The ability of particles to tunnel through barriers is an essential property of quantum mechanical systems. The extent of the effect is strongly dependent on the properties of the barrier.
As early as 1929, theoretical physicist Oscar Klein suggested that a relativistic particle can penetrate a potential barrier with 100% transmission upon normal incidence on the barrier. Scientists considered this exotic and counterintuitive phenomenon the “Klein tunneling” theory. In the accompanying 100 odd years, scientists tried different ways to deal with Klein tunneling tentatively; however, the endeavors were unsuccessful, and direct experimental evidence is still lacking.
For the first time, scientists from the University of Hong Kong experimentally proved a century-old quantum theory that relativistic particles can pass through a barrier with 100% transmission.
Scientists experimented with artificially designed phononic crystals with triangular lattice. The lattice’s linear dispersion properties make it possible to mimic the relativistic Dirac quasiparticle by sound excitation, which led to the successful experimental observation of Klein tunneling.
Professor Xiang Zhang, President of the University of Hong Kong (HKU), said, “This is an exciting discovery. Quantum physicists have always tried to observe Klein tunneling in elementary particle experiments, but it is difficult. We designed a phononic crystal similar to graphene that can excite the relativistic quasiparticles. Still, unlike the natural material of graphene, the geometry of the human-made phononic crystal can be adjusted freely to precisely achieve the ideal conditions that made it possible to the first direct observation of Klein tunneling.”
The outcomes represent a breakthrough in fundamental physics. It also shows a new platform for exploring emerging macroscale systems in applications such as on-chip logic devices for sound manipulation, acoustic signal processing, and sound energy harvesting.
Dr. Xue Jiang, a former member of Zhang’s team and currently an Associate Researcher at the Department of Electronic Engineering at Fudan University, said, “In current acoustic communications, the transmission loss of acoustic energy on the interface is unavoidable. If the transmittance on the interface can be increased to nearly 100%, acoustic communications efficiency can be greatly improved, thus opening up cutting-edge applications. This is especially important when the surface or the interface plays a role in hindering the accuracy acoustic detection, such as underwater exploration. The experimental measurement is also conducive to the future development of studying quasiparticles with topological property in phononic crystals which might be difficult to perform in other systems.”
Scientists noted, “The research findings might also benefit the biomedical devices. It may help to improve the accuracy of ultrasound penetration through obstacles and reach designated targets such as tissues or organs, which could improve the ultrasound precision for better diagnosis and treatment.”
- Xue Jiang, Chengzhi Shi, Zhenglu Li, Siqi Wang, Yuan Wang, Sui Yang, Steven G. Louie, Xiang Zhang. Direct observation of Klein tunneling in phononic crystals. Science, 2020 DOI: 10.1126/science.abe2011