Can free charge carriers induce structural polarization?

Free charge carriers are often detrimental to ferroelectricity due to the screening of long range coulomb interactions. Thus the design of a metal that undergoes a ferroelectric-like phase transition (from a centrosymmetric to a structurally phase) is challenging. Although recent studies has shown the mutual existence of ferroelectricity and metallicity in several systems, the carrier-enhanced/induced ferroelectricity is still extremely rare. In this talk, we show that in a family of layered stannate oxides (hybrid-improper ferroelectric Ruddlesden-Popper phases) the polarization can actually be strengthened and even induced through the introduction of free carriers via electrostatic gating. We also investigate the microscopic mechanism and design criteria for this kind of gating-induced `metallic-ferroelectricity' from the point of view of first-principles calculations. The possibility of expanding this phenomenon to other compounds are also discussed.