A new class of exotic materials could find its way into next-generation technologies that efficiently convert waste heat into electrical current according to new research. Both the exotic materials and the means by which they generate electricity rely on a hybrid of advanced concepts—including string theory combined with black holes combined with cutting-edge condensed matter physics.
But the end result is straightforward: A strip of the material niobium phosphide (NbP), in the presence of strong magnetic fields, appears to be good at harvesting thermal energy and translating that into possibly usable current.
NbP represents a new class of material that’s neither metal nor semiconductor but a little bit of both, says Johannes Gooth, research scientist at IBM in Zurich. “Classically we have materials like metals, semiconductors, and insulators; this is the toolbox we use to make devices,” Gooth says. But Weyl semimetals, named after the physicist Hermann Weyl who first began to describe the strange physics these materials obey, are “exactly in the middle between metal and semiconductor. It has [conduction] bands, but they touch. The band gap is basically zero.”
Which means a Weyl semimetal like NbP occupies a sort of intermediate zone between true conductive metal and pure semiconductor. And as a material in no man’s land, bridging two different regimes of physical properties, it might also find applications no one has yet imagined, Gooth says.
Since their discovery in 2015, Weyl semimetals have been the subject of some curiosity and speculation. And this is for good reason, says Karl Landsteiner, a string theorist at the Instituto de Fisica Teorica UAM/CSIC in Madrid, Spain. He’s one of the co-authors, along with Gooth, of a letter in this week’s issue of Nature that reveals the discoveries they made about NbP.
Before collaborating on this latest study, Landsteiner had been studying the physical laws that quantum mechanics sometimes allows to be broken. And until recently he thought these violations happened in too rarified environments to be observed in the lab—let alone potentially finding their way into future generations of technologies.
“For me this is amazing,” Landsteiner says. “When we started working on these kinds of problems, we never thought there would be any practical way of doing this in the lab. We always thought about the beginning of the universe, very exotic states of matter heated up trillions of degrees. But now we find all our equations and everything we did applies equally to this exciting class of new materials.”
“For us, whenever we build transistors, we are always bound to conservation laws,” says IBM’s Gooth. “These define and limit everything. And now suddenly we have materials where these high-energy, quantum mechanics equations allow for us to break some of these laws. It opens up a completely new playground for device design. Because it’s simply new physics, which circumvents classical limits.”
The quantum equations Gooth references concern the sort of law-bending that quantum physics—with its uncertainty relations enabling mischief at the fringes at sub-atomic scales—has become known for. For instance, the flash memory at the heart of our smartphones and other portable electronics is based on quantum particles tunneling across barriers they wouldn’t be able to cross if the laws of classical physics always prevailed.
In this case, the quantum lawbreaking comes in via the currents of electrons traveling through a Weyl semimetal. According to standard, common sense, conservation laws, electrons should normally travel through a material in such a way that their number is conserved. That is, the number that goes in is the same number that comes out, minus any electrons that the material ate up as it passed through, plus any extra electrons the material itself gave off. (Unfortunately, there’s an additional complication here, though. Each electron’s spin adds a second ledger in the account books. So technically, two currents are conserved: The current with electron spins aligned in the direction of travel is conserved, and completely separately, the current with electron spins anti-aligned with their direction of travel is also conserved.)
The present discovery steals a page from string theory and black hole physics. Theorists in these disciplines have found quantum exceptions to conservation laws like the above. For instance, they’ve established that strong gravitational and magnetic fields together allow for sometimes breaking conservation of both kinds of currents—the kind where spin is parallel to travel direction and where spin is anti-parallel to travel direction.
And here is where Landsteiner presumed his and his colleagues’ work would remain untouched by practical applications. But thanks to work tracing back to the 1960s, a useful analogy has been developed over the years that gravitational fields sometimes behave strikingly similarly to thermal gradients. So when the string and black hole theory idea emerged that “gravitational” fields can bend the conservation laws of current in the presence of strong magnetic fields, Gooth realized he might be able to apply the thermal analogy.
Gooth thought he might try to mimic the same gravitational quantum anomaly with just a simple thermal gradient: In this case, a strip of Weyl semimetal (NbP) that’s really hot on one end and cool on the other. Put this Weyl semimetal inside a superconducting magnet, one that can generate strong (9 Tesla) fields sufficient to generate the quantum effect, and see if the thermal gradient can be converted into extra streams of electrons. In other words, use the above quantum trick to transform thermal energy into electrical current.
And it worked. Now Gooth and Landsteiner say they’ll be busy finding ways to tweak the recipe. Both practical applications like thermal energy harvesters and more fundamental physical research are in their sights now.
“You now can use physics from outer space to create new applications—it’s fantastic,” Gooth says. “It opens a new world.”
Says Subir Sachdev, a solid state physicist at Harvard unaffiliated with the discovery, this discovery opens a door to a new kind of material and a new approach to studying materials. “This experiment is an important step in a wider field of the study of ‘quantum materials,’ ” Sachdev said via email. “And I think advances here could have a strong impact on future developments in this wider field.”