Elastic Soft Mode and Electric Quadrupole Response in Excitonic Insulator Candidate (Ta0.952V0.048)2NiSe5: Contribution of Electron–Phonon Interaction

We performed ultrasonic measurements on (Ta0.952V0.048)2NiSe5 to investigate the contribution of acoustic phonons at the zone center to structural and possible excitonic transitions. The transverse elastic constant C55 with irreducible representation B2g exhibits softening over 90% above a transition temperature of Ts = 280 K, indicating the soft mode of corresponding TA phonons. Our analysis based on the quadrupole response indicates that the electron–phonon interaction contributes to the excitonic transition in addition to the hybridization between the conduction and valence bands if the excitonic transition is realized.