Optical properties of layered cobaltates

Layered NaxCoO2 system has generated great interests in the condensed matter community for it realizes a triangular lattice with partially filled strongly correlated electronic states. With change of Na content x, a rich phase diagram was revealed. We present in-plane optical spectroscopy measurement on this system and address features specific to different phases in the phase diagram. We find that the optical spectral weight increases monotonously with decreasing x from the band insulator x=1. The evolution suggests against a doped Mott insulator for NaxCoO2 system. For x∼0.5 charge-ordering sample, a small energy gap forms below TMIT. In addition, an electronic resonance develops near 800cm-1 below 100 K. We suggest that this feature corresponds to a new transverse plasma mode. For x=0.85 crystal in the A-type antiferromagnetic phase or misfit-layered Bi2Sr2Co2Oy compound, the low temperature reflectance is suppressed at the energy similar to x∼0.5 sample in the charge-ordering state. While for x≤0.3 compounds, a pseudogap structure similar to underdoped high-Tc cuprates develops at low temperature