QCN researchers have performed extensive measurements of the electric polarization on Sr and Li doped La₂CuO₄ single crystals. It is shown that La_1.999Sr_0.001CuO₄ exhibits distinct ferroelectric behavior along different crystallographic directions. The magnetic field dependence of the electric polarization is anisotropic and it can be tuned through the Dzyaloshiskii – Moriya interaction.
In La₂Li_xCu_1-xO₄ (x=0.01 and x=0.04), the measured electric polarization is in the order of μC cm^-2 i.e., several times higher than for La_1.999Sr_0.001CuO₄ and La₂CuO_4+x The electric polarization in La₂Li_xCu_1-xO₄ is also tunable by an applied magnetic field, similarly to La_1.999Sr_0.001CuO₄ and La₂CuO_4+x.
Combined with previous electric polarization studies in oxygen doped La₂CuO_4+x single crystals (Z. Viskadourakis et al., Phys. Rev. B 85, 214502 (2012)), these results demonstrate that the ferroelectric phase is generic to underdoped La-214 cuprates and is prompted by charge carrier doping.
It is proposed that ferroelectricity originates from a mechanism that breaks inversion symmetry by local structural distortions of the CuO₆ octahedra due to the presence of the dopant ions and/or clustering of the added holes. Such local symmetry breaking induces electric dipoles which couple and order at low temperature.