title: Infrared spectroscopy of the charge ordering transition of Na0.5CoO2.
abstract: We report infrared reflectivity study of charge ordering in a Na0.5CoO2 single crystal. In comparison with x=0.7 and 0.85 compounds, we found that the effective carrier density increases systematically with decreasing Na contents. The charge ordering transition only affects the optical spectra below 1000 cm(-1). A hump near 800 cm(-1) develops below 100 K, which is accompanied by the appearance of new lattice modes as well as the strong antiresonance feature of phonon spectra. These observations signify a polaronic characteristic of charge carriers. Below T(co), an optical gap develops at the magnitude of 2Delta approximately 3.5k(B)T(co) (T<<T(co)), evidencing an insulating charge density wave ground state.
authors.affiliation: Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People's Republic of China. nlwang@aphy.iphy.ac.cn
authors.forename: N L
authors.initials: NL
authors.lastname: Wang
authors.forename: Dong
authors.initials: D
authors.lastname: Wu
authors.forename: G
authors.initials: G
authors.lastname: Li
authors.forename: X H
authors.initials: XH
authors.lastname: Chen
authors.forename: C H
authors.initials: CH
authors.lastname: Wang
authors.forename: X G
authors.initials: XG
authors.lastname: Luo