Acoustic waves in gases, liquids and solids generally travel at an almost constant speed of sound. Rotons are an exception: their speed of sound changes dramatically with wavelength, and it is also possible for the waves to travel backwards. Researchers at the Karlsruhe Institute of Technology (KIT) are studying the possibilities of using rotons in man-made materials. These computer-designed metamaterials, produced by ultra-precise 3D laser printing, could be used in the future to manipulate or direct sound in ways that had never been possible before. A report on the researchers’ work has been published in Nature Communication.
Rotons are quasi-particles, which means that they behave in the same way as free particles. Unlike ordinary sound waves in gases, liquids, and solids, the speed of sound changes dramatically with wavelength. In addition, some frequencies generate three different partial waves. “The slowest of these is a back wave: the energy flow and the wave fronts go in exactly opposite directions,” says Professor Martin Wegener of the Institute of Applied Physics (APH) and the KIT Institute of Nanotechnology (INT). Understanding and taking advantage of quasiparticles such as rotons is one of the great challenges of quantum physics. Physicist Lev Landau, who won a Nobel Prize in 1962 for his groundbreaking work, predicted their existence against the background of superfluidity, a condition in which a fluid loses its internal friction and becomes thermally conductive in an almost ideal way. Until now, rotons could only be observed under special quantum physics conditions at very low temperatures – and therefore were not suitable for technical applications.
Rotons without any quantum effect
This could change in the future: in KIT’s 3D Matter Made to Order pole of excellence and Heidelberg University, a group of researchers are working on metamaterials that “grow” rotons. Metamaterials exhibit optical, acoustic, electrical or magnetic properties that are not found in nature. Scientists come up with an artificial material capable of producing rotons without any quantum effect under normal ambient conditions and at almost random frequencies or wavelengths. Thus, it may be possible in the future to better manipulate sound waves in air or in materials, for example, to bounce them, redirect them or create echoes. These materials have not yet been demonstrated experimentally; however, it should be possible to produce them using technologies such as ultra-precise 3D laser printing. “We have even manufactured some of these metamaterials in the meantime,” explains Prof. Martin Wegener. “We are currently working intensively on direct experimental evidence for the existence of rotons.”
3D printing – the gateway from the digital world to the physical world
Lead author Dr Yi Chen explains that the researchers relied on a combination of thinking, lots of discussion, numerical simulations, and optimizations to design the computer-aided virtual design of materials with such novel properties. . His work as a post-doctoral researcher at KIT is funded by the Alexander von Humboldt Foundation and is part of a Helmholtz program called “Material Systems Engineering” launched in 2021. “Usually our dream is to design materials on the computer and then design them. turn directly into reality – without years of trial and error. 3D printing is therefore just an automated converter, so to speak, from the digital world to the physical world, ”explains Professor Martin Wegener. (or)
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