Tunable metal-insulator transition, Rashba effect and Weyl Fermions in a relativistic charge-ordered ferroelectric oxide

Controllable metal–insulator transitions (MIT), Rashba–Dresselhaus (RD) spin splitting, and Weyl semimetals are promising schemes for realizing processing devices. Complex oxides are a desirable materials platform for such devices, as they host delicate and tunable charge, spin, orbital, and lattice degrees of freedoms. Here, using first-principles calculations and symmetry analysis, we identify an electric-field tunable MIT, RD effect, and Weyl semimetal in a known, charge-ordered, and polar relativistic oxide Ag 2 BiO 3 at room temperature. Remarkably, a centrosymmetric BiO 6 octahedral-breathing distortion induces a sizable spontaneous ferroelectric polarization through Bi 3+ /Bi 5+ charge disproportionation, which stabilizes simultaneously the insulating phase. The continuous attenuation of the Bi 3+ /Bi 5+ disproportionation obtained by applying an external electric field reduces the band gap and RD spin splitting and drives the phase transition from a ferroelectric RD insulator to a paraelectric Dirac semimetal, through a topological Weyl semimetal intermediate state. These findings suggest that Ag 2 BiO 3 is a promising material for spin-orbitonic applications.