釔鋇銅氧/鉍鐵氧異質結構之超快動力學

Abstract

本論文利用極化雙色激發探測光譜研究YBCO/BFO異質結構之超快動力學行為，從不同溫度的瞬時反射率變化(ΔR/R)來研究YBCO在超導態與正常態的光激載子弛緩行為，且藉由改變BFO的厚度來探討YBCO/BFO異質結構的光激載子弛緩異向性。首先分別量測單層YBCO薄膜及單層BFO薄膜之ΔR/R，以釐清YBCO/BFO異質結構中複雜的ΔR/R光譜訊號。我們在YBCO/BFO異質結構上同時觀察到正的ΔR/R訊號與負的ΔR/R訊號，經過指數衰減函數的擬合分析，我們發現正的ΔR/R訊號其行為與單層YBCO薄膜的結果相似。至於負的ΔR/R訊號，則是受YBCO底下的BFO影響所造成，而BFO的條狀電偶極疇域(沿[1-10]方向)及其疇壁上的淨磁矩更進一步造成YBCO中準粒子在弛緩(回復成古柏對的過程)上的異向性，此結果可讓我們理解[001]方向的YBCO超導臨界溫度較[1-10]方向低(同時轉變範圍也較寬)的微觀機制。

In this thesis, we study the ultrafast dynamics of YBCO/BFO heterostructures by using dual-color pump-probe spectroscopy. The relaxation behavior of photoexcited carriers in superconducting state and normal state of YBCO can be revealed by measuring the transient reflectivity changes (ΔR/R) at various temperatures; meanwhile, the anisotropy of quasiparticle relaxation in YBCO/BFO heterostructures can be observed through varying the thickness of the BFO layer. At first, we measured the ultrafast spectra of a single-layer YBCO thin film and a single-layer BFO thin film to verify the complicate ΔR/R signals in YBCO/BFO heterostructures, which are composed of two components, i.e., positive (ΔR/R>0 before 15 ps) and negative (ΔR/R<0 after 15 ps) signals. By fitting with the exponential decay functions, we found that the positive component in ΔR/R is similar to that of a single-layer YBCO thin film. On the other hand, the negative component in ΔR/R is due to the BFO layer under the YBCO layer. Moreover, the BFO layer with stripped ferroelectric domains and the net magnetic moments at the domain walls would further lead to the anisotropy of quasiparticle relaxation and superconducting transition temperatures (Tc) in YBCO/BFO heterostructures.