastered, on the other hand, is applying an electric field to manipulate charge on a chip – but until now we had no way of using this to control spin. Topological insulators square that circle. In these materials, applying an electric field to the electrons creates their initial motion, and through the resulting spin-orbit interaction controls where electrons of opposite spins flow.
For physicists, the interest in topological insulators does not end with their spintronic promise. Besides revolutionising the behaviour of electrons in our smartphones and laptops, the materials may be a breeding ground for a whole menagerie of exotic physical beasts – something that could make them the next big hope in the quest to build a computer that fully harnesses the awesome power of quantum mechanics (see “Topological breeds”).
“Besides revolutionising smartphones and laptops, these materials could help to harness the power of quantum computing”
While such curiosities are sure to keep physicists busy, work is already under way to move topological insulators from the wine cooler to the laptop. That requires some further thought, though. For one thing, although electrons on a topological superhighway can’t perform U-turns, they can seek the exit in other ways, by diving down into the crystal when they encounter an impurity, for example.
Such problems can be solved by making the crystals as pure as possible, something that Cava and others are working on. Meanwhile, other materials are in the frame: the possibility of topological insulators based on lead and germanium is also being explored (), and a consortium of chip-makers and other technology companies are funding research to apply the technology in consumer devices. The spin superhighway could soon be open for traffic.
Topological breeds
There has certainly been no shortage of predictions of exotic species lurking within the newly discovered topological insulator materials. For instance, Xiao-Liang Qi and colleagues at , a Microsoft-sponsored research group working on topological quantum computing at the University of California, Santa Barbara, have proposed that we might find magnetic monopoles at their surface – equivalent to finding a magnetic north pole without its accompanying south pole ().
Together with Shou-Cheng Zhang at Stanford University in California, Qi has also speculated that we can use topological insulators to characterise hypothetical elementary particles called axions, which some believe make up dark matter. The equations that describe axions at the grand scale of the cosmos are exactly the same kind of equations that describe excitations within a topological insulator. “These materials are like a little baby universe,” says Zhang. “We can study lots of effects that have been predicted but have been hard to access experimentally.”
But the most exotic beast of all – and potentially most useful – is the Majorana fermion. If theorists are right, putting a slab of topological insulator next to a slab of conventional superconductor may create a perfect nursery for them.
Majorana fermions are truly odd characters – as mysterious as the Italian physicist Ettore Majorana after whom they are named, who in the 1930s at the height of his career. These particle-like objects only ever appear in pairs, and they are their own antiparticles. Because of that, they are potentially an ideal memory resource for a future quantum computer.
Such computers, though powerful, are vulnerable to their information being destroyed by the surrounding environment. Majorana fermions could make them resilient: imprint any information on the quantum state of one of these particles, and you imprint the same information on its partner. That remains the case even if the two particles are subsequently separated, meaning the information cannot be lost without destroying the state of both particles. That is like hiding a second key to your safe deposit box – and could be a vital step forward for quantum computing.
