Ferroaxial order breaks mirror symmetries while preserving time-reversal and inversion. It's a rotation-like ordering — the order parameter is an axial vector — but because it preserves the symmetries that most probes are sensitive to, it's invisible to standard diffraction, magnetization, and polarization measurements. A “hidden order” not because it doesn't exist but because the usual detectors can't see it.
The paper detects ferroaxial order in 1T-TiSe₂ using elastoresistivity — measuring how electrical resistance changes when the crystal is strained. The key observable: an off-diagonal linear elastoresistivity that appears below the charge density wave transition at ~200 K. This component is forbidden by the undistorted crystal's symmetry and permitted only if mirror symmetry is broken, which is exactly what ferroaxial order does.
The detection becomes more convincing with dynamics. Applying engineered strain combinations moves ferroaxial domain walls, producing hysteretic resistance changes. The domains — regions of different ferroaxial orientation — respond to mechanical stress because the order parameter couples to strain. Nonlinear elastoresistivity coefficients diverge near the transition temperature, matching theoretical predictions for ferroaxial susceptibility.
An additional phase transition appears tens of Kelvin below the main CDW transition, detected through elastocaloric measurements. The CDW state of TiSe₂ is not one ordered phase but at least two nested phases, the inner one associated with ferroaxial ordering that was undetectable until the right probe — strain, not light or magnetism — was applied. The order was hidden; the detector was wrong.