Schematic diagram of possible reconstruction processes. (a) Stacking of and layers, where is the electric field and the electrostatic potential. In this case, the potential diverges since there is no reconstruction. (b) A structure in which the top layer of the V oxide portion of the system reconstructs to a charge . The electrostatic potential does not diverge in this case. (c) Another example where there is no divergence. In this case, the surface is reconstructed to a charge . The competition between (b) and (c) is the key observation reported by Higuchi et al. .