Charge-Order-Induced Ferroelectricity in ${\mathrm{LaVO}}_3/{\mathrm{Sr}\mathrm{VO}}_3$ Superlattices

The structure and properties of the $1:1$ superlattice of ${\mathrm{LaVO}}_3$ and ${\mathrm{SrVO}}_3$ are investigated with a first-principles density-functional-theory-plus-$U$ ($\mathrm{DFT}+U$) method. The lowest energy states are antiferromagnetic charge-ordered Mott-insulating phases. In one of these insulating phases, layered charge ordering combines with the layered La/Sr cation ordering to produce a polar structure with a large nonzero spontaneous polarization normal to the interfaces. This polarization, comparable to that of conventional ferroelectrics, is produced by electron transfer between the ${\mathrm{V}}^{3+}$ and ${\mathrm{V}}^{4+}$ layers. The energy of this normal-polarization state relative to the ground state is only 3 meV per vanadium. Under tensile strain, this energy difference can be further reduced, suggesting that the normal-polarization state can be induced by an electric field applied normal to the superlattice layers, yielding an antiferroelectric double-hysteresis loop. If the system does not switch back to the ground state on removal of the field, a ferroelectric-type hysteresis loop could be observed.

Figure 1

Schematics of the charge-order patterns for ${({\mathrm{LaVO}}_3)}_1/{({\mathrm{SrVO}}_3)}_1$ superlattice. For descriptive purposes the octahedral rotations and tilts are omitted and the Jahn-Teller distortions are exaggerated. The left, middle, right panels denote the checkerboard-type charge order with the $(\pi ,\pi ,\pi )$ ordering vector, the stripe charge order with the $(0,\pi ,\pi )$ ordering vector, and the layered charge order with the $(0,0,\pi )$ ordering vector, respectively.

Figure 2

(a) Atomic arrangement in the ${({\mathrm{LaVO}}_3)}_1/{({\mathrm{SrVO}}_3)}_1$ superlattice. (b) The Jahn-Teller distortions and $t_{2g}$ level splittings for ${\mathrm{V}}^{4+}$ (left) and ${\mathrm{V}}^{3+}$ (right).

Figure 3

Distance between V and neighboring O atoms of ${\mathrm{V}}^{4+}$ and ${\mathrm{V}}^{3+}$ cations compared for CCO, SCO, and LCO phases. The labels $x_{\pm }$, $y_{\pm }$, and $z_{\pm }$ designate oxygen atoms by their octahedron positions in the high-symmetry ideal-perovskite reference structure to which the actual structure is related by small distortions.

Figure 4

Total energy with respect to the lowest-energy configuration versus the charge ordering and magnetic ordering patterns. The symbols G, C, A, and FM represent the type of magnetic ordering of V cations illustrated in the right column.

Figure 5

(a) Density of states of CCO (black solid line), SCO (red dashed line), and LCO (blue dotted line) phases with C-AFM ordering. The positive and negative signs of the vertical axis correspond to spin up and down states. The shaded (from $-6$ to $-1.15\mathrm{eV}$) and unshaded regions (from $-1.15$ to 5 eV) are oxygen $p$- and vanadium $d$-derived states, respectively. (b)–(c) PDOS in the LCO phase with C-AFM ordering for V $t_{2g}$ with positive magnetic moment. (b) PDOS of the contracted octahedron (${\mathrm{V}}^{4+}$) with spin-up polarization. (c) As in (b), on the elongated octahedron (${\mathrm{V}}^{3+}$).

Figure 6

Total energy per vanadium with respect to the lowest energy configuration as a function of (001) epitaxial strain. The lines are obtained from quadratic fits to the data points. The zero-strain in-plane lattice constant is taken as the cube root of the volume of bulk ${\mathrm{LaVO}}_3$ per formula unit obtained from the $\mathrm{GGA}+U$ calculation.

Figure 7

(a) Total energy per vanadium (blue solid line) with respect to the lowest energy configuration and band gap (red dashed line) for structures linearly interpolated between the SCO phase ($\lambda =0$) and the LCO phase ($\lambda =1$). (b) $\lambda$ dependence of magnetic moments of the two V ions that have different nominal valence in SCO and LCO.