Heterostructure oxides offer the opportunity to build in electric fields by precise control of chemistry on the atomic scale, used recently to generate modulation doping of two- dimensional electron gases (2DEG) in oxides. The origin of the 2DEG, whether in pristine or defected materials, is under debate. I will discuss the role of surface redox reactions, in particular O vacancies, as the source of mobile carriers, and also discuss their role in the switching of ferroelectricity in ultra-thin films.
While electric charges can be screened by mobile carriers, the same is not true of strain fields, which have intrinsic long-range interactions that cannot be screened. When strain fields are produced as a secondary order parameter in phase transitions - as for example in ferroelectrics - this produces unexpected consequences for the dynamics of order parameter fluctuations, including the generation of a gap in what would otherwise have been expected to be Goldstone modes. In some cases, eg manganites and nickelates, other intra-cell modes can nonlinearly screen the order parameter, which produces a strong sensitivity of ordering to octahedral rotations, essentially a jamming transition.