Anomalous phonon Gr\"uneisen parameters in semiconductor Ta$_2$NiS$_5$

TL;DR

This study investigates how strain affects lattice vibrations and electronic properties in semiconductor Ta$_2$NiS$_5$, revealing large, negative Gr"uneisen parameters and proximity to structural instability through Raman spectroscopy and first-principles calculations.

Contribution

It provides the first detailed analysis of strain effects on phonons and electronic structure in Ta$_2$NiS$_5$, highlighting anomalous phonon behavior and proximity to structural instability.

Findings

Large negative Gr"uneisen parameters for specific phonons

Strain increases the semiconductor band gap

Proximity to structural instability similar to Ta$_2$NiSe$_5$

Abstract

Strain tuning is a powerful experimental method in probing correlated electron systems. Here we study the strain response of the lattice dynamics and electronic structure in semiconductor Ta$_2$NiS$_5$ by polarization-resolved Raman spectroscopy. We observe an increase of the size of the direct semiconducting band gap. Although the majority of the optical phonons show only marginal dependence to applied strain, the frequency of the two B$_{2g}$ phonon modes, which have quadrupolar symmetry and already anomalously soften on cooling under zero strain, increases significantly with tensile strain along the $a$ axis. The corresponding Gr\"uneisen parameters are unusually large in magnitude and negative in sign. These effects are well captured by first-principles density functional theory calculations and indicate close proximity of Ta$_2$NiS$_5$ to a structural instability, similar to that encountered in excitonic insulator candidate Ta$_2$NiSe$_5$.