Artificial charge-modulationin atomic-scale perovskite titanate superlattices

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Abstract

The nature and length scales of charge screening in complex oxides are fundamental to a wide range of systems, spanning ceramic voltage-dependent resistors (varistors), oxide tunnel junctions and charge ordering in mixed-valence compounds1,2,3,4,5,6. There are wide variations in the degree of charge disproportionation, length scale, and orientation in the mixed-valence compounds: these have been the subject of intense theoretical study7,8,9,10,11, but little is known about the microscopic electronic structure. Here we have fabricated an idealized structure to examine these issues by growing atomically abrupt layers of LaTi3+O3 embedded in SrTi4+O3. Using an atomic-scale electron beam, we have observed the spatial distribution of the extra electron on the titanium sites. This distribution results in metallic conductivity, even though the superlattice structure is based on two insulators. Despite the chemical abruptness of the interfaces, we find that a minimum thickness of five LaTiO3 layers is required for the centre titanium site to recover bulk-like electronic properties. This represents a framework within which the short-length-scale electronic response can be probed and incorporated in thin-film oxide heterostructures.

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Figure 1: Annular dark field (ADF) image of LaTiO3 layers (bright) of varying thickness spaced by SrTiO3 layers.
Figure 2: Electron energy-loss spectra (EELS) for La and Ti simultaneously recorded across a 2-unit-cell LaTiO3 layer in SrTiO3.
Figure 3: Spatial distribution of the Ti3+ signal in the vicinity of the LaTiO3 layer and bilayer.
Figure 4: Summary of the electronic properties in SrTiO3/LaTiO3 superlattices as a function of the number of LaTiO3 unit cells.

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Acknowledgements

We thank G. E. Blumberg, R. de Picciotto, B. I. Halperin, D. R. Hamann, S. H. Simon, C. M. Varma and N. Zhitenev for discussions. A.O. acknowledges partial support by the Nishina Memorial Foundation, Japan.

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Correspondence to H. Y. Hwang.

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Ohtomo, A., Muller, D., Grazul, J. et al. Artificial charge-modulationin atomic-scale perovskite titanate superlattices. Nature 419, 378–380 (2002) doi:10.1038/nature00977

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