The ability to grow atomically flat oxide thin films by laser molecular beam epitaxy allows us to study the transport behavior of carriers accumulated in two-dimensional layers at delta-doped interfaces. Left figure shows examples of such heterostructures where a single or double layer of LaTiO₃ was grown on a SrTiO₃ and capped with another SrTiO₃ layer. Each La atom donates an extra electron, some of which will dope the adjacent SrTiO₃ layers by charge transfer. We find that multiple electron populations form in this structure, with some electrons localized at the interface, some forming a metallic quantum well, and some spreading deep into the substrate. The relative sizes of these different populations can be tuned by adjusting the total La number in the doping layer, by utilizing surface depletion, or using electrostatic gating. We can observe distinct changes in the magnetotransport behavior when the relative electron population sizes change, as illustrated in right figure, which shows the variation of in-plane magnetoresistance when the carrier number is changed.

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