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.