  Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are
introducing new directions in the 2D materials landscape. The coexistence of
ferroelectricity and spin-orbit interactions play a key role in their
optoelectronic properties. We perform a detailed study on a recently
synthesized ferroelectric 2D-HOIP, (AMP)PbI$_4$ (AMP =
4-aminomethyl-piperidinium). The calculated polarization and Rashba parameter
are in excellent agreement with experimental values. We report a striking new
effect, i.e., an extraordinarily large Rashba anisotropy that is tunable by
ferroelectric polarization: as polarization is reversed, not only the spin
texture chirality is inverted, but also the major and minor axes of the Rashba
anisotropy ellipse in k-space are interchanged - a pseudo rotation. A $k \cdot
p$ model Hamiltonian and symmetry-mode analysis reveal a quadrilinear coupling
between the cation-rotation modes responsible for the Rashba ellipse
pseudo-rotation, the framework rotation, and the polarization. These findings
may provide new avenues for spin-optoelectronic devices such as spin valves or
spin FETs.
