Polar oxide surfaces

In the light of recent experimental as well as theoretical studies, we summarize our present understanding of polar oxide surfaces and examine fundamental issues regarding their stability. The focus is on the surface atomic configurations (relaxations, reconstructions, nonstoichiometry, etc) obtained under specific preparation conditions and their associated electronic structure. We discuss several mechanisms at work on polar surfaces, such as relaxation effects, change of covalency in the surface layers, partial filling of surface states, and stoichiometry variations, and try to assess their actual efficiency for cancelling the polarity.