Disproportionation and critical interaction strength in NaxCoO2:: Concentration dependence

We present results of studies of charge disproportionation (CD) and spin differentiation in NaxCoO2 using the correlated band theory approach [local density approximation (LDA)+Hubbard U:LDA+U]. The simultaneous CD and gap opening can be followed through a first-order charge disproportionation transition. By comparison with experiments, we propose a value of the Coulomb repulsion strength U(x) that has significant dependence on the carrier concentration x, for which we obtain a phase diagram. The connection between the Co-O bond length and effects of on-site correlation is also reported. (C) 2006 American Institute of Physics.