  Electrons and holes can spontaneously form excitons and condense in a
semimetal or semiconductor, as predicted decades ago. This type of Bose
condensation can happen at much higher temperatures in comparison with dilute
atomic gases. Two-dimensional (2D) materials with reduced Coulomb screening
around the Fermi level are promising for realizing such a system. Here we
report a change in the band structure accompanied by a phase transition at
about 180 K in single-layer ZrTe2 based on angle-resolved photoemission
spectroscopy (ARPES) measurements. Below the transition temperature, gap
opening and development of an ultra-flat band top around the zone center are
observed. This gap and the phase transition are rapidly suppressed with extra
carrier densities introduced by adding more layers or dopants on the surface.
The results suggest the formation of an excitonic insulating ground state in
single-layer ZrTe2, and the findings are rationalized by first principles
calculations and a self-consistent mean-field theory. Our study provides
evidence for exciton condensation in a 2D semimetal and demonstrates strong
dimensionality effects on the formation of intrinsic bound electron-hole pairs
in solids.
