Perfect sound transmission through a barrier is difficult to achieve, if not impossible based on our existing knowledge. This is also true for other forms of energy such as light and heat.
A research team led by Professor Xiang Zhang, president of the University of Hong Kong (HKU) while a professor at the University of California, Berkeley (UC Berkeley), first experimentally proved a centuries-old quantum theory that relativistic particles can cross the barrier with 100 % by leaking. The research findings were published in a top academic journal Science.
Just as it would be difficult for us to jump over a thick high wall without enough accumulated energy. In contrast, it is predicted that a microscopic particle in the quantum world can pass through a barrier and beyond its energy, regardless of the height or width of the barrier, as if it were “transparent”.
As early as 1929, theoretical physicist Oscar Klein suggested that a relativistic particle could penetrate a potential barrier with 100% leakage at a normal fall on the barrier. Scientists have called this exotic and counterintuitive phenomenon the “Klein tunneling” theory. Over the next 100 odd years, scientists tried different approaches to the experimental study of Klein tunneling, but the attempts were unsuccessful and direct experimental evidence is still lacking.
Professor Zhang’s team performed the experiment in artificially designed background crystals with a triangular lattice. The properties of the linear lattice dispersion make it possible to mimic the relativistic Dirac quasiparticle by sound excitation, which led to a successful experimental observation of Klein tunneling.
“This is an exciting discovery. Quantum physicists have always tried to observe Klein tunneling in elementary particle experiments, but it’s a very difficult task. We designed a graphene-like background crystal that can excite relativistic quasiparticles, but unlike natural graphene material, the phonon crystal geometry man-made can be freely adapted to precisely achieve the ideal conditions that allowed the first direct observation of Klein’s tunneling, ”Professor Zhang said.
The achievement not only represents a breakthrough in fundamental physics, but also represents a new platform for exploring emerging macro-dimensional systems to be used in applications such as on-chip logic devices for sound manipulation, audio signal processing, and sound energy collection.
“In current acoustic communications, loss of acoustic energy transmission at the interface is inevitable. If bandwidth at the interface can be increased to almost 100%, the efficiency of acoustic communications can be greatly improved, opening up state-of-the-art applications. This is especially important when surface or interface plays a role. in interfering with acoustic detection accuracy, such as underwater research. Experimental measurement also favors the future development of the study of quasiparticles with topological properties in background crystals, which could be difficult to perform in other systems, “said Dr. Xue Jiang, a former member of Zhang’s team and currently a research associate in the Department of Electronic Engineering at Fudan University.
Dr. Jiang stressed that the research findings could also use biomedical devices. This can help improve the accuracy of ultrasound penetration through obstacles and achieve certain goals such as tissues or organs, which can improve the accuracy of ultrasound for better diagnosis and treatment.
Based on current experiments, researchers can control the mass and dispersion of a quasiparticle by exciting background crystals at different frequencies, thus achieving a flexible experimental configuration and control and on / off of Klein tunneling. This approach can be extended to other artificial structures for the study of optics and thermothics. It enables unprecedented control of a quasiparticle or wavefront and contributes to the research of other complex quantum physical phenomena.
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Material provided University of Hong Kong. Note: Content can be edited for style and length.