Visualizing the atomic-scale electronic structure of the Ca2CuO2Cl2 Mott insulator.

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. A crucial step for solving the high temperature superconductivity puzzle is to elucidate the electronic structure of the parent compound and the behaviour of doped charge carriers. Here we use scanning tunnelling microscopy to investigate the atomic-scale electronic structure of the Ca(2)CuO(2)Cl(2) 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. Defect-induced charge carriers are found to create broad in-gap electronic states that are strongly localized in space. We show that the electronic structure of pristine Mott insulator is consistent with the Zhang-Rice singlet model, but the peculiar features of the doped electronic states require further investigations.