Electronic inhomogeneity and band structure in superstructural CuO_2 planes of infinite-layer Sr_(0.94)La_(0.06)CuO_(2+y) films

摘要

Scanning tunneling microscopy and spectroscopy are utilized to study the atomic-scale structure and electronic properties of infinite-layer Sr_(0.94)La_(0.06)CuO_(2+y) films prepared on SrRuO_3-buffered SrTiO_3(001) substrate by ozone-assisted molecular beam epitaxy. Incommensurate structural supermodulation with a period of 24.5 A is identified on the CuO_2-terminated surface, leading to characteristic stripes running along the 45° direction with respect to the Cu-O-Cu bonds. Spatially resolved tunneling spectra reveal substantial inhomogeneity on a nanometer-length scale and emergence of in-gap states at sufficient doping. Despite the Fermi level shifting up to 0.7 eV, the charge-transfer energy gap of the CuO_2 planes remains fundamentally unchanged at different doping levels. The occurrence of the CuO_2 superstructure is constrained in the surface region and its formation is found to link with oxygen intake that serves as the doping agent of holes in the epitaxial films.