Orbital physics in the perovskite Ti oxides
Orbital physics in the perovskite Ti oxides
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DOI：
10.1088/1367-2630/6/1/154
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发表时间：
2004-10
影响因子：
3.3
通讯作者：
M. Mochizuki;M. Imada
中科院分区：
文献类型：
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作者：
M. Mochizuki;M. Imada
钛化合物被认为是理解强关联电子系统中磁性和轨道耦合的关键材料。在典型的Mott-Hubbard绝缘体的钙钛矿型Ti氧化物RTiO 3（其中R表示三价稀土离子）中，3d 1态的Ti t2 g轨道和自旋通过强电子关联彼此耦合，导致丰富多样的轨道-自旋相。控制耦合的一种方法是通过改变R离子来改变TiO 6八面体的倾斜度（即GdFeO 3型畸变），通过R离子来控制电子带宽与库仑相互作用的相对比率。在这种控制下，这些莫特绝缘体表现出反铁磁到铁磁（AFM-FM）相变，这已被证明是这些材料中丰富的轨道物理的结果。这些莫特绝缘体的轨道自旋结构的起源和性质已经在实验和理论上得到了深入的研究。当Mott绝缘体中掺杂载流子时，钛酸盐显示出填充控制的Mott转变的试金石性质。在本文中，我们首先回顾了了解钙钛矿钛酸盐的性质所包含的物理的研究的最新进展。关于绝缘体的特性，我们重点讨论以下三个主题：（1）GdTiO 3和YTiO 3等R离子相对较小的化合物的铁磁性的起源和本质以及轨道有序性，（2）LaTiO_3中G型反铁磁性和轨道态的起源;（3）其他具有较大R离子的AFM（G）化合物中的轨道-自旋结构（R = Ce、Pr、Nd和Sm）。在此基础上，讨论了整个相图和磁相变机制。在微观理解的基础上的轨道自旋状态，我们表明，钛t2 g简并固有地解除钛酸盐，这使得单带描述的基态和低能量的电子结构作为一个很好的起点。我们的分析表明，这些化合物提供了良好的试金石材料所描述的单带哈伯德模型的立方晶格。从这一认识出发，我们还重新分析了空穴掺杂的钛酸盐TiO 3（其中A代表二价碱土金属离子）。实验揭示的填充和带宽依赖的属性和金属-绝缘体过渡的临界行为进行了讨论，在光的理论基础上的单带哈伯德模型。
Titanate compounds have been recognized as key materials for understanding the coupling of magnetism and orbitals in strongly correlated electron systems. In the perovskite Ti oxide RTiO3 (where R represents the trivalent rare-earth ions), which is a typical Mott–Hubbard insulator, the Ti t2g orbitals and spins in the 3d1 state couple each other through the strong electron correlations, resulting in a rich variety of orbital–spin phases. One way of controlling the coupling is to change the tiltings of the TiO6 octahedra (namely the GdFeO3-type distortion) by varying the R ions, through which the relative ratio of the electron bandwidth to the Coulomb interaction is controlled. With this control, these Mott insulators exhibit an antiferromagnetic-to-ferromagnetic (AFM–FM) phase transition, which has turned out to be a consequence of rich orbital physics in these materials. The origin and nature of orbital–spin structures of these Mott insulators have been intensively studied both experimentally and theoretically. When the Mott insulators are doped with carriers, the titanates show touchstone properties of the filling controlled Mott transition. In this paper, we first review the state of the art on the studies for understanding physics contained in the properties of the perovskite titanates. On the properties of the insulators, we focus on the following three topics: (1) the origin and nature of the ferromagnetism as well as the orbital ordering in the compounds with relatively small R ions such as GdTiO3 and YTiO3, (2) the origin of the G-type antiferromagnetism and the orbital state in LaTiO3 and (3) the orbital–spin structures in other AFM(G) compounds with relatively large R ions (R = Ce, Pr, Nd and Sm). On the basis of these discussions, we discuss the whole phase diagram together with mechanisms of the magnetic phase transition. On the basis of the microscopic understanding of the orbital–spin states, we show that the Ti t2g degeneracy is inherently lifted in the titanates, which allows the single-band descriptions of the ground-state and the low-energy electronic structures as a good starting point. Our analyses indicate that these compounds offer good touchstone materials described by the single-band Hubbard model on the cubic lattice. From this insight, we also re-analyse the hole-doped titanates TiO3 (where A represents the divalent alkaline-earth ions). Experimentally revealed filling- and bandwidth-dependent properties and the critical behaviour of the metal–insulator transitions are discussed in the light of theories based on the single-band Hubbard models.