Stereochemically Active Lone Pair Effect of Cations Triggers Giant Rashba–Dresselhaus Spin Splitting in Ferroelectric Semiconductors for Circularly Polarized Light Detection

Rashba-Dresselhaus (RD) spin splitting arising from spin-orbit coupling in spatial inversion asymmetric semiconductors is critical for realizing numerous advanced spintronic applications. However, modulating the RD spin splitting coefficient (α_RD) is challenging due to the unclear structural-property relationship. Herein, inspired by highly distorted inorganic selenites containing stereochemically active lone pair (SCALP) electrons, we selected 4-aminomorpholine cation (4AM) containing SCALP electrons to synthesize a new two-dimensional (2D) ferroelectric semiconductor (4AM)₂PbBr₄, exhibiting a giant α_RD of 2.377 eV Å with ΔE_RD = 126 meV and Δk₀ = 0.106 Å^-1 that differentiates circularly polarized light-excited carriers in the momentum space via spin-dependent optical transition selection rules with an asymmetric factor of 0.65. A geometric structure-property relationship study reveals that incorporating SCALP into the organic cations enhances their net polarity and induces substantial distortions within the inorganic sublattices that enhance the α_RD. This work reveals the regulatory mechanism of organic cations on distortion of inorganic octahedron and provides a new approach for efficient synthesis and rapid screening of 2D ferroelectric semiconductors with large α_RD.