Intrinsic Conductor

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Science  18 Oct 2013:
Vol. 342, Issue 6156, pp. 291
DOI: 10.1126/science.342.6156.291-a

The unexpected finding of a conducting interface between two insulators, LaAlO3 (LAO) and SrTiO3 (STO), can be explained by extrinsic mechanisms such as the formation of oxygen vacancies; whether this interface is intrinsically conducting is still an open question. Warusawithana et al. designed an experiment to minimize the influence of extrinsic factors and glean the influence of the stoichiometry of the LAO layer on the conductivity of the interface. Their mosaic sample was formed by cutting several STO substrates into four pieces each and mounting the pieces from the different substrates next to each other; an LAO layer was then grown on top using molecular beam epitaxy where the ratio of the La and Al fluxes was finely varied to accomplish a gradient of LAO stoichiometries across the mosaic. The authors found that the conducting samples were Al-rich. In these conditions, the excess Al substituted for the missing La, and no vacancies were formed; the natural discontinuity in polarization at the interface was relieved by the formation of a two-dimensional conducting layer. In contrast, in La-rich samples the discontinuity was neutralized by the migration of cations from the interface, enabled by vacancies created because of the off-stoichiometric composition. Similar mechanisms may be at work in other interfacial systems.

Nat. Commun. 4, 2351 (2013).

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