Atomic Control of the Electronic Structure at Complex Oxide Heterointerfaces
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Atomic Control of
the Electronic Structure at Complex Oxide Heterointerfaces Harold Y. Hwang
Abstract The following article is based on the Outstanding Young Investigator Award presentation given by Harold Y. Hwang of the University of Tokyo on March 29, 2005, at the Materials Research Society Spring Meeting in San Francisco. Hwang was cited for “innovative work on the physics of transition-metal oxides and the atomic-scale synthesis of complex oxide heterostructures.” Perovskite oxides range from insulators to superconductors and can incorporate magnetism as well as couple to phonon instabilities. The close lattice match between many perovskites raises the possibility of growing epitaxial thin-film heterostructures with different ground states that may compete or interact. The recent development of superconducting Josephson junctions, magnetic tunnel junctions, ferroelectric memory cells, and resistive switching can be considered examples within this new heteroepitaxial family. In this context, Hwang presents his studies of electronic structure at atomically abrupt interfaces grown by pulsed laser deposition. Some issues are generic to all heterointerfaces, such as the stability of dopant profiles and diffusion, interface states and depletion, and interface charge arising from polarity discontinuities. A more unusual issue is the charge structure associated with Mott insulator/band insulator interfaces. The question is, how should one consider the correlated equivalent of band bending? This semiconductor concept is based on the validity of rigid single-particle band diagrams, which are known to be an inadequate description for strongly correlated electrons. In addition to presenting an interesting scientific challenge, this question underlies the attempts to develop new applications of doped Mott insulators in device geometries. Keywords: epitaxy, heterojunctions, interfaces, pulsed laser deposition.
Introduction Perovskite oxides and structurally related compounds exhibit an incredibly broad range of physical properties— insulator, semiconductor, metal, superconductor, heavy fermion, ferromagnet, antiferromagnet, spin glass, charge/spin density wave transitions, ferroelectricity, piezoelectricity, etc.1 It is not an exaggeration to say that almost every ground state is available, and the list continues to grow by the continual development of new ma-
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terials exploring exotic superconductivity; nanoscale ordering of charge, spin, and orbitals; multiferroics, and so on.2 Many of these “emergent” states (that is, states beyond those arising from the simple components) reflect the competition between various underlying instabilities, and the possibility to microscopically couple different materials at artificial interfaces provides one of the important motivations for our research.
Needless to say, many of these physical properties are technologically useful, as can be seen by the long history of commercial utilization of complex oxides, predominantly in bulk form. Following the microelectronics revolution, a
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