Doping Evolution and Polar Surface Reconstruction of the Infinite-Layer Cuprate Sr_1-xLa_xCuO_2

We use angle-resolved photoemission spectroscopy to study the doping evolution of infinite-layer Sr1-xLaxCuO2 thin films grown by molecular-beam epitaxy. At low doping, the material exhibits a dispersive lower Hubbard band typical of the superconducting cuprate parent compounds. As carriers are added to the system, a continuous evolution from charge-transfer insulator to superconductor is observed, with the initial lower Hubbard band pinned well below the Fermi level and the development of a coherent low-energy band with electron doping. This two-component spectral function emphasizes the important role that strong local correlations play even at relatively high doping levels. Electron diffraction probes reveal a p(2 x 2) surface reconstruction of the material at low doping levels. Using a number of simple assumptions, we develop a model of this reconstruction based on the polar nature of the infinite-layer structure. Finally, we provide evidence for a thickness-controlled transition in ultrathin films of SrCuO2 grown on nonpolar SrTiO3, highlighting the diverse structural changes that can occur in polar complex oxide thin films.