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

Charge density wave (CDW) is a collective quantum phenomenon with a charge modulation in solids. Condensation of electron and hole pairs with finite momentum will lead to such an ordered state. However, lattice symmetry breaking manifested as the softening of phonon modes can occur simultaneously, which makes it difficult to disentangle the origin of the transition. Here, we report a condensed phase in low dimensional HfTe2, whereas angle-resolved photoemission spectroscopy (ARPES) measurements show a metal-insulator transition by lowering the temperature in single triatomic layer (TL) HfTe2. A full gap opening, renormalization of the bands, and emergence of replica bands at the M point are observed in the low temperatures, indicating formation of a CDW in the ground state. Raman spectroscopy shows no sign of lattice distortion within the detection limit. The results are corroborated by first-principles calculations, demonstrating the electronic origin of the CDW. By adding more layers, the phase transition is suppressed and completely destroyed at 3 TL because of the increased screening around the Fermi surface. Interestingly, a small amount of electron doping in 1 TL film during the growth significantly raises the transition temperature (Tc), which is attributed to a reduced screening effect and a more balanced electron and hole carrier density. Our results indicate a CDW formation mechanism consistent with the excitonic insulator phase in low dimensional HfTe2 and open up opportunity for realization of novel quantum states based on 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