  Biased conducting-tip atomic force microscopy (AFM) has been shown to write
and erase nanoscale metallic lines at the LaAlO3/SrTiO3 interface. Using
various AFM modes, we show the mechanism of conductivity switching is the
writing of surface charge. These charges are stably deposited on a wide range
of LaAlO3 thicknesses, including bulk crystals. A strong asymmetry with writing
polarity was found for 1 and 2 unit cells of LaAlO3, providing experimental
evidence for a theoretically predicted built-in potential.
