Wang N L, Wu Dong, Li G, Chen X H, Wang C H, Luo X G
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People's Republic of China.
Phys Rev Lett. 2004 Oct 1;93(14):147403. doi: 10.1103/PhysRevLett.93.147403. Epub 2004 Sep 27.
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.
我们报道了对Na0.5CoO2单晶中电荷有序化的红外反射率研究。与x = 0.7和0.85的化合物相比，我们发现有效载流子密度随着Na含量的降低而系统地增加。电荷有序化转变仅影响低于1000 cm(-1)的光谱。在100 K以下出现了一个位于800 cm(-1)附近的峰，同时伴随着新晶格模式的出现以及声子谱的强反共振特征。这些观察结果表明了电荷载流子的极化子特性。在T(co)以下，出现了一个大小约为2Δ ≈ 3.5k(B)T(co)（T << T(co)）的光学能隙，证明了绝缘电荷密度波基态的存在。
