具有 $t_{2g}$ 电子简并的氟钙钛矿中轨道有序与无序的迹象
Signatures of Orbital Order and Disorder in Fluoro-Perovskites with $t_{2g}$ Electronic Degeneracies
AI总结：
通过高分辨率变温粉末衍射研究，发现 NaFeF$_3$ 发生类似二阶的 C 型轨道有序相变并暗示隐藏对称性破缺，而 NaCoF$_3$ 在 4 K 下无长程轨道有序，指向一种与 $t_{2g}^5$ 电子简并相关的新型轨道无序。
摘要：
对氟钙钛矿 NaFeF$_3$ 和 NaCoF$_3$ 进行了详细的高分辨率变温粉末衍射研究，以探测其轨道有序相变。通过分析对称化宏观应变和原子畸变，我们表明 NaFeF$_3$ 发生了与 Fe$^{2+}$ 的 $t_{2g}^4$ 态相关的 C 型轨道有序相变。反直觉地，导致轨道有序的相变表现为类似二阶相变，这与电子和同对称相变的热力学要求相矛盾，暗示必然存在相关的隐藏对称性破缺。另一方面，对于 NaCoF$_3$，对对称化应变的考虑使我们能够确信地排除在 4 K 以下出现任何长程轨道有序的可能性。由于 NaCoF$_3$ 在此温度下是具有淬灭轨道角动量的绝缘体，我们的发现指向一种与 $t_{2g}^5$ 电子简并相关的新型轨道无序。
英文摘要：
A detailed high-resolution, variable temperature powder diffraction study of the fluoro-perovskites NaFeF$_3$ and NaCoF$_3$ is performed to probe their orbital ordering transitions. Through analysis of the symmetry adapted macrostrains and atomic distortions, we show that NaFeF$_3$ undergoes a C-type orbital order transition associated with the $t_{2g}^4$ states of Fe$^{2+}$. Counter-intuitively, the phase transition leading to the orbital order appears second order-like, which contradicts the thermodynamic requirements for electronic and isosymmeric phase transitions, implying that there must be an associated hidden symmetry breaking. On the other hand, for NaCoF$_3$, consideration of the symmetry adapted strains allows us to confidently rule out the occurrence of any long-range orbital orders down to 4 K. Since NaCoF$_3$ is an insulator with quenched orbital angular momentum at this temperature, our findings point towards a novel kind of orbital disorder associated to the $t_{2g}^5$ electronic degeneracy.
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