Molecular Dipole Engineering in Layered Perovskite Ferroelectrics Enables Large Rashba-Dresselhaus Spin Splitting and...

Rashba-Dresselhaus (RD) spin splitting provides a crucial physical basis for realizing various advanced spintronic applications. Nevertheless, rationally regulating the RD splitting coefficient (α_RD) continues to be a difficult task owing to the still unclear structure-property relationship. In this work, we propose a molecular dipole engineering, the core of which involves introducing halogen atoms with different electronegativities into organic cations, aiming to design and synthesize two-dimensional (2D) ferroelectric semiconductors with strong RD spin splitting. By substituting the organic spacer cations in the parent compound (PMA)₂PbCl₄ (PMA = benzylammonium), we obtained two new 2D ferroelectric semiconductors, namely, (2F4ClPMA)₂PbCl₄ (2F4ClPMA = 2-fluoro-4-chlorobenzylammonium) and (2F4BrPMA)₂PbCl₄ (2F4BrPMA = 2-fluoro-4-bromobenzylammonium). Both variants exhibit significantly enhanced RD splitting coefficients. In particular, (2F4ClPMA)₂PbCl₄ possesses a large α_RD value of 1.878 eV·Å and a persistent spin texture region, which helps amplify its circular photogalvanic effect (CPGE). Through the spin-selective optical transition rule, the effect can enable the differentiation of carriers excited by circularly polarized light (CPL) in momentum space. A photodetector fabricated based on a (2F4ClPMA)₂PbCl₄ single crystal shows a high asymmetry factor of 0.53 under excitation by UV CPL at 325 nm. This work not only confirms the effectiveness of the molecular dipole engineering in tuning RD spin splitting, but also lays an important material foundation for the development of novel low-power, high-sensitivity spin-optoelectronic devices.