Photoemission and x-ray absorption study of the two-dimensional triangular lattice superconductorNa0.35CoO2·1.3H2O

Abstract

We have investigated the electronic structure of a triangular lattice superconductor ${\mathrm{Na}}_{0.35}{\mathrm{CoO}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}1.3{\mathrm{H}}_{2}\mathrm{O}$ $({T}_{c}\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ by photoemission and x-ray absorption spectroscopy. The $\mathrm{Co}$ $2p$ x-ray absorption and $\mathrm{Co}$ $2p--3d$ resonant photoemission clarified that the valence band near the Fermi level is mainly constructed by the narrow $\mathrm{Co}$ $3d$ ${t}_{2g}$ states at the binding energy of $1.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with the low-spin configuration. In the resonant photoemission, Auger features coexist with the resonant enhancements, while the charge-transfer satellites are observed in the photoemission spectra of a $\mathrm{Co}$ $2p$ core levels. It is also expected that the narrow ${t}_{2g}$ band reflects on the importance of an electron--phonon coupling. Besides, $\mathrm{O}$ $1s$ and $\mathrm{Co}$ $1s$ x-ray absorption spectroscopy reveal that intercalated water condenses into a two-dimensional ice and showing no influence on a carrier doping.