Infrared conductivity of NaxCoO2:: Evidence of gapped states -: art. no. 024549

We present infrared ab-plane conductivity data for the layered cobaltate NaxCoO2 at three different doping levels (x=0.25, 0.50, and 0.75). The Drude weight increases monotonically with hole doping, 1-x. At the lowest hole doping level x=0.75 the system resembles the normal state of underdoped cuprate superconductors with a scattering rate that varies linearly with frequency and temperature and there is an onset of scattering by a bosonic mode at 600 cm(-1). Two higher hole-doped samples (x=0.50 and 0.25) show two different-size gaps (110 cm(-1) and 200 cm(-1), respectively) in the optical conductivities at low temperatures and become insulators. The spectral weights lost in the gap region of 0.50 and 0.25 samples are shifted to prominent peaks at 200 cm(-1) and 800 cm(-1), respectively. We propose that the two gapped states of the two higher hole-doped samples (x=0.50 and 0.25) are pinned charge-ordered states.