  In the 1960s, it was proposed that in small indirect band-gap materials,
excitons can spontaneously form because the density of carriers is too low to
screen the attractive Coulomb interaction between electrons and holes. The
result is a novel strongly interacting insulating phase known as an excitonic
insulator. Here we employ scanning tunnelling microscopy (STM) and spectroscopy
(STS) to show that the enhanced Coulomb interaction in quantum-confined
elemental Sb nanoflakes drives the system to the excitonic insulator state. The
unique feature of the excitonic insulator, a charge density wave (CDW) without
periodic lattice distortion, is directly observed. Furthermore, STS shows a gap
induced by the CDW near the Fermi surface. Our observations suggest that the
Sb(110) nanoflake is an excitonic insulator.
