Stability and Structure of Polar Surfaces and Interfaces

Simple -- and often used -- electrostatic arguments suggest that polar surfaces of insulators are unstable. Experimental studies, however, have demonstrated that polar surfaces of oxides such as MgO do exist. Moreover, the structure and growth of thin films on these polar oxide surfaces can be quite different than on the non-polar surfaces. The physics underlying the long-standing controversy regarding the inherent (in)stability of polar systems, and the interface properties of polar materials, will be discussed and illustrated by a combination of experiments and first-principles calculations for the MgO(111) surface and its interfaces with GaN and iron oxide.