Unusual e g 3d x 2−y 2 Orbital Ordering and Low-Energy Excitations in the CE Structure of NaMn7O12

Similarly to the half-doped manganese oxides with perovskite structure like La_0.5Ca_0.5MnO₃, the double perovskite compound NaMn₇O₁₂ contains an equal number of Mn³⁺ and Mn⁴⁺ ions in the corner-sharing network of MnO₆ octahedra and exhibits a CE order of the charge and spins of these ions at low temperature. Though, in NaMn₇O₁₂ the order is complete, the charge and spin ordering transitions are sharp and the system is free of disorder, phase coexistence or structural inhomogeneities thanks to the absence of chemical substitutions. Here we discuss two unusual features of the CE structure of NaMn₇O₁₂: (1) the e _g 3d x ²?y ² orbital ordering expected from the direct crystallographic observation of compressed Mn³⁺O₆ octahedra below the charge ordering transition; (2) the existence of a large amount of low-energy excitations evidenced by the low temperature behavior of the specific heat. We propose a picture of nearly degenerate spin and orbital configurations of the CE structure arising from the peculiar orbital ordering.