  We propose a physical mechanism to generate and selectively amplify
anisotropic Rudermann-Kittel-Kasuya-Yosida (RKKY) interactions between two
local spins. The idea is to combine the deflection of the carrier velocity by a
P-N interface and the locking of this velocity to the carrier spin orientation
via spin-orbit coupling. We provide analytical and numerical results to
demonstrate this mechanism on the surface of a topological insulator P-N
junction. This work identifies the P-N interface as a second knob which,
together with the carrier density, enables independent control of the strength
and anisotropy of the RKKY interaction over a wide range. These findings may be
relevant to scalable quantum computation and two-impurity quantum criticality.
