Optical Study of Nanosize Effects on Charge Ordering in Half-Doped Manganites
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- 作者：Shiming Zhou ; Yuqiao Guo ; Zhiquan Jiang ; Jiyin Zhao ; Xuan Cai ; Lei Shi
- 刊名：The Journal of Physical Chemistry C
- 出版年：2013
- 出版时间：May 2, 2013
- 年：2013
- 卷：117
- 期：17
- 页码：8989-8996
- 全文大小：376K
- 年卷期：v.117,no.17(May 2, 2013)
- ISSN：1932-7455
文摘
Optical spectra are very sensitive to investigate strong charge correlations in transition-metal oxides. Recent studies on nanosized half-doped manganites frequently reported that the robust charge ordering present in the bulk is strongly weakened or suppressed by reducing the sample size to nanoscale; however, the origin of the novel phenomenon is not clearly understood until now. Here, we study this nanosize effect through infrared spectra on Nd0.5Ca0.5MnO3 nanoparticles with different particle sizes. Optical phonon modes demonstrate that the cooperative Jahn鈥揟eller distortion associated with charge ordering is not only clearly observed in the large nanoparticles with a long-range charge-ordered transition but also visible in the small nanoparticles where such the transition is completely suppressed. Lattice distortion is interestingly found to be weakened at low temperatures but enhanced at the high-temperature paramagnetic regime by the size reduction, which is responsible for the change in magnetization with particle size. Detailed analysis on the optical density further reveals that size-induced suppression of the charge-ordered transition does not mean a close of the charge gap, which still persists with just a slight decrease in the energy and exhibits no obvious change in the dependence on the temperature. Localization of charge carriers increases with decreasing particle size, evidenced by the decrease in the effective number of carriers. Taken together, our findings give good evidence that a short-range charge-ordered state exists in the half-doped manganite nanoparticles when the long-range one is destroyed by the size reduction. More importantly, these features can be well ascribed to originate from the surface disorder effects. Our optical study provides deep insight into the nanosize effects on charge ordering in the transition-metal oxides.