Grainger College of Engineering, All Events

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 (A_2g) instability, which in turn sets the stage for a nematic (E_g) 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.