The search for spontaneously formed excitons in solid state materials has lasted for more than half a century. Despite intense experimental effort, a concurrent structural transition caused by electron-phonon interaction often complicates the search. Lately, Ta2Ni(Se,S)5 system has received increasing attention as a potential quasi-1D excitonic insulator, with a normal state continuously tunable from a semimetal to a semiconductor. Combining angle-resolved photoemission spectroscopy and first principles calculation, we discover negative electronic compressibility in charge-doped ground state, and electron-phonon interaction may be solely accountable . We also report an anomalous sudden disappearance of "pre-formed" excitons in the normal state across the semimetal-to-semiconductor transition, which contradicts Coulomb-interaction-only models . By comparing the experimental phase diagram with existing model predictions, we highlight the importance of electron-phonon interaction in the general consideration of "excitonic insulators" especially at low dimensions.
Reference:  Chen et al., arXiv:2203.06817  Chen et al., to appear