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. 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.Comment: 26 pages, 4 figures, supplementary information include

Growth and characterisation of bulk Sr2CuO2Cl2 single crystals

Large bulk single crystals of the Sr2CuO2Cl2 compound with dimensions of 15 x 6 x 4 mm(3) have been grown directly from the melt by the floating-zone method using a light-image furnace. The optimal growth conditions are found in a mixed atmosphere of 0.2 bar oxygen and 1.2 bar argon. Results of the characterisation have revealed thr high quality of the as-grown crystals. Furthermore, a possible congruent route of formation from the melt as well as the chemical stability of this compound is discussed