Novel Aspects of Correlated Electron Motion in (K/Na)xCoO2 systems Revealed by Advanced Photoelectron Spectroscopy*

Recent discovery of superconductivity, large thermopower and other unusual transport behavior in cobalt oxides (Na$_{0.3}$CoO$_{2}$.nH$_{2}$O) have generated much interests in understanding the many-body character of the electron liquid in this system. We employ advanced photoelectron spectroscopic techniques to reveal the microscopic electron dynamics in Na$_{x}$CoO$_{2}$ and K$_{x}$CoO$_{2}$ system classes. Results directly unveil an unusual and novel nature of many-body electron motion manifested through strong bandwidth suppression, heavy-fermion-like carrier mass and unusually slow movement of low-lying electrons despite the presence of a large Fermi surface indicating a two orders of magnitude departure from the conventional BCS paradigm. Temperature dependence of quasiparticles in the over-doped cobaltates shows that spectral weight remains well defined in the T-linear resistivity regime. Unusually small single-particle hopping and unconventional quasiparticle dynamics have direct implications for understanding the novel phase of matter realized in this new class of strongly interacting complex system.