利用飛秒雷射脈衝在La0.42Ca0.58MnO3薄膜中產生與偵測同調聲頻聲子之研究

Abstract

在許多材料中，舉凡超導體、半導體、磁性材料等都曾透過超快激發－探測實驗，觀察同調聲子振盪行為。特別是在本實驗室感興趣的La1-xCaxMnO3龐磁阻材料，在幾個掺鈣含量x=0、x=0.3、x=0.58，也都呈現明顯的同調聲子振盪，雖然形成同調聲子振盪的機制有幾種不同的理論模型，但互相仍有所爭議。在本論文中，我們已經運用脈衝雷射蒸鍍成長不同厚度的La0.42Ca0.58MnO3薄膜，透過超快激發－探測實驗量測不同溫度時鑭鈣錳氧薄膜的瞬時反射率變化(ΔR/R)隨延遲時間振盪情形，我們在ΔR/R訊號中，發現一個週期為數百皮秒(ps)較慢的振盪，其振盪並進ㄧ步被一個週期為數十皮秒較快的振盪所調制著。這些振盪的特性可透過由Thomsen 研究團隊所提出的「超快激發形變脈衝」理論模型[8]來做解釋。另外，透過這些實驗結果我們計算出在樣品中不同溫度時的聲速，並與頻率為幾個megahertz的傳統「脈衝-回音-重疊(pulse-echo-overlap)」技術所得到的聲速結果一致[16][17]，而且我們也觀察到在電荷有序排列相轉變溫度附近時聲速的異常行為。

Coherent acoustic phonon oscillations (CAPO) have been observed in many materials, such as superconductor, semiconductor, and magnetic materials using ultrafast pump-probe spectroscopy. Especially the colossal magnetoresistance manganates La1-xCaxMnO3 , one of interesting materials in our lab, exhibited strongly CAPO at various doping concentrations (x=0, x=0.3 and x=0.58). However, several mechanisms about the generation and detection of CAPO have been proposed and disputed each other. In this thesis, we have prepared La0.42Ca0.58MnO3 thin films with various thicknesses by pulsed laser deposition and have measured the transient reflectivity (ΔR/R) as a function of a pump-probe delay at various temperatures. We found that a slower oscillation with a period of hundreds of ps in ΔR/R is further modulated with a faster oscillation with a period of tens of ps. The characterization of these oscillations can be elucidated by the ultrafast generated strain pulse model, which was proposed by Thomsen [8]. 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 [16][17]. The anomaly of sound velocity near the charge ordering phase transition has been observed.