Probing the susceptibility of the excitonic insulator candidate Ta₂NiSe₅ through the elastocaloric effect

The excitonic insulator, a theorized phase in which electron-hole pairs form a condensate, analogous to the Cooper pairs condensate in superconductivity, has long been searched for in bulk materials, yet scant experimental evidence exists of its appearance. Ta₂NiSe₅, a near zero-gap semiconductor at high temperatures, exhibits a phase transition at 328K for which both transport and spectroscopic measurements indicate a gap forms which is consistent with a semiconductor to excitonic insulating phase transition. However, a structural orthorhombic to monoclinic q=0 phase transition occurs simultaneously, raising questions as to whether the driving mechanism for the phase is from electronic excitonic fluctuations or from purely structural degrees of freedom. Here we present elastocaloric measurements of the material, which use strain as a tuning parameter to change the entropy of the system and hence probe order parameters which couple to strain (and their associated susceptibilities). This technique effectively isolates the electronic fluctuations which drives the phase transition, and we measure the corresponding susceptibility which can be described by a Curie-Weiss form with a high Weiss temperature (T*=300K), consistent with non-structural modes being the driving mechanism of the phase transition.