Charge-Order-Induced Ferroelectricity inLaVO3/SrVO3Superlattices

Abstract

The structure and properties of the 1:1 superlattice of LaVO3 and SrVO3 are investigated with a firstprinciples density-functional-theory-plus-U (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 cation ordering to produce a polar structure with nonzero spontaneous polarization normal to the interfaces. This polarization is produced by electron transfer between the V 3+ and V 4+ layers, and is comparable to that of conventional ferroelectrics. The energy of this polar state relative to the nonpolar ground state is only 3 meV per vanadium. Under tensile strain, this energy difference can be further reduced, suggesting that the polar phase can be induced by applied electric field, yielding an antiferroelectric double-hysteresis loop. If the system does not switch back to the nonpolar state on removal of the field, a ferroelectric-type hysteresis loop could be observed. PACS numbers: 73.21.Cd, 75.25.Dk, 77.55.Px The investigation of novel mechanisms for ferroelectricity has attracted much interest in recent years, especially in the search for ferroelectrics with properties, such as ferromagnetism, that are contraindicated by the mechanism driving ferroelectricity in the prototypical perovskite titanates [1]. One approach, proposed by Khomskii [2,3] is to combine two symmetry-breaking orderings, neither of which separately lift inversion symmetry, to generate a switchable polar structure [4]. The specific orderings discussed by Khomskii are sitecentered charge ordering and bond-centered charge ordering. Ferroelectricity and multiferroelectricity induced by charge order have been proposed and reported in various magnetites, manganates, and charge transfer organic salts [2,3,5]. In some of these materials, the polarization is dominated by the electron transfer producing the charge order rather than by ionic displacements, leading to the term "electronic ferroelectricity." [6][7][8] In perovskite transition metal (TM) oxide (ABO 3 ) n (A BO 3 ) m (001) superlattices, the layered cation ordering lowers the symmetry from cubic to tetragonal and breaks up-down symmetry across BO 2 layers. Control of the TM d-orbital occupancy through choice of A cations and layer thicknesses to obtain a mixed valence leads to charge disproportionation and long-range charge ordering [9-12]. As we will discuss further below, layered charge ordering breaks the up-down symmetry across the AO and A O layers. Thus, the combination of these two symmetry-breaking orderings (TM site-centered charge ordering and layered cation ordering) can generate a switchable polar structure with polarization normal to the layers.A 1:1 superlattice composed of LaVO 3 and SrVO 3 is a promising candidate for this type of charge-order-induced ferroelectricity. The low temperature phases of orthorhombic LaVO 3 and cubic SrVO 3 are antiferromagnetic Mottinsulating and correlated metallic, respectively [9,[13][14][15]. When they f...

Electronic Reconstruction in (LaVO3)m/SrVO3 (m = 5, 6) Superlattices

“…For atom V1a (V1d) only the d xy orbital is occupied by one spinup (spin-down) electron, resulting in a magnetic moment of 1.0 μ B . [19] Figure 4e-h shows the DOS projected on the V atoms in the L2 and L3 layers following the layer with atoms V1c and V1d. For atom V1b (V1c) both the d xz and d yz orbitals are occupied by one spin-up (spin-down) electron, whereas the d xy orbital remains empty.…”

“…For the m = 6 superlattice this fact is supported by AC measurements, [7] whereas in DC measurements [6] a metallic low temperature behavior is found. [19] besides other patterns such as stripe order. A recent study of the band structure of O deficient SrVO 3 thin films points to complex band shifts due to local modifications of the crystalline structure around O vacancies.…”

“…In addition, it is known that the magnetization in (LaVO 3 ) m /SrVO 3 superlattices depends on the thickness of the LaVO 3 layer, with m = 4, 6 yielding larger saturation magnetization than m = 3, 5. [18] Distortions in vanadate superlattices generated by charge disproportionation into V 3+ and V 4+ ions have the potential to result in ferroelectricity [19] and distortions of the VO 6 octahedra can have complex effects on the orbital occupations. The mechanism responsible for the deviating behaviors of superlattices with odd and even thicknesses is not fully understood so far.…”

“…[10] without identifying a specific spatial pattern. [18] Distortions in vanadate superlattices generated by charge disproportionation into V 3+ and V 4+ ions have the potential to result in ferroelectricity [19] and distortions of the VO 6 octahedra can have complex effects on the orbital occupations. [6] The fact that in the latter case the saturation magnetization does not exceed the threshold of the substrate impurities indicates that there is no macroscopic magnetization.…”

Electronic localization in CaVO3 films via bandwidth control

“…CaVO3 film) as the spectral weight is transferred away from the valence band maximum. However, 8 there is still weight in the intra-band peak even in the insulating phase indicating metallic puddles are still present even in the insulating phase [26].…”