Uniaxial-Pressure Control of Excitonic Fluctuations and Monoclinic Distortions in Ta_{2}NiSe_{5}

Ta_{2}NiSe_{5} undergoes a phase transition characterized by the opening of an energy gap and a reduction in its crystal symmetry below T_{c}=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 Ta_{2}NiSe_{5} 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.