Artificial Charge-Modulation in Atomic-Scale Perovskite Titanate Superlattices

The nature and length scales of charge screening in complex oxides is fundamental to a wide range of systems, spanning ceramic varistors, oxide tunnel junctions, and charge-ordering in mixed valence compounds. There are wide variations in the degree of charge disproportionation, length scale, and orientation in these latter configurations, but little is known about their microscopic electronic structure, a matter of intense theoretical scrutiny. Here we have fabricated an idealized structure to examine these issues by growing atomically abrupt layers of LaTi 3+ O3 embedded in SrTi 4+ 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 arising from superlattices of two constituent insulators. Despite the chemical abruptness of the interfaces, we find that a minimum thickness of five Ti 3+ layers is required for the center titanium site to recover bulk-like electronic properties. This represents a new framework within which the short-length-scale electronic response can be probed and incorporated in thin film oxide heterostructures.