Excitonic insulator (EI) was proposed in 60's as a distinct insulating state originating from pure electronic interaction, but its material realization has been elusive with extremely few material candidates and with only limited evidence such as anomalies in transport properties, band dispersions, or optical transitions. We investigate the real-space electronic states of the low temperature phase in Ta2NiSe5 with an atomic resolution to clearly identify the quasiparticle energy gap together with the strong electron-hole band renormalization using scanning tunneling microscopy (STM) and spectroscopy (STS). These results are in good agreement with the EI transition scenario in Ta2NiSe5. Our spatially-resolved STS data and theoretical calculations reveal further the orbital inversion at band edges, which indicates the exciton condensation close to the Bardeen-Cooper-Schrieffer regime.
@article{arxiv.1811.01552,
title = {Strong interband interaction in the excitonic insulator phase of Ta$_2$NiSe$_5$},
author = {Jinwon Lee and Chang-Jong Kang and Man Jin Eom and Jun Sung Kim and Byung Il Min and Han Woong Yeom},
journal= {arXiv preprint arXiv:1811.01552},
year = {2019}
}