J_eff=1/2 Mott insulating state modification by polarization field effect doping of Sr₂IrO₄ ultrathin films

The 4d and 5d transition metals are commonly characterized by a decreased Hubbard repulsion U which diminishes correlation effects, but simultaneously by an increased spin orbit coupling to create a new type of correlation effects which have been leading to such as spin-orbit coupled Mott insulators, Weyl semimetals, axion insulators and spin liquids. This rich physics allows small perturbations to create large effects in these strongly correlated materials. The Ruddlesden-Popper series of Sr_x+1Ir_xO_3x+1 shows large differences in conductive behavior, where the n=∞ perovskite SrIrO₃ is metallic while the n=1 Sr₂IrO₄ is an insulator due to a spin-orbit coupling band splitting to a J_eff=1/2 state. This state has many similarities to the high T_C cuprate superconductors which show an S=1/2 state, which loses its antiferromagnetism and becomes superconducting upon hole doping. Likewise, under electron doping it is possible to drive Sr₂IrO₄ to a metallic state. Here the suppression of the octahedral rotations will remove the ferromagnetic moment due to canted antiferromagnetism is studied by growing a high-quality bilayer of single-phase Sr₂IrO₄ ultrafilms with the ferroelectric material PbTiO₃ as a novel way to provide electron doping by polarization field-effect doping.