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

Abstract

Charge density wave (CDW) is a collective quantum phenomenon with a charge modulation in solids1-2. Condensation of electron and hole pairs with finite momentum will lead to such an ordered state3-7. However, lattice symmetry breaking manifested as the softening of phonon modes can occur simultaneously, which makes it difficult to disentangle the origin of the transition8-14. 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.

https://www.nature.com/articles/s41567-023-02349-0 published in Nature Physics

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• Gate tuning of coupled electronic and structural phase transition in atomically thin Ta${}_2$NiSe${}_5$
• Observation of quasi-steady dark excitons and gap phase in a doped semiconductor
• Sliding phasons in Moiré Ladders
• Unusual temperature dependence of the band structure associated with local atomic distortion in monolayer 1T'-WTe2
• Stabilization of sliding ferroelectricity through exciton condensation
• Excitons and Optical Response in Excitonic Insulator Candidate TiSe${}_2$
• Raman spectroscopy of van der Waals topological magnet GdGaI
• Emergent quantum phenomena in two-dimensional 1T-TaS2
• Engineering excitonic metal-insulator transitions in ultra-thin doped copper sulfides