Infrared phonon spectra of quasi-one-dimensional Ta2NiSe5 and Ta2NiS5

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

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Infrared phonon spectra of quasi-one-dimensional Ta₂NiSe₅ and Ta₂NiS₅

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Isobe, M.
Scientific Facility Crystal Growth (Masahiko Isobe), Max Planck Institute for Solid State Research, Max Planck Society;
Department Quantum Materials (Hidenori Takagi), Max Planck Institute for Solid State Research, Max Planck Society;

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Takagi, H.
Department Quantum Materials (Hidenori Takagi), Max Planck Institute for Solid State Research, Max Planck Society;

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Keimer, B.
Department Solid State Spectroscopy (Bernhard Keimer), Max Planck Institute for Solid State Research, Max Planck Society;

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Boris, A. V.
Department Solid State Spectroscopy (Bernhard Keimer), Max Planck Institute for Solid State Research, Max Planck Society;

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Citation

Larkin, T. I., Dawson, R. D., Höppner, M., Takayama, T., Isobe, M., Mathis, Y.-L., et al. (2018). Infrared phonon spectra of quasi-one-dimensional Ta₂NiSe₅ and Ta₂NiS₅. Physical Review B, 98(12): 125113.

Cite as: https://hdl.handle.net/21.11116/0000-000E-D1EC-C

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) Ta2NiSe5 and orthorhombic (Cmcm) Ta2NiS5 single crystals. The identified dipole-active phonon modes polarized along the a and c axes are in good agreement with density functional theory (DFT) calculations. With increasing temperature the a-axis phonon modes of Ta2NiSe5 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 = 326K. In Ta2NiS5, 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 Ta2NiSe5, as opposed to isolated instances of the same in Ta2NiS5, and find this to be in good agreement with an excitonic insulator state below Tc in the former, compared to the absence of one in the latter.