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Double-exchange theory of ferroelectric polarization in orthorhombic manganites with twofold periodic magnetic texture

机译：具有正交周期磁织构的正交晶锰矿中铁电极化的双交换理论

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摘要

We argue that many aspects of improper ferroelectric (FE) activity in orthorhombic manganites can be rationalized by considering the limit of infinite intra-atomic splitting between majority- and minority-spin states (or the double-exchange limit), which reduces the problem to the analysis of a spinless double-exchange (DE) Hamiltonian. We apply this strategy to the low-energy model, derived from the first-principles electronic-structure calculations, and combine it with the Berry-phase theory of electric polarization. We start with the analysis of the simplest two-orbital model, describing the behavior of the e_g bands, and apply it to the E-type antiferromagnetic (AFM) phase, which in the DE limit effectively breaks up into one-dimensional zigzag chains. We derive an analytical expression for the electronic polarization (P~(el)) and explain how it depends on the orbital ordering and the energy splitting △ between e_g states. Then, we evaluate parameters of this model for the series of manganites. For these purposes, we start from a more general five-orbital model for all Mn 3d bands and construct a new downfolded model for the e_g bands. From the analysis of these parameters, we conclude that the behavior of P~(el) in realistic manganites always corresponds to the limit of large A. This property holds for all considered compounds even in the local-density approximation, which typically underestimates A. We further utilize this property in order to derive an analytical expression for P~(el) in a general twofold periodic magnetic texture, based on the five-orbital model and the perturbation-theory expansion for the Wannier functions in the first order of 1 /△. This expression explains the functional dependence of P~(el) on the relative directions of spins. Furthermore, it suggests that P~(el) is related to the asymmetry of transfer integrals, which should have simultaneously symmetric and antisymmetric components in the crystal-field representation. The main contribution to this asymmetry comes from the antiferro-orbital ordering in the ab plane. Finally, we explain how the FE polarization can be switched between orthorhombic a and c directions by inverting the zigzag AFM texture in every second ab plane. We argue that this property is generic and can be realized even in the twofold periodic texture.
机译：我们认为，通过考虑多数自旋态和少数自旋态之间无限原子内分裂的极限（或双交换极限），可以使正交晶锰矿中铁电（FE）活性不正常的许多方面合理化。无旋转双交换（DE）哈密顿量的分析。我们将此策略应用于源自第一性原理电子结构计算的低能模型，并将其与电极化的Berry相理论相结合。我们从分析最简单的两个轨道模型开始，描述e_g谱带的行为，并将其应用于E型反铁磁（AFM）相，该相在DE极限内有效地分解为一维之字形链。我们导出了电子极化（P〜（el））的解析表达式，并解释了它如何取决于轨道排序和e_g状态之间的能量分裂△。然后，我们评估该模型对于一系列锰的参数。为此，我们从针对所有Mn 3d波段的更通用的五轨道模型开始，并为e_g波段构造一个新的向下折叠模型。通过对这些参数的分析，我们得出结论，实际锰矿中P〜（el）的行为始终对应于大A的极限。即使在局部密度近似中（通常会低估A），该属性也适用于所有考虑的化合物。我们进一步利用这一性质，以基于五轨道模型和Wannier函数以1/1的阶的微扰理论展开式为基础，得出一般双重周期性磁织构中P〜（el）的解析表达式。 △。该表达式解释了P〜（el）对自旋相对方向的功能依赖性。此外，这表明P〜（el）与传递积分的不对称有关，传递积分在晶体场表示中应同时具有对称和反对称分量。对这种不对称性的主要贡献来自ab平面中的反铁轨道定序。最后，我们解释了如何通过每隔一个ab平面反转之字形AFM纹理来在正交a和c方向之间切换FE偏振。我们认为该属性是通用的，即使在双重周期性纹理中也可以实现。

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  • 来源
    《Physical review》 |2013年第14期|144424.1-144424.15|共15页
  • 作者

    I. V. Solovyev; S. A. Nikolaev;

  • 作者单位

    Computational Materials Science Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan,Department of Theoretical Physics and Applied Mathematics, Ural Federal University, Mira str. 19, 620002 Ekaterinburg, Russia;

    Department of Theoretical Physics and Applied Mathematics, Ural Federal University, Mira str. 19, 620002 Ekaterinburg, Russia;

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  • 正文语种 eng
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  • 关键词

    manganites; density functional theory; local density approximation; gradient and other corrections;

    机译：锰矿;密度泛函理论;局部密度近似;渐变和其他校正;

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