Infrared phonon spectra of quasi-one-dimensional Ta$_2$NiSe$_5$ and Ta$_2$NiS$_5$

TL;DR

This study investigates the infrared phonon spectra of Ta$_2$NiSe$_5$ and Ta$_2$NiS$_5$, revealing differences in phonon-electron interactions related to their structural phases and excitonic states.

Contribution

It provides experimental infrared spectra and analysis of phonon modes in both materials, linking phonon behavior to excitonic insulator states and structural phase transitions.

Findings

Ta$_2$NiSe$_5$ shows phonon modes superimposed on electronic background near phase transition.

Ta$_2$NiS$_5$ exhibits persistent broad electronic mode without structural transition.

Overlap of exciton-phonon complexes in Ta$_2$NiSe$_5$ supports excitonic insulator state.

Abstract

Using a combination of infrared ellipsometry, time-domain terahertz spectroscopy, and far-infrared reflectometry we have obtained the $ac$-plane complex dielectric function of monoclinic ($C2/c$) Ta$_2$NiSe$_5$ and orthorhombic ($Cmcm$) Ta$_2$NiS$_5$ single crystals. The identified dipole-active phonon modes polarized along $a$ and $c$ axes are in good agreement with density functional theory calculations. With increasing temperature the $a$-axis phonon modes of Ta$_2$NiSe$_5$ become poorly discernible, as they are superimposed on the electronic background which gradually fills the energy gap near the monoclinic-to-orthorhombic phase transition temperature $T_c$ = 326 K. In Ta$_2$NiS$_5$, which does not exhibit such a structural transition and remains orthorhombic down to low temperatures, the $a$-axis phonon modes are superimposed on a persistent broad electronic mode centered near 16 meV. We attribute this difference to strongly overlapping exciton-phonon complexes in Ta$_2$NiSe$_5$, as opposed to isolated instances of the same in Ta$_2$NiS$_5$, and find this to be in good agreement with an excitonic insulator state below $T_c$ in the former, as compared to the absence of one in the latter.