Visualization of the emergence of the pseudogap state and the evolution to superconductivity in a lightly hole-doped Mott insulator

Superconductivity emerges from the cuprate antiferromagnetic Mott state with hole doping. The resulting electronic structure(1) is not understood, although changes in the state of oxygen atoms seem paramount(2-5). Hole doping first destroys the Mott state, yielding a weak insulator(6,7) where electrons localize only at low temperatures without a full energy gap. At higher doping levels, the 'pseudogap', a weakly conducting state with an anisotropic energy gap and intra-unit-cell breaking of 90 degrees rotational (C-4v) symmetry, appears(3,4,8-10). However, a direct visualization of the emergence of these phenomena with increasing hole density has never been achieved. Here we report atomic-scale imaging of electronic structure evolution from the weak insulator through the emergence of the pseudogap to the superconducting state in Ca2-xNaxCuO2Cl2. The spectral signature of the pseudogap emerges at the lowest doping level from a weakly insulating but C-4v-symmetric matrix exhibiting a distinct spectral shape. At slightly higher hole density, nanoscale regions exhibiting pseudogap spectra and 180 degrees rotational (C-2v) symmetry form unidirectional clusters within the C-4v-symmetric matrix. Thus, hole doping proceeds by the appearance of nanoscale clusters of localized holes within which the broken-symmetry pseudogap state is stabilized. A fundamentally two-component electronic structure(11) then exists in Ca2-xNaxCuO2Cl2 until the C-2v-symmetric clusters touch at higher doping levels, and the long-range superconductivity appears.