Inverse design of co-functionaity: Discovery of Rashba materials that are also topological insulators and Rashba materials that are also ferroelectrics

We demonstrate design of two types of cross fuctionalities involving co existence of the Rashba effect with “effect X”=either topology or ferroelectricity. (a) Rashba topological insulators [Matter 3, 145 (2020)]: strong Rashba effect -- as measured by the Rashba coefficient α_R -- is required for various applications but the physical mechanism defining the Rashba scale, i.e., the magnitude of α_R, is unknown. We predict an intrinsic separation between strong Rashba effect associated with band anti-crossing (BAC), and weak Rashba effect with no BAC. We then design 199 novel fabricated Rashba materials. Since topological insulators (TIs) intrinsically have BACs, all non-centrosymmetric TIs have large Rashba coefficient, which establishes the cross-functionality of topological Rashba insulators illustrated here for 3 cases. We find 12 such compounds. (b) Rashba Ferroelectric [Phys. Rev. B 102, 144106 (2020)]: we demonstrate the inverse design of ferroelectric Rashba materials, in which the different electric polarization directions provided by ferroelectricity have also opposite spin-polarization. These compounds thus allow the electrical reversion of spin-polarization. We find 52 ferroelectric Rashba compounds.