A quantum cousin of the Hall effect could open the door to energy-efficient electronics, better sensors, and more-powerful quantum computers. Researchers have now broken a key barrier to its practical application by controlling the phenomenon electrically, rather than magnetically, for the first time.
The Hall effect, discovered by physicist Edwin Herbert Hall in 1879, describes a phenomenon in which applying a perpendicular magnetic field to a conductor creates a voltage that runs sideways across the material. The effect has wide-ranging applications, including sensing and spacecraft propulsion.
In 1980, researchers discovered a quantum version of the Hall effect that occurs in certain materials at very low temperatures. When a strong magnetic field is applied, the interior of the sample becomes an insulator, but an electrical current continues to flow around its edges. Crucially, when this happens, the resistance along the length of the material drops to zero, and the electrons travel around the edges without losing any energy, achieving an effect similar to that of a superconductor.
“We have repeatedly seen that if academic developments lead to a material that industry wants, then with the production might and backing of the big industry players, they find a way to produce it.” —Charles Gould, University of Würzburg, Germany
Finding ways to to exploit these dissipation-less “chiral edge currents,” as they are known, could have far-ranging applications in quantum metrology, spintronics, and topological quantum computing. The idea was given a boost by the discovery that thin films of magnetic materials exhibit similar behavior without the need for a strong external magnetic field—something known as the quantum anomalous Hall effect (QAH)—which makes building electronic devices that harness the phenomenon much more practical.
One stumbling block has been that switching the direction of these edge currents—a crucial step in many information-processing tasks—could be done only by passing an external magnetic field over the material. Now, researchers at Penn State University have demonstrated for the first time that they can switch the direction by simply applying a pulse of current.
“Electric devices based on the QAH effect have the potential to overcome the limitations imposed by Moore’s Law and mitigate heat generation in compact devices,” Cui-Zu Chang, an associate professor of physics who led the research, told IEEE Spectrum in an email.
“Achieving instantaneous electrical control over the edge current chirality [direction] in QAH materials, without the need for sweeping the external magnetic field, is indispensable for the advancement of QAH-based computation and information technologies,” he said.
Chang was the first to experimentally demonstrate the QAH effect in 2013 as a Ph.D. student at Tsinghua University, in China. He did this by taking thin films of materials known as topological insulators, in which the interior behaves as an insulator while the surface acts as a conductor, and doping the materials with magnetic particles to create QAH insulators.
The direction in which currents flow around the edge of a QAH insulator is determined by its internal magnetism, said Chang. Passing an external magnetic field over the material can switch the direction of this magnetization and therefore which way the electrons travel, but it’s a cumbersome way to do things, he added.
Other researchers had already shown that it was possible to switch the magnetization of similar materials by simply applying a current, but doing so in QAH would require a very high current density. Chang and colleagues achieved this by sandwiching a layer of undoped topological insulator between two magnetically doped layers, and then using electron-beam lithography to etch this into a rectangular structure less than 10 micrometers across.
In a paper published 19 October in Nature Materials, the researchers showed that applying a 5-millisecond current pulse to the device was enough to switch the edge currents. The ability to control the flow of current is a critical ingredient for most information-processing tasks, says Chang, so enabling near-instantaneous electrical switching could make practical applications of QAH insulators much more feasible.
“Electric devices based on the QAH effect have the potential to overcome the limitations imposed by Moore’s Law and mitigate heat generation in compact devices.” —Cui-Zu Chang, Penn State
Hansjörg Scherer, head of the electrical quantum metrology department at Germany’s national metrology institute, the Physikalisch-Technische Bundesanstalt, said the breakthrough could prove useful for carrying out ultraprecise measurements of resistance using quantum effects. At present, preparing measuring devices involves subjecting them to magnetic fields as strong as 1 tesla, he says, and being able to do this electrically instead would simplify things considerably.
Electrical quantum metrology is likely to be the first practical application of the QAH effect, agrees Charles Gould, a professor of experimental physics at the University of Würzburg, in Germany. But being able to control it using current rather than magnetic fields could open up other applications in information technologies such as spintronics and high frequency circuits. “If I have a million or any large number of devices on a chip and I want to modify some of them, applying a local magnetic field to individually address them is near impossible,” he wrote in an email to Spectrum, “whereas selectively applying a current or electric field is technologically commonplace.”
By far the biggest limitation is that today’s QAH insulators operate only at temperatures below 50 millikelvins, says Gould. Fabrication methods are also not well-suited to industrial production and have been mastered by only a handful of labs, he adds, though that is unlikely to be a fundamental barrier.
“We have repeatedly seen that if academic developments lead to a material that industry wants, then with the production might and backing of the big industry players, they find a way to produce it,” says Gould.
Edd Gent is a freelance science and technology writer based in Bengaluru, India. His writing focuses on emerging technologies across computing, engineering, energy and bioscience. He's on Twitter at @EddytheGent and email at edd dot gent at outlook dot com. His PGP fingerprint is ABB8 6BB3 3E69 C4A7 EC91 611B 5C12 193D 5DFC C01B. His public key is here. DM for Signal info.
