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## Condensed Matter / Biophysics Seminars – Chunjing Jia (UF Physics)

### Searching for excitonic insulators in low-dimensional systems

The excitonic insulator is a long-conjectured quantum phase that could occur in narrow-gap semiconductors or semimetals. In these systems, electron-hole pairs (excitons) can be created spontaneously and Bose condense into a collective state at low temperatures. Many of the EI candidates are also known to exhibit a structural transition across the putative EI phase transition, complicating the unambiguous identification of EI in these materials. In my talk, I am going to discuss two studies on the investigation of excitonic insulator candidates. One is about monolayer 1T-ZrTe2, a sister compound of excitonic insulator candidate 1T-TiSe2 in the two-dimensional limit. With a combination of theoretical calculation and angle-resolved photoemission spectroscopy measurement, exciton gas phase was observed above transition temperature Tc, suggesting a BEC type of excitonic insulator. [1] The second study is about Ta2NiSe5, a quasi-one-dimensional material. Combination of resonant inelastic x-ray scattering measurement and theoretical calculations show that gap opening in Ta2NiSe5 below transition temperature Tc is likely driven by the structural transition, although a minor contribution from excitonic insulator state may not be excluded. [2]

[1] Arxiv: 2201.11592 (2022)

[2] Phys. Rev. B. 103, 235150 (2021)