ARPES on Na${}_{0.6}$CoO${}_2$: Fermi surface, extended flat dispersion, and unusual band splitting

Abstract

The electronic structure of single crystals Na${}_{0.6}$CoO${}_2$, which are closely related to the superconducting Na${}_{0.3}$CoO${}_2$.$y$H${}_2$O ($T_c\sim 5K$), is studied by angle-resolved photoelectron spectroscopy. While the measured Fermi surface is found to be consistent with the prediction of a local density band theory, the energy dispersion is highly renormalized, with an anisotropy along the two principle axes ($\Gamma$-$K$, $\Gamma$-$M$). Our ARPES result also indicates that an extended flat band is formed slightly above $E_F$ along $\Gamma$-$K$. In addition, an unusual band splitting is observed in the vicinity of the Fermi surface along the $\Gamma$-$M$ direction, which differs from the predicted bilayer splitting.

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