  Motivated by the discovery of superconductivity in KTaO3-based
heterostructures, we study a pairing mechanism based on spin-orbit assisted
coupling between the conduction electrons and the ferroelectric modes present
in the material. We use ab initio frozen-phonon computations to show a
linear-in-momentum Rashba-like coupling with a strong angular dependence in
momentum for the lower j=3/2 manifold, deviating from the conventional
isotropic Rashba model. This implies the Rashba-like interaction with the polar
modes has substantial L=3 cubic harmonic corrections, which we quantify for
each electronic band. The strong anisotropy of the Rashba interaction is
captured by a microscopic toy model for the t2g electrons. We find its origin
to be the angular dependence in electronic momentum imposed by the kinetic term
on the degenerate j=3/2 manifold. A comparison between the toy model and ab
initio results indicate that additional symmetry allowed terms beyond
odd-parity spin-conserving inter-orbital hopping processes are needed to
describe the Rashba-like polar interaction between the electrons and the soft
ferroelectric mode.
