Strain-engineering stabilization of $\mathrm{BaTi}{\mathrm{O}}_{3}$-based polar metals

Strain-engineering stabilization of $BaTi O_{3}$ -based polar metals

Chao Ma, Kui-juan Jin, Chen Ge, and Guo-zhen Yang

Phys. Rev. B 97, 115103 – Published 1 March 2018

Abstract

Polar metals, which possess ferroelectriclike polar structure and conductivity simultaneously, have attracted wide interest since the first solid example, $LiOs O_{3}$ (below 140 K), was discovered. However, the lack of room-temperature polar metals hinders further research and applications. Thus abundant properties of polar metals are unexplored. Here, with first-principles calculations, we report that the polar metal phase can be stabilized in the strain-engineered $BaTi O_{3}$ with electron doping. The mechanism relates to the competition between the shifting of the $t_{2 g}$ energy levels and the narrowing of their bandwidth. Surprisingly, it is predicted that the ferroelectric-to-paraelectric transition temperature can be increased by electron doping when the strain is large enough, which holds potential for room-temperature polar metals. Our results indicate that strain engineering is a promising way to achieve $BaTi O_{3}$ -based polar metals, and they should have practical significance for obtaining easily accessible, ecofriendly, and potential room-temperature polar metals.

Received 27 November 2017
Revised 5 January 2018

DOI:https://doi.org/10.1103/PhysRevB.97.115103

Physics Subject Headings (PhySH)

Ferroelectricity Structural properties

Ferroelectrics Noncentrosymmetric materials Oxides Perovskites

Density functional theory

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Chao Ma^1,2, Kui-juan Jin^1,2,3,*, Chen Ge¹, and Guo-zhen Yang^1,2,3

¹Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
²School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China
³Collaborative Innovation Center of Quantum Matter, Beijing 100190, China

^*kjjin@iphy.ac.cn

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Vol. 97, Iss. 11 — 15 March 2018

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Figure 1
The diagram for the routine to obtain polar metals.
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Figure 2
The calculated $BaTi O_{3}$ crystal unit cell (a), where $B_{s}$ and $B_{l}$ represent the short and long Ti-O1 bond lengths, respectively. The heat map diagrams of $P^{'}$ (b) and $d E$ (c), respectively, as functions of the dope concentration $n_{e}$ and the in-plane lattice constant $a$ , where $d E = {E_{P}}_{4 / m m m} - {E_{P}}_{4 m m}$ . Green line in (b) represents the critical concentration $n_{C}$ , beyond which $P^{'} = 0$ . Orange shadow in (b),(c) represents the region for obtaining the polar metal phase with nonzero $n_{e}$ and $P^{'}$ .
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Figure 3
(a) $a$ , $c$ , and unit cell volume versus $n_{e}$ . (b) The ratio $c$ / $a$ and cation-oxygen relative displacements versus $n_{e}$ . In (a),(b), the squares are calculated results. The dashed lines represent the boundaries of two adjacent regions, in which the green one also represents the critical concentration $n_{C}$ . Orbital resolved DOS of three $t_{2 g}$ orbitals with $n_{e}$ equals 0.03 $e$ /u.c. (c), 0.1 $e$ /u.c. (d), and 0.17 $e$ /u.c. (e), corresponding to the regions I–III in (a),(b), respectively. In (c)–(e), the zero point of energy is placed at the Fermi level ( $E_{F}$ ). The yellow areas with yellow arrows represent the doping electrons. The isosurface demonstration of charge density distributions of the doping electrons is drawn with the same scale in the inserted pictures. Note that the DOS of $d_{y z}$ and $d_{x z}$ orbitals always overlaps because these orbitals are degenerate due to the crystal symmetry.
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Figure 4
(a) Diagram of the $t_{2 g}$ levels shifting and the consequent DOS shifting without narrowing of bandwidth with strain. The calculated orbital resolved DOS of three $t_{2 g}$ orbitals with $a$ equals 3.93 Å (b), 3.9 Å (c), and 3.8 Å (d), respectively, without doping. $E_{Δ}$ denotes the energy difference of the band bottoms for $d_{y z} / d_{x z}$ with respect to that for $d_{x y}$ . In (b)–(d), the zero point of energy is placed at the CBM. The yellow areas with yellow arrows schematically represent the doping range corresponding to the weak-coupling region. The inserted pictures show the corresponding unit cell structures. The $d_{y z}$ and $d_{x z}$ orbitals are degenerate, so their DOS always overlaps.
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Figure 5
The calculated orbital resolved DOS of three $t_{2 g}$ levels with in-plane lattice constant $a = 3.88 Å$ but various doping (a)–(c) and with $n_{e} = 0.3 e / u . c$ . but various $a$ (d)–(f). The zero point of energy is placed at the Fermi level ( $E_{F}$ ). The $d_{y z}$ and $d_{x z}$ orbitals are degenerate, so their DOS always overlaps.
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Physical Review B

covering condensed matter and materials physics

Strain-engineering stabilization of $BaTi O_{3}$ -based polar metals

Chao Ma, Kui-juan Jin, Chen Ge, and Guo-zhen Yang

Phys. Rev. B 97, 115103 – Published 1 March 2018

Abstract

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Physical Review B

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Strain-engineering stabilization of BaTiO3-based polar metals

Chao Ma, Kui-juan Jin, Chen Ge, and Guo-zhen Yang

Phys. Rev. B 97, 115103 – Published 1 March 2018

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Strain-engineering stabilization of $BaTi O_{3}$ -based polar metals