Uniaxial-Pressure Control of Excitonic Fluctuations and Monoclinic Distortions in Ta2NiSe5

Ta2NiSe5 undergoes a phase transition characterized by the opening of an energy gap and a reduction in its crystal symmetry below Tc=326 K. The primary cause of this transition is debated to be either electron-hole correlations, in line with the excitonic insulator hypothesis, or structural instability. The chain structure of Ta2NiSe5 enables effective control over its electronic and lattice properties by applying uniaxial strain. In this study, we utilize polarization-resolved Raman spectroscopy to demonstrate that the electronic and structural order parameters respond differently to uniaxial pressure applied along the Ta-Ni chains. Compressive strain reduces monoclinic distortions while enhancing excitonic fluctuations. Such disparate behavior suggests that many-body effects may significantly amplify exciton fluctuations even when monoclinic distortions are minimized, thereby supporting the excitonic nature of the phase transition.