The parent compound of cuprate high temperature superconductors is widely believed to be a charge-transfer-type Mott insulator. A key question concerning the pairing mechanism is the behavior of doped holes in the antiferromagnetic (AF) Mott insulator background, which is a prototypical quantum many-body problem. Scanning tunneling microscopy (STM) represents an ideal experimental technique to address these questions owing to its capability of atomic-scale imaging of local electronic states.

In this work, we use STM to visualize the electronic structure of diluted holes doped into the Ca₂CuO₂Cl₂ parent Mott insulator of cuprates. We find that a single hole exhibits an in-gap electronic state and clover-shaped spatial distribution reminiscent of a Zhang-Rice singlet. For multiple dopants lying in close proximity, the overlap of wavefunctions generates stripe- and ladder-shaped molecular orbitals, accompanied by the opening of a precursory energy gap around the Fermi level. With increasing doping, the molecular patterns proliferate in space and gradually form densely packed plaquettes with typical size around 4 a₀. A full-fledged superconducting gap develops smoothly on top of the stripe-like molecular orbitals, and display a systematic evolution of spectral lineshape from U-shape to V-shape. As the spatial occupation of the plaquette exceeds a threshold, long-range phase coherence is established and the system enters the superconducting state. The molecular orbital picture also gives natural explanations to several key issues in cuprates including the microscopic inhomogeneity, electronic nematicity, pair density wave, and nodal Fermi arc. These results demonstrate that the molecular orbital is the fundamental low energy electronic state formed by doping holes into the AF Mott insulator, and it holds the key for elucidating the pairing mechanism of cuprates.