Infrared Spectroscopy of the Charge Ordering Transition of N a 0.5 C o O 2
2004; American Physical Society; Volume: 93; Issue: 14 Linguagem: Inglês
10.1103/physrevlett.93.147403
ISSN1092-0145
AutoresN. L. Wang, Dong Wu, Gang Li, Xianhui Chen, C. H. Wang, X. G. Luo,
Tópico(s)Magnetic and transport properties of perovskites and related materials
ResumoWe report infrared reflectivity study of charge ordering in a ${\mathrm{N}\mathrm{a}}_{0.5}{\mathrm{C}\mathrm{o}\mathrm{O}}_{2}$ 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 $1000\text{ }\text{ }{\mathrm{c}\mathrm{m}}^{\ensuremath{-}1}$. A hump near $800\text{ }\text{ }{\mathrm{c}\mathrm{m}}^{\ensuremath{-}1}$ 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 ${T}_{\mathrm{c}\mathrm{o}}$, an optical gap develops at the magnitude of $2\ensuremath{\Delta}\ensuremath{\approx}3.5{k}_{B}{T}_{\mathrm{c}\mathrm{o}}$ ($T\ensuremath{\ll}{T}_{\mathrm{c}\mathrm{o}}$), evidencing an insulating charge density wave ground state.
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