Abstract
Ferroelectric polarization and metallic conduction are two seemingly irreconcilable properties that cannot normally coexist in a single system, as the latter tends to screen the former. Polar metals, however, defy this rule and have thus attracted considerable attention as a new class of ferroelectrics exhibiting novel properties. Here, we fabricate a new polar metal film based on the typical ferroelectric material BaTiO3by combining chemical doping and epitaxial strain induced by a substrate. The temperature dependences of the c-axis lattice constant and the second harmonic generation intensity of La-doped BaTiO3films indicate the existence of polar transitions. In addition, through La doping, films become metallic at the polar phase, and metallicity enhancement at the polar state occurs in low-La-doped films. This intriguing behaviour is effectively explained by our first-principles calculations. Our demonstration suggests that the carrier doping to ferroelectric material with epitaxial strain serves as a new way to explore polar metals.
Highlights
The coexistence of two seemingly incompatible states in materials such as multiferroics[1, 2] and magnetic superconductors[3, 4] has attracted considerable interest
The dotted curve denotes the extrapolated lattice constant of BTO single crystals in the cubic phase. (b) Temperature dependence of the optical second harmonic generation (SHG) intensity of the films. (c) La doping ([n]/Ti) dependence of in-plane strain ε. (d) Non-polar to polar transition temperature TC as a function of the in-plane strain ε of non-doped and La-doped BTO films on GSO, of non-doped BTO films on GSO and of DSO26 with a theoretical line based on the phase-field simulation[32]
The temperature dependences of the c-axis lattice constant and of SHG intensity indicate that the polar transition temperature of La-doped BTO (La-BTO) decreases as La doping levels increase
Summary
The coexistence of two seemingly incompatible states in materials such as multiferroics (showing ferromagnetic and ferroelectric order)[1, 2] and magnetic superconductors[3, 4] has attracted considerable interest. Anderson and Blount proposed in the 1960s that a continuous structural phase transition is possible, even in a polar metal[5], it is rare to see such a transition[6,7,8] In addition to such contradictions between polar and metallic states, it is interesting to note the non-centrosymmetric structure itself in solids. Electron-doped BaTiO3-δ bulk single crystals have been studied to understand the relationship between the crystal structure and transport properties. These studies have revealed the coexistence of a ferroelectric-like lattice distortion and metallic phase[15,16,17]. We found that a characteristic change in the band structure occurs during phase transition under a low doping regime via first-principles calculations, resulting in the enhancement of metallicity
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