  In this work, we find by means of first principle calculations a new physical
mechanism to generate a two dimensional electron gas, namely, the breaking of
charge ordering at the surface of a charge ordered semiconductor due to the
incomplete oxygen environment of the surface ions. The emergence of the 2D gas
is independent of the presence of oxygen vacancies or polar discontinuities;
this is a self-doping effect. This mechanism might apply to many charge ordered
systems, in particular, we study the case of BaBiO3(001). In bulk, this
material is a prototype of a "forbidden valence" compound in which the formal
"metallic" Bi4+ state is skipped exhibiting a charge disproportionated Bi3+ -
Bi5+ ordered structure. At room temperature, this charge disproportionation
together with the breathing distortions gives rise to a Peierls semiconductor
with monoclinic crystal structure. At higher temperature (T > 750 K) or upon
doping, it turns cubic and metallic. Interestingly, doped BaBiO3 was one of the
first non-cuprate high-Tc superconductors discovered. The outer layer of the
Bi-terminated simulated surface turns more cubic- like and metallic while the
inner layers remain in the insulating monoclinic state. On the other hand, the
metallization does not occur for the Ba termination, a fact that makes this
system appealing for nanostructuring. Finally, this finding sets another
possible route for future exploration: the potential scenario of 2D
superconductivity at the BaBiO3 surface.
