Concurrence of ferroelectricity and metallicity in perovskite ultrathin films

Ferroelectricity and metallicity are generally considered mutually exclusive phenomena, as free electrons in metals prevent the occurrence of Coulomb interactions necessary for stabilizing long-range ferroelectric order. Here, we realized the coexistence of ferroelectricity and metallicity in epitaxial (001)-oriented NdNiO3 ultrathin films within an optimal thickness range of 10~16 unit-cell. These films exhibited Fermi liquid-like behavior at room temperature, confirming their metallic nature. Their ferroelectricity is supported by a spontaneous polarization of 27 μC cm-2 and a Curie temperature of approximately 150°C, as determined via electrical and optical measurements. On the basis of first-principles calculations and experimental evidence, the ferroelectricity of NdNiO3 ultrathin films was attributed to switchable charge ordering states formed by Ni3+δ and Ni3-δ ions with particularly low energy barriers, thereby facilitating the coexistence of ferroelectric and metallic behaviors. These findings establish NdNiO3 ultrathin films as promising platforms for metallic ferroelectrics, with potential applications in next-generation quantum electronic devices.