paper

Infrared phonon spectra of quasi-one-dimensional Ta $_{2}$ NiSe $_{5}$ and Ta $_{2}$ NiS $_{5}$

arXiv:1807.10035 · doi:10.1103/PhysRevB.98.125113

Abstract

Using a combination of infrared ellipsometry, time-domain terahertz spectroscopy, and far-infrared reflectometry we have obtained the $a c$ -plane complex dielectric function of monoclinic ( $C 2/ c$ ) Ta $_{2}$ NiSe $_{5}$ and orthorhombic ( $C m c m$ ) 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.

10 pages, 7 figures

Infrared phonon spectra of quasi-one-dimensional Ta2​NiSe5​ and Ta2​NiS5​

Abstract

Infrared phonon spectra of quasi-one-dimensional Ta $_{2}$ NiSe $_{5}$ and Ta $_{2}$ NiS $_{5}$