Disproportionation, metal-insulator transition, and critical interaction strength in Na(1/2)CoO2 |
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Authors: | Lee K-W Kunes J Novak P Pickett W E |
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Institution: | Department of Physics, University of California, Davis, California 95616, USA. |
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Abstract: | Charge disproportionation (CD) and spin differentiation in Na(1/2)CoO2 are studied using the correlated band local-density approximation + Hubbard U (LDA+U) approach. The simultaneous CD and gap opening seen previously is followed in detail through a first-order charge disproportionation transition 2Co(3.5+)-->Co3++Co4+. Disproportionation in the Co a(g) orbital results in half of the ions (Co3+) becoming electronically and magnetically dead, transforming the quarter-filled a(g) system into a half-filled subsystem that subsequently undergoes the observed charge ordering or metal-insulator transition. Comparing with data in the x approximately 0.3 regime suggests the system has moved into the multiband regime where the effective Coulomb repulsion U-->U(eff)=U/sqrt3] strongly lessens correlation effects. |
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