Doping without Disorder: Digital Synthesis of Manganite Superlattices*

We have synthesized digital superlattices of (LaMnO$_{3})_{m}$(SrMnO$_{3})_{n }$ using ozone-assisted oxide MBE. This technique of \textit{digital synthesis} allows the realization of superlattices with the same overall stoichiometry as random alloys of composition La $_{m/m+n}$ Sr $_{n/m+n}$MnO$_{3}$, without introducing the random $A$-site disorder associated with bulk synthesis. In these digital superlattices, the `doping' or charge transfer occurs at well ordered coherent interfaces, whose electronic and magnetic properties may be studied by local probes, scattering techniques, and transport and magnetization studies. We shall present our results on a series of samples synthesized for various values of $m$ and $n$, exploring both metallic and insulating phases of the nominal phase diagram for the bulk counterpart. The results will be discussed in the context of the interfacial states that arise in structures obtained with digital synthesis.