Physics – Condensed Matter – Strongly Correlated Electrons
Scientific paper
2004-05-11
Phys. Rev. Lett. 93, 147403 (2004) (Minor changes were made in the journal paper)
Physics
Condensed Matter
Strongly Correlated Electrons
4 pages, 3 figures
Scientific paper
10.1103/PhysRevLett.93.147403
We report infrared spectra of a Na$_{0.5}$CoO$_2$ single crystal which exhibits a sharp metal-insulator transition near 50 K due to the formation of charge ordering. In comparison with x=0.7 and 0.85 compounds, we found that the spectral weight associated with the conducting carriers at high temperature 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 anti-resonance feature of phonon spectra. At lower temperature $T_{co}$, an optical gap develops at the magnitude of 2$\Delta\approx3.5k_BT_{co}$, evidencing an insulating charge density wave ground state. Our experimental results and analysis unequivocally point towards the importance of charge ordering instability and strong electron-phonon interaction in Na$_x$CoO$_2$ system.
Chen Hua-Xing
Li Gaojun
Luo X. G.
Wang Chang H.
Wang Nan-Lin
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