标题： 具有 $t_{2g}$ 电子简并性的氟化钙钛矿中轨道有序与无序的特征 显示英文标题

标题： Signatures of Orbital Order and Disorder in Fluoro-Perovskites with $t_{2g}$ Electronic Degeneracies

摘要： 对氟化钙钛矿 NaFeF$_3$和 NaCoF$_3$进行了详细的高分辨率、可变温度的粉末衍射研究，以探测它们的轨道有序转变。 通过对对称适应的宏观应变和原子畸变的分析，我们表明 NaFeF$_3$发生了一种与 Fe$^{2+}$的$t_{2g}^4$态相关的 C 型轨道有序转变。 出人意料的是，导致轨道有序的相变表现出类似二阶的特性，这与电子和同构相变的热力学要求相矛盾，这意味着必然存在一种相关的隐藏对称性破缺。 另一方面，对于 NaCoF$_3$，考虑对称适应的应变使我们能够排除在 4K 温度以下出现任何长程轨道序的可能性。由于 NaCoF$_3$在这个温度下是一种具有淬灭轨道角动量的绝缘体，我们的研究结果指向一种与$t_{2g}^5$电子简并性相关的新型轨道无序现象。 显示英文摘要

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.