  Charge transfer in superlattices consisting of SrIrO$_3$ and SrMnO$_3$ is
investigated using density functional theory. Despite the nearly identical work
function and non-polar interfaces between SrIrO$_3$ and SrMnO$_3$, rather large
charge transfer was experimentally reported at the interface between them.
Here, we report a microscopic model that captures the mechanism behind this
phenomenon, providing a qualitative understanding of the experimental
observation. This leads to unique strain dependence of such charge transfer in
iridate-manganite superlattices. The predicted behavior is consistently
verified by experiment with soft x-ray and optical spectroscopy. Our work thus
demonstrates a new route to control electronic states in non-polar oxide
heterostructures.
