STM probe of charge ordering and 2D carrier confinement in mixed valence manganites

One property common to most layered transition metal perovskite oxides is their highly anisotropic transport properties. The in-plane resistivity is metallic, whereas electron transport perpendicular to the layers is more characteristic of a semiconductor. We present vacuum tunneling spectroscopy on cleaved La$_{1.4}$Sr$_{1.6}$Mn$_{2}$O$_{7}$ single crystals using a scanning tunneling microscope (STM) in ultrahigh vacuum. We find a gap E$_{g}$ that is associated with the room temperature c-axis conductivity. Surprisingly, E$_{g}$ remains finite when the temperature is lowered through the metal-insulator transition at 90 Kelvin. This remarkable result suggests that the intrinsic electron transport between adjacent perovskite bilayers is thermally activated at all temperatures, with a characteristic activation energy E$_{g}$. We further find no evidence of electronic phase separation, as E$_{g}$ is spatially homogeneous at all temperatures. We shall discuss these spectroscopy data and atomically resolved STM micrographs in the context of colossal magnetoresistance (CMR), and their contribution to our understanding of CMR manganites.