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

<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mtext>Ta</a:mtext><a:mn>2</a:mn></a:msub><a:mrow><a:msub><a:mrow><a:mi>NiSe</a:mi></a:mrow><a:mrow><a:mn>5</a:mn></a:mrow></a:msub></a:mrow></a:math> undergoes a phase transition characterized by the opening of an energy gap and a reduction in its crystal symmetry below <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msub><c:mrow><c:mi>T</c:mi></c:mrow><c:mrow><c:mi>c</c:mi></c:mrow></c:msub><c:mo>=</c:mo><c:mn>326</c:mn><c:mtext> </c:mtext><c:mtext> </c:mtext><c:mi mathvariant="normal">K</c:mi></c:mrow></c:math>. 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 <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mrow><f:msub><f:mrow><f:mi>Ta</f:mi></f:mrow><f:mrow><f:mn>2</f:mn></f:mrow></f:msub></f:mrow><f:mrow><f:msub><f:mrow><f:mi>NiSe</f:mi></f:mrow><f:mrow><f:mn>5</f:mn></f:mrow></f:msub></f:mrow></f:math> 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.