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arXiv · 1807.10035

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

T. I. Larkin·R. D. Dawson·M. Höppner·T. Takayama·M. Isobe·Y. -L. Mathis·H. Takagi·B. Keimer·A. V. Boris
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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.

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T. I. Larkin, R. D. Dawson, M. Höppner, T. Takayama, M. Isobe, Y. -L. Mathis, H. Takagi, B. Keimer, A. V. Boris. 2018-07-26. Infrared phonon spectra of quasi-one-dimensional Ta$_2$NiSe$_5$ and Ta$_2$NiS$_5$. https://doi.org/10.1103/physrevb.98.125113

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