The conducting meat in the insulating sandwich

In 2004, it was discovered that when a layer of LaAlO3 (LAO) is in contact with a layer of SrTiO3 (STO), an ultrathin layer of highly conducting material is formed where they contact one another, despite the fact that both LAO and STO are insulators. The underlying physics responsible for this phenomenon is still much disputed, despite a worldwide concerted research drive since then to explain it. Using x-rays, the atomic structure of the interface between LAO and STO has been revealed. For the first time, the exact positions and chemical compositions of each atomic layer were defined. Using simple arguments regarding electrostatic energy minimization and the known sizes of the contributing ions, it was shown that the conducting layer consists of about three monolayers of a graded mixture of STO and LAO, which is predicted to be conducting. Thus this fascinating and potentially technologically important phenomenon could be explained based on structural arguments alone.

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