  The parent compound of the copper-oxide high temperature superconductors is a
Mott insulator. Superconductivity is realized by doping an appropriate amount
of charge carriers. How a Mott insulator transforms into a superconductor is
crucial in understanding the unusual physical properties of high temperature
superconductors and the superconductivity mechanism. Here we report high
resolution angle-resolved photoemission measurement on heavily underdoped
Bi2Sr2-xLaxCuO6+d system. The electronic structure of the lightly-doped samples
exhibit a number of characteristics: existence of an energy gap along the nodal
direction, d-wave-like anisotropic energy gap along the underlying Fermi
surface, and coexistence of a coherence peak and a broad hump in the
photoemission spectra. Our results reveal a clear insulator-superconductor
transition at a critical doping level of ~0.10 where the nodal energy gap
approaches zero, the three-dimensional antiferromagnetic order disappears, and
superconductivity starts to emerge. These observations clearly signal a close
connection between the nodal gap, antiferromagnetism and superconductivity.
