  Identifying the essence of doped Mott insulators is one of the major
outstanding problems in condensed matter physics and the key to understanding
the high-temperature superconductivity in cuprates. We report real space
visualization of Mott transition in Sr1-xLaxCuO2+y cuprate films that cover the
entire electron- and hole-doped regimes. Tunneling conductance measurements
directly on the cooper-oxide (CuO2) planes reveal a systematic shift in the
Fermi level, while the fundamental Mott-Hubbard band structure remains
unchanged. This is further demonstrated by exploring atomic-scale electronic
response of CuO2 to substitutional dopants and intrinsic defects in a sister
compound Sr0.92Nd0.08CuO2. The results could be better explained in the
framework of self-modulation doping, similar to that in semiconductor
heterostructures, and form a basis for developing any microscopic theories for
cuprate superconductivity.
