以時間解析光譜研究鑭鍶銅氧薄膜異向性之超快動力學

一般認為，銅氧化物超導體的銅氧平面在高溫超導機制上扮演了極重要的角色。鑭鍶銅氧(LSCO: La2-xSrxCuO4)其晶格為具高對稱性並含有銅氧平面的tetragonal結構。近來， J. Wu et al團隊發現了鑭鍶銅氧薄膜的電阻率變化隨著量測角度的改變會表現二重對稱，且此二重對稱的強度與鑭鍶銅氧中的鍶摻雜量有關，此一發現打破了對tetragonal晶格的對稱性及高溫超導體電傳輸性質的認知。更者，我們團隊亦利用了飛秒雷射時間解析光譜之激發-探測光偏振方向技術，成功的發現了La1.95Sr0.06CuO4 在低溫時的二重對稱，此結果與2017年J. Wu et al團隊等人之發表文獻結果相符。故此論文中，我們嘗試其他不同摻雜之La2-xSrxCuO4 (x=0.02、x=0.05、x=0.16)，以時間解析光譜探測並比較不同摻雜之LSCO低溫到高溫時的時間解析光譜變化，而LSCO (x = 0.02, 0.05)在低溫之時析光譜皆顯示出一低谷訊號，且在100K時該低谷訊號皆完全消失。另外，我們也在La1.95Sr0.05CuO4中觀察到二重對稱，而藉由物理公式擬合，我們發現在30K時，此二重對稱主要來源於負峰值項，而當溫度大於50K小於100K時，此二重對稱來自於樣品內部載子受到激發後，弛緩程度不同導致。

關鍵字

超快雷射；超導體；時間解析光譜；鑭鍶銅氧

並列摘要

Copper oxide planes play an important role in cuprate superconductors. La2-xSrxCuO4 (LSCO) is a tetragonal crystal system which has four-fold symmetry in the crystal structure. Recently, J. Wu et al found that the resistivity change of LSCO thin film showed the two-fold symmetry through varying the measurement current angles, and the magnitude of two-fold symmetry will vary with the doping level of LSCO. This discovery breaks with the conventional concept of the symmetry of the tetragonal lattice systems. Furthermore, our research team successfully observed the nematicity (two-fold symmetry) of La1.95Sr0.05CuO4 at low temperatures by two color pump probe system time resolved spectroscopy. This result agrees with the conclusion in 2017 by J. Wu et al. Therefore, in this thesis we tried other La2-xSrxCuO4 (x=0.02、x=0.05、x=0.16) with various doping by using two color pump probe system and compared the difference between low and high temperatures. The time-resolved spectroscopy results of LSCO (x = 0.02, 0.05) showed the negative signals in low temperatures, whereas all of these negative signals disappeared above 100K. We found that nematicity also exists in La1.95Sr0.05CuO4 thin film. Through the model fitting, at 30K, the nematicity is due to the negative term. When the temperature is between 50K and 100K, the electronic component plays an important role.