利用超快光激發探測技術研究La0.42Ca0.58MnO3薄膜

Abstract

在本論文中，我們透過超快光激發探測系統量測La0.42Ca0.58MnO3薄膜隨溫度及探測波長變化時，鑭鈣錳氧薄膜的瞬時反射率變化 隨著延遲時間的變化情形。並且利用Y. Tokura團隊所採用的數學擬合公式搭配數學軟體Origin做數學擬合的工作，針對四個不同的弛緩分量做解析。 前兩個弛緩分量分別為弛緩時間次皮秒到數個皮秒和數百個皮秒的快、慢弛緩行為。其中，快弛緩行為的振幅和電阻以及中子散射隨溫度變化極為相似，是對應相關聯極化子的行為。而慢弛緩振幅則和磁化強度隨溫度變化有類似的轉折點，應該和「自旋-晶格」的耦合行為相關。 另外兩個弛緩分量為週期數十皮秒和數百皮秒的快、慢振盪。振盪的特性透過C. Thomsen團隊提出的「超快激發形變脈衝」理論模型來解釋，並計算出聲速在薄膜中隨溫度變化的情形，和利用傳統「脈衝-回音-重疊」技 術所得到的聲速結果一致。

In this thesis, we have used ultrafast optical pump optical probe spectroscopy to measure the transient reflectivity change in La0.42Ca0.58MnO3 thin films as a function of delay time at various temperatures and wavelengths. A typical transient reflectivity change curve can be separated into four components; Namely, fast component, slow component, fast oscillation component and slow oscillation component. We found that the relaxation time of the fast and the slow components were several ps and hundreds of ps respectively. The amplitude of the fast component exhibits the similar temperature dependence with that of the resistance and the neutron scattering intensity due to nanoscale correlated polarons and the amplitude of the slow component exhibits the similar temperature dependence with that of the magnetization due to 「spin-lattice」coupling. In addition, the slow oscillation with period of hundreds of ps in ΔR/R is further modulated with fast oscillation with a period of tens of ps in the curves. The characterization of these oscillations can be elucidated by the ultrafast generated strain pulse model. Moreover, the ultrasound velocity at various temperatures also has been calculated in this material and is consistent with that generated by a conventional pulse-echo-overlap technique with a frequency of several megahertz.