Tunable Metallicity at the Surface of La$_{5/8}$Ca$_{3/8}$MnO$_{3 }$Thin Films*

A series of in-situ STM studies of La$_{5/8}$Ca$_{3/8}$MnO$_{3}$(001) thin film reveals that the surface metallicity can be tuned by extrinsic oxygen doping at the surface. By in-situ annealing with or without oxygen, we can convert the surface back and forth between a ({\AA},,2 $\times $ {\AA},,2)R45 reconstructed surface and a (1 $\times $ 1) surface. Electrical properties of the surfaces are investigated by scanning tunneling spectroscopy (STS). I-V curves clearly show that the oxygen doping renders the surface insulating while the (1 $\times $ 1) surface without the oxygen doping is metallic. Structural models and their correlation to the surface metallicity have been proposed.