  High temperature superconductivity in cuprates is realized by doping the Mott
insulator with charge carriers. A central issue is how such an insulating state
can evolve into a conducting or superconducting state when charge carriers are
introduced. Here, by in situ vacuum annealing and Rb deposition on the
Bi2Sr2Ca0.6Dy0.4Cu2O8+delta (Bi2212) sample surface to push its doping level
continuously from deeply underdoped (Tc=25 K, doping level p-0.066) to the near
zero doping parent Mott insulator, angle-resolved photoemission spectroscopy
measurements are carried out to observe the detailed electronic structure
evolution in lightly hole-doped region for the first time. Our results indicate
that the chemical potential lies at about 1 eV above the charge transfer band
for the parent state at zero doping which is quite close to the upper Hubbard
band. With increasing hole doping, the chemical potential moves continuously
towards the charge transfer band and the band structure evolution exhibits a
rigid band shift-like behavior. When the chemical potential approaches the
charge transfer band at a doping level of -0.05, the nodal spectral weight near
the Fermi level increases, followed by the emergence of the coherent
quasiparticle peak and the insulator-superconductor transition. Our
observations provide key insights in understanding the insulator-superconductor
transition in doping the parent cuprate compound and for establishing related
theories.
