  Ferroelectric Rashba semiconductors (FERSC) are a novel class of
multifunctional materials showing a giant Rashba spin splitting which can be
reversed by switching the electric polarization. Although they are excellent
candidates as channels in spin field effect transistors, the experimental
research has been limited so far to semiconducting GeTe, in which ferroelectric
switching is often prevented by heavy doping and/or large leakage currents.
Here, we report that CsBiNb2O7, a layered perovskite of Dion-Jacobson type, is
a robust ferroelectric with sufficiently strong spin-orbit coupling and spin
texture reversible by electric field. Moreover, we reveal that its topmost
valence band's spin texture is quasi-independent from the momentum, as a result
of the low symmetry of its ferroelectric phase. The peculiar spin polarization
pattern in the momentum space may yield the so-called "persistent spin helix",
a specific spin-wave mode which protects the spin from decoherence in diffusive
transport regime, potentially ensuring a very long spin lifetime in this
material.
