利用飛秒光譜量測La0.85Ca0.15MnO3薄膜中的相干極化子及超快動力行為之研究

Abstract

在本論文中，我們利用脈衝雷射蒸鍍法製備了La0.85Ca0.15MnO3薄膜，並利用超快激發探測實驗量測不同溫度下的瞬時反射率變化ΔR/R隨著延遲時間變化的情形。我們觀察皮秒快速弛緩行為的振幅隨著溫度變化的情形，在電荷有序溫度TCO以上，發現振幅和相干極化子的數目隨溫度的變化有類似的趨勢，和之前La0.7Ca0.3MnO3的情形相符;而在TCO以下的趨勢，在K. Miyasaka團隊的Nd0.5Sr0.5MnO3的研究結果中發現振幅和電荷有序的超晶格強度變化相符，而我們的結果也是有此趨勢。整體趨勢來說，快速弛緩行為都和相干極化子的數目隨溫度變化相同，因此證實快速弛緩行為對應的仍是相干極化子的動力行為。我們也分析快速弛緩時間隨溫度的變化，其對應的是電荷有序機制的強弱。在鐵磁有序溫度TC以上，並沒有隨著溫度明顯變化的趨勢；到了TC以下，電荷有序機制受到鐵磁相的壓迫而變弱，因此回復時間(弛緩時間)變長。

In this thesis, we have prepared La0.85Ca0.15MnO3 thin films by pulsed laser deposition, and have measured the transient reflectivity change ΔR/R as a function of a pump-probe delay at various temperatures. We analyze the amplitude of picosecond fast relaxation behavior at various temperatures. Above the charge ordering temperature TCO, we observe the similar temperature dependence between the amplitude of the fast component and the correlated polaron population. This result is consistent with that of La0.7Ca0.3MnO3 case, which we reported previously. Below TCO, the temperature dependence of the amplitude are similar to that of the charge ordering superlattice peak intensity, which is consistent with the results of Nd0.5Sr0.5MnO3 reported by K. Miyasaka et al. In addition, we analyze the relaxation time of the fast component, which corresponds to the strength of charge ordering. Above TC, the relaxation time is weakly temperature dependent. On the other hand, below TC, the charge ordering mechanism are suppressed by the prevailing of ferromagnetism, so the recovery time (relaxation time) is slow. Below TCO, the charge ordering mechanism is stronger, so the recovery time becomes faster.