Li Gang, Huang Huiyu, Peng Shaoqin, Xiong Ying, Xiao Yongguang, Yan Shaoan, Cao Yanwei, Tang Minghua, Li Zheng
School of Materials Science and Engineering, Xiangtan University Xiangtan Hunan 411105 China
School of Mathematics and Computational Science, Xiangtan University Xiangtan 411105 China.
RSC Adv. 2019 Nov 1;9(61):35499-35508. doi: 10.1039/c9ra06209b. eCollection 2019 Oct 31.
Polar metals, commonly defined by the coexistence of polar structure and metallicity, are thought to be scarce because free carriers eliminate internal dipoles that may arise owing to asymmetric charge distributions. By using first-principle electronic structure calculations, we explored the possibility of producing metallic states in the polar/nonpolar KNbO/BaTiO superlattice (SL) composed of two prototypical ferroelectric materials: BaTiO (BTO) and KNbO (KNO). Two types of polar/nonpolar interfaces, p-type (KO)/(TiO) and n-type (NbO)/(BaO), which can be constituted into two symmetric NbO/BaO-NbO/BaO (NN-type) and KO/TiO-KO/TiO (PP-type) SL, as well as one asymmetric KO/TiO-NbO/BaO (PN-type) SL. The spatial distribution of ferroelectric distortions and their conductive properties are found to be extraordinarily sensitive to the interfacial configurations. An insulator-to-metal transition is found in each unit cell of the symmetric interfacial SL models: one exhibiting quasi-two-dimensional n-type conductivity for NN-type SL, while the other being quasi-two-dimensional p-type conductivity for PP-type SL. The anisotropic coexistence of in-plane orientation of free carriers and out-of-plane orientation of ferroelectric polarization in KNO/BTO SL indicates that in-plane free carriers can not eliminate the out-of-plane dipoles. Our results provide a road map to create two-dimensional polar metals in insulating perovskite oxide SL, which is expected to promote applications of new quantum devices.
极性金属通常由极性结构和金属性的共存来定义，被认为是稀缺的，因为自由载流子会消除由于不对称电荷分布而可能产生的内部偶极子。通过使用第一性原理电子结构计算，我们探索了在由两种典型铁电材料：钛酸钡（BTO）和铌酸钾（KNO）组成的极性/非极性KNbO/BaTiO超晶格（SL）中产生金属态的可能性。两种类型的极性/非极性界面，p型（KO）/（TiO）和n型（NbO）/（BaO），可以构成两种对称的NbO/BaO-NbO/BaO（NN型）和KO/TiO-KO/TiO（PP型）超晶格，以及一种不对称的KO/TiO-NbO/BaO（PN型）超晶格。发现铁电畸变的空间分布及其导电性质对界面构型异常敏感。在对称界面超晶格模型的每个晶胞中都发现了绝缘体到金属的转变：一种在NN型超晶格中表现出准二维n型导电性，而另一种在PP型超晶格中表现为准二维p型导电性。KNO/BTO超晶格中自由载流子的面内取向和铁电极化的面外取向的各向异性共存表明，面内自由载流子不能消除面外偶极子。我们的结果为在绝缘钙钛矿氧化物超晶格中创建二维极性金属提供了路线图，有望促进新型量子器件的应用。
