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arXiv 2010.03433cond-mat.str-el
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关联电子材料中Hund交换耦合磁涨落的轨道各向同性

Orbital isotropy of magnetic fluctuations in correlated electron materials induced by Hund's exchange coupling

Evgeny A. Stepanov, Yusuke Nomura, Alexander I. Lichtenstein, Silke Biermann

摘要:

描述具有强电子库仑相互作用的材料中的非局域磁涨落仍然是现代凝聚态理论的主要挑战之一。在这项工作中,我们解决的空间对称性和轨道结构的钙钛矿材料中的磁波动。为此,我们开发了一个一致的多轨道图形扩展的动力学平均场理论,我们适用于各向异性三轨道模型的立方$t_{2g}$对称性。我们发现,空间自旋涨落的形式是由当地的洪德耦合。对于小的耦合值,磁涨落在轨道空间中是各向异性的,这反映了所考虑的$t_{2g}$模型的对称性。大洪德耦合增强了集体自旋激发,混合了轨道和空间自由度,磁波动变得轨道各向同性。值得注意的是,这种效应只能在两粒子的量中看到;单粒子的观测量对于任何洪德耦合值都是各向异性的。重要的是,我们发现,轨道各向同性可以诱导,在半填充和$4$电子的情况下,每个晶格站点,其中磁不稳定性与不同的,反铁磁和铁磁模式,分别。

英文摘要:

Characterizing non-local magnetic fluctuations in materials with strong electronic Coulomb interactions remains one of the major outstanding challenges of modern condensed matter theory. In this work we address the spatial symmetry and orbital structure of magnetic fluctuations in perovskite materials. To this aim, we develop a consistent multi-orbital diagrammatic extension of dynamical mean field theory, which we apply to an anisotropic three-orbital model of cubic $t_{2g}$ symmetry. We find that the form of spatial spin fluctuations is governed by the local Hund's coupling. For small values of the coupling, magnetic fluctuations are anisotropic in orbital space, which reflects the symmetry of the considered $t_{2g}$ model. Large Hund's coupling enhances collective spin excitations, which mixes orbital and spatial degrees of freedom, and magnetic fluctuations become orbitally isotropic. Remarkably, this effect can be seen only in two-particle quantities; single-particle observables remain anisotropic for any value of the Hund's coupling. Importantly, we find that the orbital isotropy can be induced both, at half-filling and for the case of $4$ electrons per lattice site, where the magnetic instability is associated with different, antiferromagnetic and ferromagnetic modes, respectively.

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