标题： 锰氧化物钙钛矿中Jahn-Teller畸变与结构自由度在赝立方态之间的相互作用 显示英文标题

标题： The interplay between Jahn-Teller distortions and structural degrees of freedom on pseudocubic states in manganite perovskites

摘要： 固溶体 LaMn$_{1-x}$Ga$_x$O$_3$ (LMGO) 的平均结构已通过利用同步辐射 X 射线和中子粉末衍射技术的对称性驱动方法进行研究。 我们通过实验表明，剪切应变、八面体旋转和$C$类轨道有序模式之间的三线性耦合项 ($\Gamma_5^+$M$_2^+$M$_3^+$) 是导致在成分范围 0.5$<$ $x$ $<$ 0.6 内发生的正交到赝立方相变的原因。我们的蒙特卡罗模拟阐明了这种与剪切应变的耦合的宏观起源，并指出了其在控制轨道有序-无序转变中的重要性。 我们发现，伪立方态的出现可以通过考虑这个三线性项与非相位一致的八面体倾斜的线性二次项之间的竞争来合理解释 ($\Gamma_5^+$(R$_5^-$)$^2$)。 为了说明这些结果的普遍性，我们构建了一个简单的函数，该函数捕捉了在有序-无序转变中朗道自由能的变化，在参数上很容易与锰氧化物钙钛矿中Jahn-Teller活性物种的浓度、温度、容忍因子和晶胞应变相关联。 我们的结果表明，远非伪立方态是轨道无序的症状，而是在许多情况下更正确地将其视为原因。 这些结果对钙钛矿材料中轨道有序物理的研究以及通过化学和物理控制参数来调节纠缠物理性质的丰富性具有广泛的影响。 显示英文摘要

摘要： The average structure of the solid solution LaMn$_{1-x}$Ga$_x$O$_3$ (LMGO) has been investigated from a symmetry-motivated approach utilizing synchrotron x-ray and neutron powder diffraction techniques. We show experimentally that a trilinear coupling term ($\Gamma_5^+$M$_2^+$M$_3^+$) between shear strain, octahedral rotation, and the $C$-type orbital ordering mode is responsible for driving the orthorhombic to pseudocubic phase transition occurring in the composition range 0.5 $<$ $x$ $<$ 0.6. Our Monte Carlo simulations elucidate the macroscopic origin of this coupling to shear strain, and point to its importance with respect to controlling the orbital order-disorder transitions. We find that the emergence of the pseudocubic state can be rationalized by considering the competition between this trilinear term and a linear-quadratic term of the out-of-phase octahedral tilting with strain ($\Gamma_5^+$(R$_5^-$)$^2$). Illustrating the general nature of these results, we construct a simple function that captures the change in Landau free energy at the order-disorder transition, in parameters that are trivial to relate to the concentration of Jahn--Teller active species, temperature, tolerance factor and unit cell strain, for a broad range of manganite perovskites. Our results point to the fact that far from the pseudocubic state being a symptom of orbital disorder, it is in many cases more correctly to view it as a cause. The results have a broad impact on the study of orbital ordering physics in the perovskite materials and on chemical and physical control parameters through which to tune the richness of the intertwined physical properties.