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An international team of researchers has demonstrated a new mechanism by which distinct vibrations in a crystal – normally decoupled by symmetry – can be dynamically linked. Using a light scattering technique, the team showed that in a special class of crystals with a built-in sense of rotation, known as ferroaxial materials, collective fluctuations of this ordered state act as a dynamical bridge between otherwise independent vibrational modes. This unconventional channel, called resonant chiral dressing, has also been fully explained theoretically. The findings, published in Nature Physics, open new routes to detect and control exotic quantum phases with light.
Symmetry is one of the most fundamental principles in nature. It describes the rules that make an object look unchanged after a rotation, reflection, or other transformations. In materials, symmetry governs how atoms and electrons are arranged, and how they move together. Crucially, symmetry can even prevent certain collective atomic motions (vibrations) from interacting at all: some are simply forbidden to talk to each other. But what if those symmetry restrictions are not as rigid as they seem?
A new study in the journal Nature Physics shows that these constraints can be partially lifted. Researchers at the University of Texas at Austin and the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg found that electronic fluctuations can dynamically bridge vibrations that symmetry would normally keep separate. Led by Edoardo Baldini's group at UT Austin, the study reveals how light, vibrations, and electrons become intertwined in a special type of crystal known as ferroaxial, opening new opportunities for controlling quantum states with light.
The researchers focused on a layered material that at room temperature develops an exotic quantum state. Ions and electrons rearrange together into a static, wave-like pattern known as a charge-density wave (CDW), which manifests as a tiling of star-of-David clusters (see Figure). These stars can orient in two distinct ways, giving the crystal a built-in sense of handedness (also known as planar chirality). The resulting quantum state is known as ferroaxial order, and it is notoriously difficult to study. Unlike a ferromagnet, whose magnetization responds to a magnetic field, ferroaxial order does not couple directly to either electric or magnetic fields. Standard optical experiments cannot reach it.
However, this ordered pattern is not frozen in place. The star-of-David clusters can vibrate collectively, periodically modulating the strength (or amplitude) of the CDW: physicists call this coordinated motion an amplitudon. A central question then becomes: can this peculiar oscillation influence the other vibrations of the crystal, and if so, how?
To answer this, the team used helicity-resolved light scattering, a technique that measures how crystal vibrations respond to light with well-defined helicity, meaning with polarization rotating clockwise or counterclockwise. Applying this approach to ferroaxial crystals, the researchers found that certain vibrations react more strongly when the handedness of the light matches that of the crystal, leading to an intensity imbalance between the two polarizations.
"By looking at how vibrations respond to left- and right-circularly polarized light, we can see the handedness of the CDW and map individual ferroaxial domains," explains Xinyue Peng, a graduate student at UT Austin.
By varying the temperature, the researchers could tune the energy of the amplitudon. The imbalance between left- and right-handed responses grew strongest at one specific condition: when the energy of an ordinary vibration matched that of the amplitudon.
"When these two energies align, the vibrational response changes," says Francesco Barantani, a lead author of the paper. "Our observations show that CDW fluctuations can actively connect crystal vibrations that symmetry would normally keep apart."
To explain these findings, the team in Angel Rubio's group at the MPSD in Hamburg developed a microscopic theory, in collaboration with Lara Benfatto at Sapienza University of Rome.
"The amplitudon acts as a resonant bridge between vibrations of different symmetry, linking the lower energy of atomic motions with the higher energy of the electronic sector," adds theorist Emil Viñas Boström.
Additional measurements from Michael Rübhausen's group at the University of Hamburg confirmed the model's robustness. Because the effect operates at room temperature, resonant chiral dressing offers a practical new route to probe and potentially control ferroaxial states. Ultrafast laser pulses tuned to specific energies could selectively activate interactions that symmetry would normally forbid. The approach points toward new ways to manipulate quantum states across a broad class of materials.
Emil Viñas Boström - emil.vinas-bostrom@mpsd.mpg.de
Angel Rubio - angel.rubio@mpsd.mpg.de
https://www.nature.com/articles/s41567-026-03241-3
https://www.mpsd.mpg.de/1188203/2026-05-bostroem-rubio-nature-physics
Illuminating the crystal with light reveals regions where clusters adopt opposite orientations and ...
Source: Jörg M. Harms
Copyright: Jörg M. Harms
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