Bastardis Roland, Guihéry Nathalie, Suaud Nicolas, de Graaf Coen
Laboratoire de Physique Quantique, IRSAMC/UMR5626, Université Paul Sabatier, 118 Route de Narbonne, F-31062 Toulouse Cedex 4, France.
J Chem Phys. 2006 Nov 21;125(19):194708. doi: 10.1063/1.2375119.
A truncated Hubbard model is developed for the description of the electronic structure of odd-electron TM-L-TM units (TM=transition metal and L=ligand). The model variationally treats both the double exchange and purely magnetic Heisenberg configurations. This Hubbard model can either be mapped on a purely magnetic Heisenber model in which the bridging oxygen is also magnetic or on a double exchange model owing to the hybridization of the magnetic and ligand or bitals. The purely magnetic Heisenberg model is analytically solved in the general case of two metals (having n magnetic orbitals) bridged by a magnetic oxygen. The comparison of the analytical expressions of the Heisenberg energies to those of the double exchange model reveals that the two model spectra are identical except for one state which does not belong to the model space of the double exchange Hamiltonian. Consequently, the fitting of the model spectra to accurate ab initio spectra does not discriminate between the physically different models. These concepts are illustrated for the Mn-O-Mn unit (or Zener polaron) found in the half-doped manganite Pr(0.6)Ca(0.4)MnO3. It is shown that in the present case the projections of the ab initio ground state wave function onto both model spaces are almost identical provided that one uses properly localized orbitals, proving that the magnetic description of the Zener polaron and the double exchange viewpoint of the electronic structure are equally valid.
为描述奇数电子TM-L-TM单元（TM = 过渡金属，L = 配体）的电子结构，开发了一种截断的哈伯德模型。该模型以变分法处理双交换和纯磁海森堡构型。此哈伯德模型既可以映射到其中桥连氧也具有磁性的纯磁海森堡模型上，也可以由于磁性和配体轨道的杂化而映射到双交换模型上。在由磁性氧桥连的两种金属（具有n个磁性轨道）的一般情况下，对纯磁海森堡模型进行了解析求解。将海森堡能量的解析表达式与双交换模型的解析表达式进行比较，结果表明，除了一个不属于双交换哈密顿量模型空间的态之外，这两个模型的谱是相同的。因此，将模型谱与精确的从头算谱进行拟合，并不能区分这两种物理上不同的模型。这些概念通过在半掺杂锰酸盐Pr(0.6)Ca(0.4)MnO3中发现的Mn-O-Mn单元（或齐纳极化子）进行了说明。结果表明，在当前情况下，只要使用适当局域化的轨道，从头算基态波函数在两个模型空间上的投影几乎相同，这证明了齐纳极化子的磁性描述和电子结构的双交换观点同样有效。
