作为顺磁EuTiO3磁性和绝缘源的局域位置和自旋对称性破缺
Local positional and spin symmetry breaking as a source of magnetism and insulation in paramagnetic EuTiO3
摘要：
我们认为理论上的顺磁相的EuTiO 3表示由两组微观自由度（m-自由度）的配置：位置对称性破坏由于八面体旋转和磁对称性破坏由于自旋无序。通过考虑具有所需标称全局平均对称性的足够大的（超级）细胞，评估这些组的m-自由度对电子结构和对位相的性质的影响，然而，允许降低局部位置和磁对称性。我们发现，局部对称性破缺的趋势，可以监测以下的总能量降低平均场一样的密度泛函理论，没有追索权的强关联效应。虽然大多数名义上的立方ABO 3钙钛矿因其B原子亚晶格的对称性破缺而闻名，但EuTiO 3中的f电子磁性的情况与A亚晶格对称性破缺及其与结构畸变的耦合有关。我们发现（i）顺磁性立方相EuTiO_3具有磁性和位置对称性破缺的内在趋势，而顺磁性四方相EuTiO_3相对于低温反铁磁四方相仅具有磁性对称性降低而没有明显的位置对称性降低。(ii)适当建模的顺磁性四元和立方EuTiO 3在每个Eu离子上具有非零的局部磁矩，这与在显著高于Néel温度的EuTiO 3的对位相中的局部磁性的实验观察一致。有趣的是，（iii）在短程有序的对位相的局部位置畸变模式是从长程有序的低温反铁磁基态相继承。
英文摘要：
We consider theoretically the paramagnetic phases of EuTiO3 that represent configurations created by two sets of microscopic degrees of freedom (m-DOF): positional symmetry breaking due to octahedral rotations and magnetic symmetry breaking due to spin disorder. The effect of these sets of m-DOFs on the electronic structure and properties of the para phases is assessed by considering sufficiently large (super) cells with the required nominal global average symmetry, allowing, however, the local positional and magnetic symmetries to be lowered. We find that tendencies for local symmetry breaking can be monitored by following total energy lowering in mean-field like density functional theory, without recourse for strong correlation effects. While most nominally cubic ABO3 perovskites are known for their symmetry breaking due to the B-atom sublattice, the case of f-electron magnetism in EuTiO3 is associated with A- sublattice symmetry breaking and its coupling to structural distortions. We find that (i) paramagnetic cubic EuTiO3 has an intrinsic tendency for both magnetic and positional symmetry breaking, while paramagnetic tetragonal EuTiO3 has only magnetic symmetry lowering and no noticeable positional symmetry lowering with respect to low-temperature antiferromagnetic tetragonal phase. (ii) Properly modeled paramagnetic tetragonal and cubic EuTiO3 have a nonzero local magnetic moment on each Eu ion, consistent with the experimental observations of local magnetism in the para phases of EuTiO3 significantly above the Néel temperature. Interestingly, (iii) the local positional distortion modes in the short-range ordered para phases are inherited from the long-range ordered low-temperature antiferromagnetic ground state phase.
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