Anomalous electronic raman scattering in NaxCoO2. yH2O

Raman scattering experiments on NaxCoO2·yH2O single crystals show a broad electronic continuum with a pronounced peak around 100  cm-1 and a cutoff at approximately 560  cm-1 over a wide range of doping levels. The electronic Raman spectra in superconducting and nonsuperconducting samples are similar at room temperature, but evolve in markedly different ways with decreasing temperature. For superconducting samples, the low-energy spectral weight is depleted upon cooling below T*∼150  K, indicating the opening of a pseudogap that is not present in nonsuperconducting materials. Weak additional phonon modes observed below T* suggest that the pseudogap is associated with charge ordering.