  Although the mechanism of superconductivity in the cuprates remains elusive,
it is generally agreed that at the heart of the problem is the physics of doped
Mott insulators. The cuprate parent compound has one unpaired electron per Cu
site, and is predicted by band theory to be a half-filled metal. The strong
onsite Coulomb repulsion, however, prohibits electron hopping between
neighboring sites and leads to a Mott insulator ground state with
antiferromagnetic (AF) ordering. Charge carriers doped into the CuO2 plane
destroy the insulating phase and superconductivity emerges as the carrier
density is sufficiently high. The natural starting point for tackling high Tc
superconductivity is to elucidate the electronic structure of the parent Mott
insulator and the behavior of a single doped charge. Here we use a scanning
tunneling microscope to investigate the atomic scale electronic structure of
the Ca2CuO2Cl2 parent Mott insulator of the cuprates. The full electronic
spectrum across the Mott-Hubbard gap is uncovered for the first time, which
reveals the particle-hole symmetric and spatially uniform Hubbard bands. A
single electron donated by surface defect is found to create a broad in-gap
electronic state that is strongly localized in space with spatial
characteristics intimately related to the AF spin background. The unprecedented
real space electronic structure of the parent cuprate sheds important new light
on the origion of high Tc superconductivity from the doped Mott insulator
perspective.
