Generic ferroelectric ground state in underdoped La-214 cuprates
QCN researchers have performed extensive measurements of the electric polarization on Sr and Li doped La2CuO4 single crystals. It is shown that La1.999Sr0.001CuO4 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 La2LixCu1-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 La1.999Sr0.001CuO4 and La2CuO4+x The electric polarization in La2LixCu1-xO4 is also tunable by an applied magnetic field, similarly to La1.999Sr0.001CuO4 and La2CuO4+x.
Combined with previous electric polarization studies in oxygen doped La2CuO4+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.