Collectivization at Oxide Heterointerfaces:
Interconnection of Structural, Charge, Spin, and Orbital Flexibility
Warren E. Pickett, University of California Davis
Disruption of periodic order in ionic materials (simplest examples are flat surfaces and abrupt
heterointerfaces) often involve polar layers that necessitate readjustments in the valence of
constituent ions. Recently studied examples include, for example, the SrTiO3/LaAlO3 (001)
interface between two band insulators, and the SrTiO3/LaTiO3 (001) interface between a
band and a correlated insulator. Often the resulting layer is seen to be conducting, with
high mobility, magnetic order, and even superconductivity.
Given the fractional charge (± 1 e) per interface unit cell, perhaps it is the insulating
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interfaces that are the most unusual. While Mott insulating (correlated electron) behavior
is to be expected at oxide interfaces involving open shell 3d ions (which will be addressed
also), calculations including correlation effects within the oxygen 2p states account for (and
seem to be necessary to explain) the observed insulating behavior at ‘charge-mismatched’
p-type LaAlO3/SrTiO3 interfaces. Magnetic O ions, if verified, will add a new dimension to
the phenomena that emerge at heterointerfaces.
While the systems with open-shell d shells should be anticipated to be candidates for
charge order, magnetism, and orbital order and interactions between these (as occurs in
bulk manganites and other transition metal oxides), we have recently been finding that
‘simple’ structural relaxation of ions near the interface can alter the band lineups across the
interface by 1-2 eV, a remarkably large effect that feeds back into the interplay between
charge, spin, and orbital order at the interface. Examples of this complex collectivization
of degrees of freedom will be drawn from the VO2/TiO2 system as well as the interfaces
mentioned above.
Acknowledgments: R. Pentcheva (Munich), V. Pardo (Santiago de Compostela)
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