Â鶹ÒùÔº

The discovery of "hidden orders," organization patterns in materials that cannot be detected using conventional measurement tools, can yield valuable insight, which can in turn support the design of new materials with advantageous properties and characteristics. The hidden orders that condensed matter physicists hope to uncover lie within so-called charge density waves (CDWs).

CDWs are periodic wave-like modulations of the electronic charge inside a crystal. CDWs in rare-earth tellurides, compounds containing tellurium and other rare-earth elements, have been found to sometimes give rise to unusual physical phenomena that are not observed in the absence of these wave-like states of matter.

Researchers at Boston College, Cornell University and other institutes recently observed a ferroaxial order in rare-earth tellurides that appears to originate from a combination of coupled orbital and charge patterns.

Their paper, in Nature Â鶹ÒùÔºics, uniquely combined experimental tools to unveil subtle broken symmetries in quantum materials.

"My group has long been interested in understanding how to detect and understand emergent phases of matter," Ken Burch, senior author of the paper and Rourke professor of physics at Boston College, told Â鶹ÒùÔº.

"Our approach has been to focus on the new quasiparticles they produce and study their properties as a way to uniquely identify and understand them."

Three years ago, Burch and his colleagues detected the very first axial Higgs mode (i.e., a unique type of collective vibration of a material's electronic order) in a CDW system. These types of collective vibrations can emerge when systems enter new phases of matter.

"The mode we observed also had a 'handedness' and we set about to find out why," said Burch. "This paper was our effort to understand what hidden symmetries had been broken and what in the material was the cause."

As part of their study, Burch and his colleagues performed various optical experiments. In these experiments, they observed that the color and polarization of light coming out of their sample differed from what they were when the light entered the sample.

"By carefully measuring the change with respect to rotating the crystal, we could uncover the broken symmetries," explained Burch.

"We also looked at specific colors coming out to uncover whether the change was primarily electronic or in the atoms. The optical experiments clearly point to an electronic origin. We further tested this by looking with an electron microscope, which found the 'ferroaxial' component was extremely weak in the lattice, thus proving it was electronic in origin."

After observing a ferroaxial order in their rare-earth telluride sample and determining that it was of an electronic origin, the researchers collected muon spin relaxation measurements.

These measurements allowed them to confirm that the handedness (i.e., preferred orientation or direction) in the system did not emerge from broken time reversal (i.e., electrons going around in a circle in a magnet).

Overall, this recent study demonstrates the potential of probing hidden phases of matter in materials by studying the symmetries of emergent quasiparticles within them.

In the future, the team's findings could help to improve theoretical models in condensed matter physics, while also potentially inspiring other similar experimental efforts.

"Our paper establishes a long-held belief that Higgs modes in particular could provide unambiguous signatures of such phases and help us to understand their origins (e.g., magnetic, lattice, electronic, etc.)," added Burch.

"We are now working to understand how to achieve single ferroaxial domains, and how this order affects other electronic properties of these materials. In particular, their electronic transport and nonlinear responses."

© 2025 Science X Network