The terahertz (THz) optical conductivity of charge-ordered nickelate ${\mathrm{SmNiO}}_3$ thin films depicts a unique dependence of the charge-density-wave (CDW) type instability on the film thickness. Here, we report a rare observation of two dominant resonance modes in the THz conductivity spectrum. Corroborated by structural and electrical conductivity data, we demonstrate that these doublet excitations are the attribution of the CDW type pinned and bound modes. Both the peak position and the peak strength can be controlled by the film thickness and the defect density associated with oxygen stoichiometry. The oxygen vacancies in these films act like defects/impurities and induce charge oscillation around them which get coupled with unperturbed CDW mode of the charge-ordered phase. Thus, an additional bound mode emerges alongside the pinned mode. The defect density as controlled by the film thickness allows modulation of the resonance excitations in the desired THz frequency range.

中文翻译：

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太赫兹光谱揭示了SmNiO3中电荷密度波型集体激发的束缚和束缚模式

电荷排序的镍酸盐的太赫兹（THz）光电导率 ${\mathrm{镍锰}}_3$薄膜描绘了电荷密度波（CDW）类型的不稳定性对膜厚度的独特依赖性。在这里，我们报告了太赫兹电导率谱中两个主要共振模式的罕见观察。通过结构和电导率数据的证实，我们证明了这些双重激发是CDW型固定和键合模式的归因。峰位置和峰强度都可以通过与氧化学计量有关的膜厚度和缺陷密度来控制。这些薄膜中的氧空位起缺陷/杂质的作用，并在其周围引起电荷振荡，这与电荷有序相的CDW模态无关。因此，在绑定模式旁边会出现一个附加的绑定模式。