Physics has always been about seeing beyond the obvious—finding patterns where others see only chaos. From the orbits of planets to the invisible dance of electrons inside an atom, physics thrives on revealing hidden orders. In condensed matter physics, this pursuit often leads researchers into the mysterious world of quantum materials, where electronic charges, atomic vibrations, and symmetries intertwine in unexpected ways.
One of the most intriguing frontiers in this field is the search for hidden orders—subtle patterns of organization in materials that cannot be detected with conventional tools. These hidden orders are not just scientific curiosities. They shape the physical behavior of matter, endowing it with properties that could be harnessed to design new technologies, from superconductors to advanced electronics.
The Wave-Like Dance of Charge Density Waves
At the heart of this story lie charge density waves, or CDWs. A CDW is a periodic modulation of electronic charge within a crystal—imagine the electrons arranging themselves in a repeating wave-like pattern instead of spreading out uniformly. These ripples in electron density can dramatically alter the material’s properties, leading to strange and often beautiful phenomena that would not exist without the wave-like ordering.
Rare-earth tellurides, compounds containing tellurium combined with rare-earth elements, are particularly fertile ground for such studies. Within these crystals, CDWs sometimes give rise to phases of matter that defy conventional explanation, hinting at deeper hidden orders waiting to be uncovered.
A New Kind of Symmetry: The Ferroaxial Order
Recently, researchers from Boston College, Cornell University, and collaborating institutions made a breakthrough in this search for hidden patterns. In a study published in Nature Physics, they reported the discovery of a ferroaxial order in rare-earth tellurides—a subtle electronic pattern that arises from coupled orbital and charge arrangements.
This was not just another incremental step forward. The ferroaxial order they observed revealed a broken symmetry inside the material, one that had long escaped detection. Unlike magnetism, where the orientation of spins breaks time-reversal symmetry, this ferroaxial order stems from electronic organization itself. It is a hidden order, deeply woven into the crystal’s electronic fabric, and it reshapes how physicists think about the interplay between charge, symmetry, and collective vibrations in matter.
Listening to the Music of Quasiparticles
To uncover this order, the team turned to quasiparticles—emergent entities in condensed matter systems that behave like particles but arise from collective excitations. Three years ago, Ken Burch and his colleagues had already made headlines by detecting the first-ever axial Higgs mode in a CDW system. Like the Higgs boson in particle physics, this collective vibration signals the emergence of a new phase of matter. Remarkably, the axial Higgs mode they observed also exhibited a form of “handedness,” or chirality, that begged for deeper explanation.
The new study was driven by a desire to answer why this handedness appeared. Was it the result of broken time-reversal symmetry, as in magnetism? Or did it come from a more subtle rearrangement of electronic order?
Seeing Symmetry Through Light
The researchers used optical experiments as their window into the hidden order. By shining light into the rare-earth telluride sample and analyzing the light that emerged, they could detect subtle differences in color and polarization—clues that the material’s symmetries had been broken.
By carefully rotating the crystal and measuring the resulting changes, they were able to isolate the origin of these effects. The evidence pointed clearly toward an electronic cause, rather than one rooted in the motion of atoms. To confirm this, they turned to electron microscopy, which revealed only a very weak ferroaxial component in the lattice itself. This proved that the phenomenon originated in the electrons, not in the atoms of the crystal structure.
Probing Handedness Without Magnetism
To deepen their understanding, the team conducted muon spin relaxation experiments, a technique that can reveal whether time-reversal symmetry is broken. If the handedness of the Higgs mode had come from electrons circulating like tiny magnets, the muons would have detected it. But the results showed otherwise—the handedness was not magnetic in origin.
This reinforced the conclusion that the ferroaxial order was electronic, a new kind of symmetry breaking tied directly to the charge and orbital patterns within the crystal.
Why Hidden Orders Matter
The discovery goes beyond the excitement of identifying a new phase of matter. It demonstrates that hidden orders can be revealed by studying quasiparticles and their symmetries—a powerful strategy for condensed matter physics. This approach provides unambiguous signatures of emergent phases, helping researchers to distinguish whether they arise from magnetic, lattice, or electronic origins.
Understanding these subtle electronic patterns could one day enable the design of new materials with remarkable properties. Hidden orders might influence how electrons move through a material, how it responds to light, or how it conducts electricity. Each discovery pushes the boundaries of what is possible in material science and technology.
The Road Ahead
For Ken Burch and his team, the journey is far from over. They are now working on ways to control ferroaxial domains—regions in the crystal where the hidden order aligns in a specific direction. Achieving single-domain control could open the door to manipulating the electronic properties of these materials with precision, paving the way for novel devices.
The team also aims to explore how ferroaxial order affects electronic transport and nonlinear responses—key factors that could unlock applications in next-generation electronics, quantum information, and beyond.
A Window Into Nature’s Subtleties
The discovery of ferroaxial order in rare-earth tellurides is a reminder of how much lies hidden in the world around us. Matter is not just static and inert; it is alive with patterns, rhythms, and symmetries that reveal themselves only under the right conditions. By learning to see and understand these hidden orders, physicists are not only advancing fundamental science but also opening new horizons for technology and human progress.
What began as curiosity about the handedness of a collective vibration has blossomed into a new chapter in the story of quantum materials. And as researchers continue to probe deeper into the hidden structures of matter, they will likely find even more surprises—each one a step closer to fully understanding the invisible architecture of the universe.
More information: Birender Singh et al, Ferroaxial density wave from intertwined charge and orbital order in rare-earth tritellurides, Nature Physics (2025). DOI: 10.1038/s41567-025-03008-2.
