Elastic Soft Mode and Electric Quadrupole Response in Excitonic Insulator Candidate (Ta$_{0.952}$V$_{0.048}$)$_2$NiSe$_5$: Contribution of Electron-Phonon Interaction

Abstract

We performed ultrasonic measurements on (Ta 0.952 V 0.048 ) 2 NiSe 5 to investigate the contribution of acoustic phonons at the zone center to structural and possible excitonic transitions. The transverse elastic constant C 55 with irreducible representation B 2g exhibits softening over 90% above a transition temperature of T s = 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.