  A 2D electron gas system in an oxide heterostructure serves as an important
playground for novel phenomena. Here, we show that, by using fractional
delta-doping to control the interface's composition in LaxSr1-xTiO3/SrTiO3
artificial oxide superlattices, the filling-controlled 2D insulator-metal
transition can be realized. The atomic-scale control of d-electron band
filling, which in turn contributes to the tuning of effective mass and density
of the charge carriers, is found to be a fascinating route to substantially
enhanced carrier mobilities.
