Effect of Uniaxial Strain on the Phononic and Electronic Excitations of Ta2NiS5

Semiconductor Ta2NiS5 exhibits a structural transition from orthorhombic phase to monoclinic phase at 120 K, driven by acoustic instability [Phys. Rev. B 104, 045102(2021)]. Two Raman-active phonon modes, which have the same symmetry as the order parameter (B2𝑔 symmetry), show continuous frequency softening on cooling from 300K to 20 K. Moreover, a sharp exciton mode in the B2𝑔 symmetry channel has been observed at 0.3 eV. We study the phonon modes and interband excitations of Ta2NiS5 under uniaxial strain at low temperature. With increasing tensive strain along crystallographic a axis, the frequency of the two B2𝑔-symmetry phonon modes and the band gap of this semiconductor both increase, with a 6.5% frequency increase of the lowest-energy B2𝑔 phonon mode corresponding to a 4.1% increase of the gap size. On the contrary, the frequency change of the non-softening phonons is less than 1%. By analyzing the phonon intensity, we further find that the magnitude of the order parameter, and in turn the phase transition temperature, increases with the tensive strain. These experimental results demonstrate Ta2NiS5 as a suitable platform to explore the manipulation of lattice dynamics and electronic structure by applying uniaxial strain.