Observation of possible excitonic charge density waves and metal-insulator transitions in atomically thin semimetals

This paper reports on a condensed phase observed in low-dimensional HfTe₂. Angle-resolved photoemission spectroscopy (ARPES) measurements reveal a metal-insulator transition at low temperatures in single triatomic layer (TL) HfTe₂, characterized by a full gap opening, band renormalization, and the emergence of replica bands. Raman spectroscopy shows no lattice distortion, suggesting an electronic origin for the charge density wave (CDW). First-principles calculations corroborate these findings, indicating an excitonic insulator phase. The transition is suppressed in thicker layers due to increased screening, while electron doping enhances the transition temperature. The results suggest a CDW formation mechanism consistent with exciton condensation.

Qiang Gao, Yang-hao Chan, Pengfei Jiao, Haiyang Chen, Shuaishuai Yin, Kanjanaporn Tangprapha, Yichen Yang, Xiaolong Li, Zhengtai Liu, Dawei Shen, Shengwei Jiang, Peng Chen