Highest clock rates leave electronics cold
Researchers from Germany, Japan and Russia utilize light wave electronics to use the fastest controllable alternating field that exists in nature: the carrier wave of light. If one uses this alternating field as a clock generator, one should be able to accelerate electronics a thousand times – instead of measuring clock rates in gigahertz, one would measure clock rates in tera- or even petahertz. But only in theory, because then the electrons would also collide more frequently with crystal atoms, which would generate even more heat. In order to prevent this, the researchers have gone deep into the tricks of modern physics: instead of the usual semiconductor material silicon, they use so-called topological insulators, the unusual properties of which have only been known for a few years.
This field of physics became known to the public mainly through the Nobel Prize for Physics 2016 to Kosterlitz, Haldane and Thouless: On the surface of these materials all electrons moving in one direction should align their own angular momentum, the so-called spin, in the same way, while the spins of opposing electrons should point in the opposite direction. If electrons were to change their direction of motion due to scattering, their spin would also have to turn over. Since this is not easily possible from a quantum physics point of view, such electrons seldom scatter and thus hardly develop heat.
Now the researchers have combined light wave electronics with topological insulators for the first time. To do this, they focused light pulses from the Regensburg high-field terahertz source on a topological insulator and accelerated the electrons on its surface. However, the acceleration occurs only for the extremely short period of half a light oscillation. In order to observe this electron movement alone, the physicists also had to develop a new measuring method. The key to success was a method that had been promoted for years by a group at the University of Marburg. The term is time-resolved photoelectron spectroscopy. During acceleration, the scientists use ultraviolet light pulses to release electrons from the surface of the topological insulator and take snapshots of their velocity. Finally, such snapshots can be used to compose entire “slow motion films” that show how the electrons on the surface of the topological insulator move on the time scale for less than a single light oscillation.
The physicists found that the electrons behave similarly to particles brought close to the speed of light in a large accelerator. Even more important: Despite the rapid acceleration, the theoretically expected coupling between direction of motion and spin works so well that the electrons move completely ballistically over long distances without scattering at the lattice and thus generating heat. “It’s like a billiard ball rolling straight ahead as long as it’s not deflected by any other ball – only much, much faster,” explains Professor Rupert Huber of the University of Regensburg, who is pleased: “Topological lightwave electronics are fast, lossless and compact – and therefore possibly the technology of the future.
Scientists from the University of Hiroshima and the Russian Academy of Sciences in Novosibirsk were also involved in this research. The research group reports on their results in the current issue of the journal Nature.