Engineering an Insulating Ferroelectric Superlattice with a Tunable Band Gap from Metallic Components

The recent discovery of “polar metals” with ferroelectriclike displacements offers the promise of designing ferroelectrics with tunable energy gaps by inducing controlled metal-insulator transitions. Here we employ first-principles calculations to design a metallic polar superlattice from nonpolar metal components and show that controlled intermixing can lead to a true insulating ferroelectric with a tunable band gap. We consider a $2/2$ superlattice made of two centrosymmetric metallic oxides, ${\mathrm{La}}_{0.75}{\mathrm{Sr}}_{0.25}{\mathrm{MnO}}_3$ and ${\mathrm{LaNiO}}_3$, and show that ferroelectriclike displacements are induced. The ferroelectriclike distortion is found to be strongly dependent on the carrier concentration (Sr content). Further, we show that a metal-to-insulator (MI) transition is feasible in this system via disproportionation of the Ni sites. Such a disproportionation and, hence, a MI transition can be driven by intermixing of transition metal ions between Mn and Ni layers. As a result, the energy gap of the resulting ferroelectric can be tuned by varying the degree of intermixing in the experimental fabrication method.

Figure 1

Structural and electronic properties of $2/2$ SL. (a) and (b) High symmetry $P4/mmm$ and lowest energy $Pm$ structures, respectively. Green, orange, red, blue, and black balls represents La, Sr, Mn, Ni, and O atoms, respectively. The magnetic configuration AFA implies that Mn and Ni sublattices are aligned $A$-type AFM, while at the interface the Mn-O-Ni coupling is ferromagnetic. (c) Relative energetics of dominant distortions with respect to $a^{-}{a}^{-}{c}^0$ ($Q_T$) distortion. Here, in and out represent the in-phase and out of phase sense of rotation of ${\mathrm{BO}}_6$ octahedra at the interface (Supplemental Material II [28]). The polar space groups are shown in green. (d) Density of states calculated for $U_{\text{Mn}}=0.5$, $U_{\text{Ni}}=6.0$, and $J=1.0\mathrm{eV}$.

Figure 2

(a) Schematic depiction of the ferroelectriclike mode and (b) carrier concentration (Sr content) dependency of the FEL mode distortion. Green, orange, red, blue, and black spheres represent La, Sr, Mn, Ni, and O atoms, respectively.

Figure 3

(a) Engineering a metal-insulator transition in a superlattice of two metallic oxides. (a) Schematic representation of the problem. (b) Column intermixing configuration. (c) Two nonequivalent ${\mathrm{NiO}}_6$ octahedra arising from charge disproportionation. At the ${\mathrm{Ni}}_1$ sites, the ${\mathrm{NiO}}_6$ octahedron is slightly contracted (as indicated by white arrows), while at the ${\mathrm{Ni}}_2$ sites, the octahedron is slightly elongated in the crystallographic $ab$ plane. White arrows highlight the compressed ${\mathrm{NiO}}_6$ octahedra. (c) DOS for intermixed structure computed for $U_{\text{Mn}}=0.5$, $U_{\text{Ni}}=6.0$, and $J=1.0\mathrm{eV}$. Here, green, orange, red, blue, and black spheres represent La, Sr, Mn, Ni, and O atoms, respectively.