Charge disproportionation driven polar magnetic metallic double-layered perovskite Sr$_3$Co$_2$O$_7$

Abstract

Strong coupling among spontaneous structural symmetric breaking, magnetism and metallicity in an intrinsic polar magnetic metal can give rise to novel physical phenomena and holds great promise for applications in spintronics. Here, we elucidate the mechanism of metallic ferroelectricity in the recently discovered polar metal Sr$_3$Co$_2$O$_7$. Our first-principles calculations reveal that both the spontaneous ferroelectric displacements and the metallicity originate from charge disproportionation of Co ions. This is characterized by an inverted ligand-field splitting of the Co $t_2g$ orbitals at one site, while the metallic behavior is preserved by the t$_2g$ orbitals at both sites. The charge disproportionation stabilizes the asymmetric phase Within the framework of the on-site Hubbard U interaction. We thus propose that in related transition metal oxides, charge disproportionation within specific orbitals can concurrently drive metallicity and ferroelectricity, enabling strong coupling between these properties. More remarkably, this mechanism allows for the coexistence of magnetism, as evidenced in Sr$_3$Co$_2$O$_7$. Our findings highlight a promising avenue for realizing polar magnetic metals and provide a new design principle for exploring multifunctional materials.

Figures (4)

• Figure 1: (a) a $1\times1\times1$ tetragonal unit cell of Sr$_3$Co$_2$O$_7$. Co1/Co2 correspond to the two Co sites in one double-layered structure respectively, and Co1' and Co2' are the corresponding Co sites in the other double-layered structure. In (a), Co1/Co2, (b) The inter-atomic distances along the $c$-axis for the symmetric (SYM) and asymmetric ferroelectric (FE) phases respectively in units of Å. Oz, O1z/O2z, and O1xy/O2xy correspond to the apical oxygen connecting Co1 and Co2, the apical oxygen atoms connecting Co1/Co2 respectively, and equatorial oxygen atoms in the CoO$_2$ layers centered by Co1/Co2, respectively. Red numbers indicate the displacements between Co1/Co2 and O1xy/O2xy within the same CoO$_2$ layer. The occupied $d_{xy}$ and $d_{xz/yz}$ orbitals, showing the charge disproportionation in FE phase are indicated.
• Figure 2: The projected density of states (PDOS) of Co($3d$) orbitals, including $t_{2g}$ ($d_{xz}, d_{yz}$ and $d_{xy}$) and $e_g$ ($d_{z^2}$ and $d_{x^2-y^2}$) orbitals for the (a) SYM+FM phase (b) FE+FM phase. The Fermi energy is set to zero. The energy levels and electron filling of $t_{2g}$ orbitals for Co1 and Co2 in both cases is shown schematically.
• Figure 3: The on-site energy levels (lines) and electron occupancies (numbers) of the atomic orbitals associated with polarization [Co1($t_{2g}$), Oz($p_{\pi}$), Co2($t_{2g}$)] for the (a) FE+FM phase (b) SYM+FM phase (c) FE+A-AF phase. Red and blue signifies the spin down and spin up channel, respectively. $\uparrow$ and $\downarrow$ refers to spin majority and minority relative to Co's local moment. Double-arrows indicate the virtual exchange pathways via $pd\pi$ bonding. Dashed lines in (c) indicate the exchange pathways with all orbitals fully occupied.
• Figure 4: The relative total energies (per Co) and the corresponding net magnetization of structures under different locations of the oxygen vacancies labeled in Fig.\ref{['fig:structure']}. Different colors represent vacancies at different locations. Hollow circles, solid circles and hollow diamonds denote non-polar (NP), FE and anti-ferroelectric (AFE) phase respectively.