Lattice dynamics of the excitonic insulator Ta2Ni(Se1-x Sx)5

Recently, we employed electronic polarization-resolved Raman spectroscopy to reveal the strongly correlated excitonic insulator (EI) nature of Ta2NiSe5 [Volkov, npj Quant. Mater. 6, 52 (2021)10.1038/s41535-021-00351-4], and also showed that for Ta2Ni(Se1-xSx)5 alloys the critical excitonic fluctuations diminish with sulfur concentration x, exposing a cooperating lattice instability that takes over for large x [Volkov, arXiv:2104.07032]. Here we focus on the lattice dynamics of the EI family Ta2Ni(Se1-xSx)5 (x=0,..., 1). We identify all Raman-active optical phonons of Ag (fully symmetric) and B2g (ac-quadrupole-like) symmetries (D2h point group) and study their evolution with temperature and sulfur concentration. We demonstrate the change of selection rules at temperatures below the orthorhombic-to-monoclinic transition at Tc(x) that is related to the EI phase. We find that Tc(x) decrease monotonically from 328 K for Ta2NiSe5 to 120 K for Ta2NiS5 and that the magnitude of lattice distortion also decreases with the sulfur concentration x. For x<0.7, the two lowest frequency B2g phonon modes show strongly asymmetric line shapes at high temperatures due to Fano interference with the broad excitonic continuum present in a semimetallic state. Within the framework of extended Fano model, we develop a quantitative description of the interacting exciton-phonon excitation line shape, enabling us to derive the intrinsic phonon parameters and determine the exciton-phonon interaction strength that affects the transition temperature Tc(x). While at low temperatures the intrinsic phonon parameters are in good agreement with the ab initio calculations and the anharmonic decay model, their temperature dependencies show several anomalous behaviors: (i) Frequencies of B2g phonons harden pronouncedly upon cooling in vicinity of Tc(x) for x<0.7 semimetals and, in contrast, soften monotonically for Ta2NiS5 semiconductor; (ii) the lifetime of certain phonons increases strongly below Tc(x) for x<0.7, revealing the gap opening in the broken symmetry phase; and (iii) for most modes, the intensity shows rather strong temperature dependence that we relate to the interplay between electronic and phononic degrees of freedom. For Ta2NiSe5 we also observe signatures of the acoustic mode scattered assisted by the structural domain walls formed below Tc. Based on our results, we additionally present a consistent interpretation of the origin of oscillations observed in time-resolved pump-probe experiments.