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arXiv · 2007.01723

Phononic soft mode and strong electronic background behavior across the structural phase transition in the excitonic insulator Ta$_2$NiSe$_5$ (with Erratum)

Min-Jae Kim·Armin Schulz·Tomohiro Takayama·Masahiko Isobe·Hidenori Takagi·Stefan Kaiser
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Abstract

Ta$_2$NiSe$_5$ became one of the most investigated candidate materials for hosting an excitonic insulator ground state. Many studies describe the corresponding phase transition as a condensation of excitons breaking a continuous symmetry. This view got challenged recently pointing out the importance of the loss of two mirror symmetries at a structural phase transition that occurs together with the semiconductor-excitonic insulator transition. For such a scenario an unstable optical zone-center phonon at low energy is proposed to drive the transition. Here we report on the experimental observation of such a soft mode behavior using Raman spectroscopy. In addition we find a novel spectral feature, likely of electronic or joint electronic and phononic origin, that is clearly distinct from the lattice dynamics and that becomes dominant at Tc. This suggests a picture of joint structural and electronic order driving the phase transition.

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Min-Jae Kim, Armin Schulz, Tomohiro Takayama, Masahiko Isobe, Hidenori Takagi, Stefan Kaiser. 2020-07-03. Phononic soft mode and strong electronic background behavior across the structural phase transition in the excitonic insulator Ta$_2$NiSe$_5$ (with Erratum). https://doi.org/10.1103/physrevresearch.2.042039

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