利用飛秒激發探測光譜量測鑭鈣錳氧薄膜中的相干極化子

Abstract

在本論文中，我們用超快激發檢測系統(optical pump-probe spectroscopy, OPOP)來探測掺鈣鑭錳氧(La0.7Ca0.3MnO3)薄膜中，極化子(polaron)的動力學。瞬時反射率的變化 主要由兩個部份所構成：弛緩時間在次皮秒級(sub-picosecond)的快弛緩行為，以及弛緩時間在數十到百個皮秒的慢弛緩行為。其中慢弛緩行為的部份是對應自旋－晶格(spin-lattice interaction)的弛緩行為，與自旋比熱有關，且已經被數個團隊討論，但是關於快弛緩行為卻鮮少被討論，我們發現隨溫度變化的快弛緩行為與電阻隨溫度的變化、極化子造成的中子散射強度的變化相似，因此，由我們超快激發檢測的實驗結果推測，快反射率變化的成因，是由於Jahn-Teller distortion極化子，被入射光激發之後，獲得足夠的能量巡游，再被晶格捕抓住的過程，故可藉由OPOP實驗釐清在絕緣-導體/順磁-鐵磁相變中，極化子所扮演的角色；以及樣品在從高溫過轉折溫度Tc到低溫，相干極化子(correlated polaron)在光子激發之後的弛緩過程。

In this work, we use ultrafast optical pump-optical probe (OPOP) spectroscopy to probe the correlated polaron dynamics in La0.7Ca0.3MnO3 (LCMO) thin film. The temporal evolution in transient reflectivity change □R/R is composed of two relaxing parts: a fast component with time sub picoseconds and a slow component with time constant ranging from tens of ps to hundreds of ps. The slow component, which corresponds to the spin-lattice interaction and the temperature dependence of the slow relaxation time follows the spin specific heat, has discussed by many groups. On the part of the amplitude of the fast component, though has been rarely discussed before, exhibits the similar temperature dependence with that of the resistivity, and the neutron scattering intensity due to nanoscale correlated polarons. The results strongly suggest that the fast photoinduced reflectivity change may have been due to the photoexcitation and trapping process of correlated Jahn–Teller (J-T) polarons in the paramagnetic (PM) and ferromagnetic (FM) phases.