BEGIN:VCALENDAR VERSION:2.0 PRODID:Data::ICal 0.22 BEGIN:VEVENT DESCRIPTION:Peter Littlewood\, Argonne National Laboratory and University o f Chicago\n\n
\n The boundary between metal and insulator remains a frui tful source of emergent phenomena in materials\, ranging from oxides\, to cold atoms. Typically the insulating side of this boundary is occupied by an electronic crystal (though often disordered)\, and at higher temperatur es a polaronic liquid or bad metal. While the paradigm Hamiltonian for thi s transition involves only short &ndash\;range electronic correlations\, i n practice the transition is tuned by disorder\, by screening of longer ra nge Coulomb forces\, and by coupling to the lattice. \; These lectures will discuss a few of these phenomena in real oxide systems including bul k and interface transition metal oxides.
\n\n Heterostructure oxides offer the opportunity to build in electric fields by precise control of c hemistry on the atomic scale\, used recently to generate modulation doping of two- dimensional electron gases (2DEG) in oxides. The origin of the 2D EG\, whether in pristine or defected materials\, is under debate. I will d iscuss the role of surface redox reactions\, in particular O vacancies\, a s the source of mobile carriers\, and also discuss their role in the switc hing of ferroelectricity in ultra-thin films.
\n\n While electric ch arges can be screened by mobile carriers\, the same is not true of strain fields\, which have intrinsic long-range interactions that cannot be scree ned. When strain fields are produced as a secondary order parameter in pha se transitions - as for example in ferroelectrics - this produces unexpect ed consequences for the dynamics of order parameter fluctuations\, includi ng the generation of a gap in what would otherwise have been expected to b e Goldstone modes. In some cases\, eg manganites and nickelates\, other in tra-cell modes can nonlinearly screen the order parameter\, which produces a strong sensitivity of ordering to octahedral rotations\, essentially a jamming transition. This is relevant for tuning entropic effects at phase transitions\, perhaps to enhance electro-caloric effects.
\n DTSTART:20150320T193000Z LOCATION:Physics Building\, Room 204 SUMMARY:Screening of charge and structural motifs in oxides END:VEVENT END:VCALENDAR