Direct observation of excitonic instability in Ta<sub>2</sub>NiSe<sub>5</sub>.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 33785740.
- Also identified by DOI 10.1038/s41467-021-22133-z and PMC identifier 8010035.
- Licence recorded as CC BY.
- The licence permits redistribution, so the abstract is shown in full and the full text is available from the publisher.
Abstract
Coulomb attraction between electrons and holes in a narrow-gap semiconductor or a semimetal is predicted to lead to an elusive phase of matter dubbed excitonic insulator. However, direct observation of such electronic instability remains extremely rare. Here, we report the observation of incipient divergence in the static excitonic susceptibility of the candidate material Ta<sub>2</sub>NiSe<sub>5</sub> using Raman spectroscopy. Critical fluctuations of the excitonic order parameter give rise to quasi-elastic scattering of B<sub>2g</sub> symmetry, whose intensity grows inversely with temperature toward the Weiss temperature of T<sub>W</sub> ≈ 237 K, which is arrested by a structural phase transition driven by an acoustic phonon of the same symmetry at T<sub>C</sub> = 325 K. Concurrently, a B<sub>2g</sub> optical phonon becomes heavily damped to the extent that its trace is almost invisible around T<sub>C</sub>, which manifests a strong electron-phonon coupling that has obscured the identification of the low-temperature phase as an excitonic insulator for more than a decade. Our results unambiguously reveal the electronic origin of the phase transition.