Ordering Phenomena in Transition Metal Oxides

Theory of charge disproportionation and application to Na_xCoO₂   

Theoretical understanding of charge ordering, specifically charge disproportionation and spin differentiation that accompanies or precedes ordering, became a more interesting undertaking as underdoped cuprates were studied ever more carefully. Charge/ spin/orbital ordering have since been found to occur extensively in the CMR manganites and also in vanadates, and most recently in the Na-intercalated cobaltates that superconduct when hydrated. We will describe the application of correlated band theory (LDA+U), which gives a first-order charge/spin disproportionation transition versus U. In Na_xCoO₂, the transition occurs in the range 3.2 < U < 3.5 eV for x=1/3, 1/2, and 2/3. The origin of this transition in the electronic structure and the energetics will be presented in detail. Unusual behavior in the new monovalent-Ni compound will also be described.