The direction that the phonons revolve depends on the intrinsic chirality of the quartz crystal. The photons of light possess angular momentum, which they transfer to the atomic lattice, launching the vibrations into their corkscrew motion. In their experiment, circularly polarised light shines on quartz.
"It doesn't usually work like this in science!" This was complemented with supporting theoretical descriptions of how the process would create and enable the detection of chiral phonons from groups at the ETH Zurich (Carl Romao and Nicola Spaldin) and MPI Dresden (Jeroen van den Brink). To do this, they used a technique known as resonant inelastic X-ray scattering (RIXS) at the Diamond Light Source in the UK. Using quartz, one of the best-known minerals whose atoms - silicon and oxygen - form a chiral structure, they showed how circularly polarised light coupled to chiral phonons. It was using such light that the researchers could make their proof. The researchers are interested in manipulating chiral modes of materials using chiral light - light that is circularly polarised. "It is because we are at the juncture between ultrafast X-ray science and materials research that we could approach the problem from a different angle," he says. It is this possibility that motivated the group of Urs Staub at PSI, who led the study. If phonons can revolve in this way, like the coil of wire that forms a solenoid, perhaps they could create a magnetic field in a material. This corkscrew motion is one of the reasons there has been such a drive to discover the phenomenon. In the new study, the atomic vibrations dance a twist that moves forwards like a corkscrew. What makes phonons chiral is the steps of their dance. Yet, experimental proof for their existence has remained elusive. With the rapid rise in recent years of research into topological materials that exhibit curious electronic and magnetic surface properties, interest in chiral phonons has grown. Physicists have predicted that if phonons can demonstrate chirality they could have important implications on the fundamental physical properties of materials. Now, thanks to a new study led by researchers at Paul Scherrer Institute PSI, we know that phonons can also possess this property.Ī phonon is a quasiparticle that describes the collective vibrational excitations of the atoms in a crystal lattice imagine it as the Irish Riverdance of the atoms. Consider the pharmacological disasters caused by administering the wrong drug enantiomer or, at a subatomic scale, the importance of the concept of parity in particle physics. Imagine trying to eat a sandwich with two hands that were not enantiomers - non-superimposable mirror images - of each other. Throughout nature, at all scales, you can find examples of chirality - or handedness.
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