  High temperature superconductivity in cuprates arises from doping a parent
Mott insulator by electrons or holes. A central issue is how the Mott gap
evolves and the low-energy states emerge with doping. Here we report
angle-resolved photoemission spectroscopy measurements on a cuprate parent
compound by sequential in situ electron doping. The chemical potential jumps to
the bottom of the upper Hubbard band upon a slight electron doping, making it
possible to directly visualize the charge transfer band and the full Mott gap
region. With increasing doping, the Mott gap rapidly collapses due to the
spectral weight transfer from the charge transfer band to the gapped region and
the induced low-energy states emerge in a wide energy range inside the Mott
gap. These results provide key information on the electronic evolution in
doping a Mott insulator and establish a basis for developing microscopic
theories for cuprate superconductivity.
