In the version of this Letter originally published, in two instances in Fig. 1 the layers in the cross-sectional view of the (001) interface were incorrectly labelled: in Fig. 1b SrO+ should have read SrO0; in Fig. 1c LaO+, AlO2-, LaO+, TiO20, SrO+, TiO20 should have read LaO33-, Al3+, LaO33-, Ti4+, SrO34-, Ti4+. In Fig. 3c the upper-right equation read -σs = -e/2a2 but should have read -σs = e/2a2 and in Fig. 3f the lower-right equation read -σs = -e/2√3a2 but should have read σs = -e/2√3a2. These errors have now been corrected in the online version of the Letter.