La0.8Ba0.2MnO3 與 La0.7MnO3 薄膜之光譜性質研究
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2007
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我們研究 30 nm 和 150 nm 厚度之 La0.8Ba0.2MnO3 薄膜以及三種熱退火條件 (無熱退火、在 O2 中熱退火、在 Ar 中熱退火) 之 350 nm 厚度 La0.7MnO3 薄膜的光譜性質，分別探討應力形變之於薄膜厚度、和不同熱退火條件下對錳氧化物之晶格與電子結構造成的影響。
我們觀察到 30 nm 厚度 La0.8Ba0.2MnO3 薄膜在遠紅外光區的 Mn-O 伸張振動模，其頻率位置低於 150 nm 厚度的薄膜，紅移現象指出 c 軸方向之 Mn-O 距離變長，顯示 30 nm 厚度薄膜可能受到比 150 nm 厚度薄膜更大的壓縮應力形變，導致薄膜厚度愈薄，其居禮溫度愈高。此外，在 O2 中熱退火 La0.7MnO3 薄膜的Mn3+ eg → Mn4+ eg 吸收峰頻率最低，顯示電荷躍遷機率變大。中紅外光區吸收峰頻率隨著溫度下降都有紅移現象且伴隨居德權重增加，顯示樣品低溫下趨向金屬態。
最後，我們使用大、小極化子模型擬合樣品的紅外光區光學電導率，發現 La0.8Ba0.2MnO3 薄膜的實驗數據與自我侷限大極化子模型的擬合曲線吻合，而 La0.7MnO3 薄膜適合以小極化子模型分析。
We study the optical properties of 30- and 150-nm-thick La0.8Ba0.2MnO3 (LBMO) thin films and 350-nm-thick La0.7MnO3 (LMO) thin films with different annealing processes. Our goal is to understand the effects of the strain and different annealing treatments of these manganites thin films on the lattice and charge dynamics. The frequency of Mn-O stretching mode of the 30-nm-thick LBMO thin film is lower than that of 150 nm one, indicating a decreased force constant by c-axis dilatation. This reveals the former one has larger compressive strain effect, resulting in higher Curie temperature. In addition, the absorption band between Mn3+ eg → Mn4+ eg states of LMO thin films with annealing in O2 shows a shift of the peak position to lower frequencies, that reveals the probability increasing of charge transition. With decreasing temperature, the energy of two midinfrared peaks of these thin films decreases, and a corresponding systematic enhancement of Drude spectral weight, indicating these thin films become more metallic at low temperature. Finally, the pronounced resonance in the midinfrared frequency range of the LBMO and LMO thin films can be well understood in the framework of self-trapped large polaron and small polaron models, respectively.
We study the optical properties of 30- and 150-nm-thick La0.8Ba0.2MnO3 (LBMO) thin films and 350-nm-thick La0.7MnO3 (LMO) thin films with different annealing processes. Our goal is to understand the effects of the strain and different annealing treatments of these manganites thin films on the lattice and charge dynamics. The frequency of Mn-O stretching mode of the 30-nm-thick LBMO thin film is lower than that of 150 nm one, indicating a decreased force constant by c-axis dilatation. This reveals the former one has larger compressive strain effect, resulting in higher Curie temperature. In addition, the absorption band between Mn3+ eg → Mn4+ eg states of LMO thin films with annealing in O2 shows a shift of the peak position to lower frequencies, that reveals the probability increasing of charge transition. With decreasing temperature, the energy of two midinfrared peaks of these thin films decreases, and a corresponding systematic enhancement of Drude spectral weight, indicating these thin films become more metallic at low temperature. Finally, the pronounced resonance in the midinfrared frequency range of the LBMO and LMO thin films can be well understood in the framework of self-trapped large polaron and small polaron models, respectively.
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LBMO, LMO