  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.
