The excitonic insulator is a condensate of electron-hole pairs, conceptually analogous to the Cooper pair condensate in superconductors. Long postulated as an electronic instability in small-gap semiconductors and semimetals, its unambiguous identification has remained challenging, particularly in bulk materials.
Two obstacles are especially formidable: the charge-neutral nature of the electron-hole condensate makes it invisible to conventional electric and magnetic probes, while the inevitable coupling between electronic and lattice degrees of freedom obscures the primary driving mechanism.
In this talk, I will demonstrate how in-situ strain serves as a symmetry-resolved tuning knob to probe and control the order parameters of two leading excitonic insulator candidates: Ta2NiSe5 and 1T-TiSe2.
In Ta2NiSe5, applying mirror-symmetry-breaking shear strain couples directly to the order parameter, and elastocaloric measurements reveal a thermodynamic susceptibility that follows a Curie-Weiss law with a Curie temperature lying only ~10% below the structural transition temperature.
This proximity corroborates a non-structural driving mechanism and strengthens the case for an excitonic origin. Remarkably, Ta2NiSe5 emerges as a prototypical realization of ferroaxial order - a state that breaks all vertical mirror planes while preserving inversion and the horizontal mirror symmetry, and which is notoriously difficult to detect by conventional means.
In 1T-TiSe2, where a chiral charge density wave has long been debated, symmetry-resolved elastoresistivity measurements reveal an intrinsic antisymmetric off-diagonal response satisfying m12 = −m21, a bulk transport signature unambiguously identifying ferroaxial rather than chiral order.
Combined with elastocaloric mapping across strain and temperature, our results establish a clear symmetry-breaking hierarchy: a primary CDW transition is followed by a ferroaxial (A2g) instability, which in turn sets the stage for a nematic (Eg) transition deep within the ordered phase.
These findings reframe the symmetry landscape of 1T-TiSe2 and reconcile previously conflicting surface-sensitive and bulk measurements. Taken together, the two systems illustrate how strain-based techniques serve as essential and discriminating probes for uncovering hidden symmetry breaking in quantum materials.