Laser-excited ultrahigh-resolution photoemission spectroscopy of superconducting Na0.35CoO2·1.3H2O

We have studied the electronic structure near the Fermi level of the layered cobaltate superconductor Na0.35CoO2·1.3H2O using laser-excited ultrahigh-resolution photoemission spectroscopy (PES). In the normal state, we observed the formation of a pseudogap with an energy scale of ∼20 meV. Across the superconducting transition, we observed the leading-edge shift at 3.5 K compared to that at 10 K, indicating superconducting gap (SC gap) opening having a gap value Δ of 0.5–0.7 meV. The energy scale of the SC gap is ∼20 times smaller than that of the pseudogap, suggesting the existence of competing order parameters at low temperatures.