Laser-excited ultrahigh-resolution photoemission spectroscopy of Na x CoO 2 · y H 2 O : Evidence for pseudogap formation

We have studied the temperature $(T)$-dependent electronic structure near the Fermi level of the layered cobaltate superconductor, ${\mathrm{Na}}_{0.35}{\mathrm{CoO}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}1.3{\mathrm{H}}_{2}\mathrm{O}$, and related materials, using laser-excited ultrahigh-resolution photoemission spectroscopy. We observe the formation of a pseudogap (PG) with an energy scale of $\ensuremath{\sim}20\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ in ${\mathrm{Na}}_{0.35}{\mathrm{CoO}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}1.3{\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{Na}}_{0.35}{\mathrm{CoO}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}0.7{\mathrm{H}}_{2}\mathrm{O}$, which is clearly absent in ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_{2}$. The energy scale of the PG is larger than the expected value for the superconducting gap, suggesting an additional competing order parameter at low $T$. We discuss implications of the PG in relation to available transport and magnetic susceptibility results.