BEGIN:VCALENDAR VERSION:2.0 PRODID:Data::ICal 0.22 BEGIN:VEVENT DESCRIPTION:CANCELED Peter Littlewood\, Argonne National Laboratory\n\n
\ n The boundary between metal and insulator remains a fruitful source of em ergent phenomena in materials\, ranging from oxides\, to cold atoms. Typic ally the insulating side of this boundary is occupied by an electronic cry stal (though often disordered)\, and at higher temperatures a polaronic li quid or bad metal. While the paradigm Hamiltonian for this transition invo lves only short &ndash\;range electronic correlations\, in practice the tr ansition is tuned by disorder\, by screening of longer range Coulomb force s\, and by coupling to the lattice. \; These lectures will discuss a f ew of these phenomena in real oxide systems including bulk and interface t ransition metal oxides.
\n\n Heterostructure oxides offer the opport unity to build in electric fields by precise control of chemistry on the a tomic scale\, used recently to generate modulation doping of two- dimensio nal electron gases (2DEG) in oxides. The origin of the 2DEG\, whether in p ristine or defected materials\, is under debate. I will discuss the role o f surface redox reactions\, in particular O vacancies\, as the source of m obile carriers\, and also discuss their role in the switching of ferroelec tricity in ultra-thin films.
\n\n While electric charges can be scre ened by mobile carriers\, the same is not true of strain fields\, which ha ve 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 f or 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 ca n nonlinearly screen the order parameter\, which produces a strong sensiti vity of ordering to octahedral rotations\, essentially a jamming transitio n. This is relevant for tuning entropic effects at phase transitions\, per haps to enhance electro-caloric effects.
\n DTSTART:20141114T203000Z LOCATION:Physics Building\, Room 204 SUMMARY:Screening of charge and structural motifs in oxides END:VEVENT END:VCALENDAR