How chemistry controls electron localization in 3d1 perovskites:: a Wannier-function study -: art. no. 188
How chemistry controls electron localization in 3d1 perovskites:: a Wannier-function study -: art. no. 188
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DOI：
10.1088/1367-2630/7/1/188
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发表时间：
2005-09-02
影响因子：
3.3
通讯作者：
Andersen, OK
中科院分区：
文献类型：
--
作者：
Pavarini, E;Yamasaki, A;Andersen, OK
在3d(t(2g))(1)钙钛矿系列SrVO3-CaVO3-LaTiO3-YTiO3中，过渡金属d电子越来越局域化，在CaVO3和LaTiO3之间发生Mott跃迁。通过在t(2g) LDA波段的万尼尔函数的基础上定义一个低能哈伯德哈密顿量，并在单点动态平均场（DMFT）近似中求解它，它最近被证明(Pavarini et al . (2004) Phys。在莫特跃迁的同时，由于t（2g）能级的分裂，轨道波动受到了强烈的抑制。本文回顾和扩展了这一工作，特别是在利用n阶松饼轨道（NMTO）方法生成的从头算LDA万尼尔函数揭示潜在化学机制的方向上。占据的氧-p带的万尼尔函数说明了氧-p对沿序列渐进的gdfeo3型畸变的大阳离子-d共价的重要性。与阳离子成键的氧轨道与与过渡金属t（2g）轨道成键的氧轨道相同。因此，t（2g）波段的万尼尔函数表现出过渡金属t（2g）、大阳离子-d和氧-p态之间的剩余共价。这种沿级数增加的剩余共价不仅导致了t（2g）能级的分裂，而且导致了跳变积分的非三次扰动，这两者都是莫特跃迁的决定性因素。我们发现所有四种材料的光学和光电发射光谱，最近测量的钛酸盐的晶体场分裂和轨道极化，以及LaTiO3的金属化体积（压力）都有很好的一致性。预测了YTiO3的金属化体积，并讨论了jhn - teller （JT）畸变的作用。为了在未来的多体计算中使用，我们将所有四种材料的t（2g）现场和跳跃矩阵元素制表，并给出k + Q表示中的正交哈密顿量的解析表达式。利用传统的超交换理论，我们的现场和跳跃矩阵元素再现了观测到的LaTiO3和YTiO3的磁序，但结果对细节很敏感，特别是对于YTiO3，如果没有JT畸变，磁序将是反铁磁性的C-或a型，而不是铁磁性的。决定性的是，随着gdfeo3型畸变的增大，最低阶和最高阶万尼尔函数之间的近邻跳变强于最低阶万尼尔函数之间的跳变。最后，我们证明了导致这种行为的非立方扰动使得将正交t(2g) LDA带结构展开到伪立方区成为可能。在这个区域，最低的带与其他两个带之间有一个直接的间隙，宽度为W- 1，明显小于整个t（2g）带的宽度W。因此，渐进式gdfeo3型畸变通过减小W、增大δ /W和减小W- i /W有利于电子定位。关于gdfeo3型和JT畸变的作用，我们的结论与Mochizuki和Imada （2003 Phys）的结论一致。Rev. letter .91 67203)。
In the series of 3d(t(2g))(1) perovskites, SrVO3-CaVO3-LaTiO3-YTiO3, the transition-metal d electron becomes increasingly localized and undergoes a Mott transition between CaVO3 and LaTiO3. By defining a low-energy Hubbard Hamiltonian in the basis of Wannier functions for the t(2g) LDA band and solving it in the single-site dynamical mean-field (DMFT) approximation, it was recently shown (Pavarini et al (2004) Phys. Rev. Lett. 92 176403) that simultaneously with the Mott transition there occurs a strong suppression of orbital fluctuations due to splitting of the t(2g) levels. The present paper reviews and expands this work, in particular in the direction of exposing the underlying chemical mechanisms by means of ab initio LDA Wannier functions generated with the Nth order muffin-tin orbital (NMTO) method. The Wannier functions for the occupied oxygen-p band illustrate the importance of oxygen-p to large cation-d covalency for the progressive GdFeO3-type distortion along the series. The oxygen-porbitals which pd sigma-bond to the cations are the same as those which pd pi-bond to the transition-metal t(2g) orbitals. As a consequence, the Wannier functions for the t(2g) band exhibit residual covalency between the transition-metal t(2g), the large cation-d, and the oxygen-p states. This residual covalency, which increases along the series, turns out to be responsible not only for the splittings, Delta, of the t(2g) levels, but also for non-cubic perturbations of the hopping integrals, both of which are decisive for the Mott transition. We find good agreement with the optical and photoemission spectra for all four materials, with the crystal-field splittings and orbital polarizations recently measured for the titanates, and with the metallization volume (pressure) for LaTiO3. The metallization volume for YTiO3 is predicted and the role of the Jahn-Teller (JT) distortion is discussed. For use in future many-body calculations, we tabulate the t(2g) on-site and hopping matrix elements for all four materials and give an analytical expression for the orthorhombic Hamiltonian in the k + Q representation. Using conventional super-exchange theory, our on-site and hopping matrix elements reproduce the observed magnetic orders in LaTiO3 and YTiO3, but the results are sensitive to detail, in particular for YTiO3 where, without the JT distortion, the magnetic orderwould be antiferromagnetic C- or A-type, rather than ferromagnetic. It is decisive that upon increasing the GdFeO3-type distortion, the nearest-neighbour hopping between the lowest and the upper-level Wannier functions becomes stronger than the hopping between the lowest-level Wannier functions. Finally, we show that the non-cubic perturbations responsible for this behaviour make it possible to unfold the orthorhombic t(2g) LDA bandstructure to a pseudo-cubic zone. In this zone, the lowest band is separated from the two others by a direct gap and has a width, W-I, which is significantly smaller than that, W, of the entire t(2g) band. The progressive GdFeO3-type distortion thus favours electron localization by decreasing W, by increasing Delta/W, and by decreasing W-I/W. Our conclusions concerning the roles of GdFeO3-type and JT distortions agree with those of Mochizuki and Imada (2003 Phys. Rev. Lett.91 67203).