In-gap density of states in hole- and electron-doped CuO2 planes probed by scanning tunneling spectroscopy

The low-energy electronic structure of a c-axis SrxAyCuO2 (A is alkaline earth cation, x+y≦1, hole- and electron-doped infinite layer) thin film, grown by laser-molecular-beam epitaxy on a SrTiO3 (001) substrate, has been studied using ultrahigh-vacuum scanning tunneling microscopy/spectroscopy. Images have been obtained for co-deposited SrxAyCuO2 thin films, which show the surface consisting of flat terraces separated by steps that are unit cell high. Tunneling spectra of undoped Sr0.3Ca0.7CuO2 indicate a wide band gap of 1.8 eV which is consistent with the charge transfer gap. Hole-doped Sr0.85CuO2 shows in-gap states appearing at both the valence and conduction band edges. In contrast, for the electron-doped Sr0.9La0.1CuO2, in-gap states appear predominantly above the Fermi level, and the spectral shape becomes asymmetric around the Fermi level. When these two systems are compared, barrier-height measurements reveal that there is no apparent shift of the Fermi level measured from the vacuum level. This suggests that the framework of the rigid-band picture might break down implying a strongly correlated electron system.