使用離軸射頻磁控濺鍍技術在TiO2/SrO終端之鈦酸鍶基板上生長釔鋇銅氧薄膜的超導特性之研究

Abstract

本實驗先拋光確定鈦酸鍶(SrTiO3)(100)基板有良好的平整度，之後對基板進行熱處理，使鈦酸鍶基板有TiO2或SrO為終端的2種表面，然後用射頻磁控濺鍍系統(magnetron sputteirng)，以Tg = 720 ℃、功率90 W與壓力400 mtorr的條件下成長高溫超導體釔鋇銅氧(YBCO)薄膜，最後進行量測。經由4點量測比較3個樣品的臨界溫度(Tc)，YBCO Tc = 85.4 K、TiO2-YBCO Tc = 86.8 K、SrO-YBCO Tc = 86.7 K，可以得知在經過熱處理後的基板成長YBCO薄膜會有更好的Tc。之後利用SQUID量測進行比較發現經過熱處理的樣品一樣擁有較高的超導轉變溫度，但在將磁化強度轉換成磁化率時發現在SrO-YBCO中發現有明顯之順磁性效應，此效應即為順磁性麥斯納效應。

之後我們針對YBCO、TiO2-YBCO和SrO-YBCO的磁性量測做比較，YBCO、TiO2-YBCO和SrO-YBCO的Hc1(0)分別6.82 Oe、10.03 Oe和10.41 Oe，Hc2(0)分別為4.59 T、5.31 T和5.11 T，之後再計算出YBCO、TiO2-YBCO和SrO-YBCO的相干長度(ξ)與倫敦穿透深度(λ)，最後利用磁滯曲線計算出臨界電流密度(Jc)，利用外加磁場(H)與臨界電流密度(Jc)擬和釘扎力(Fp)與外加磁場(H)關係圖。藉由擬合釘扎力的結果可以推斷YBCO多數在二維釘札附近，與SrO-YBCO差不多，但TiO2-YBCO多數值都在一、二維混合釘札。In this experiment, the SrTiO3(100) substrates are first polished to ensure a smooth surface. Subsequently, the substrates undergo thermal treatment to produce two types of surface terminations: TiO2-terminated and SrO-terminated. The (YBCO) thin films are then grown on these substrates using a radio frequency magnetron sputtering system under the conditions of a substrate temperature of 720°C, a power of 90 W, and a pressure of 400 mtorr. Finally, measurements are performed on the prepared thin films.Through four-point measurements, the critical temperatures (Tc) of three sam-ples were compared: YBCO with Tc = 85.4 K, TiO2-YBCO with Tc = 86.8 K, and SrO-YBCO with Tc = 86.7 K. These results indicate that YBCO thin films grown on thermally treated substrates exhibit improved Tc. Subsequent SQUID measurements confirmed that samples with thermally treated substrates maintained higher super-conducting transition temperatures. However, when the magnetization was converted to magnetic susceptibility, a noticeable paramagnetic effect was observed in the SrO-YBCO sample, identified as the paramagnetic Meissner effect.Subsequently, we compared the magnetic measurements of YBCO, TiO2-YBCO, and SrO-YBCO. The lower critical fields, Hc1(0), for YBCO, TiO2-YBCO, and SrO-YBCO were determined to be 6.82 Oe, 10.03 Oe, and 10.41 Oe, respectively, while the upper critical fields, Hc2(0), were found to be 4.59 T, 5.31 T, and 5.11 T, respectively. We then calculated the coherence lengths (ξ) and London penetration depths (λ) for each sample. Additionally, the critical current densities (Jc) were de-rived from the magnetic hysteresis loops, and the pinning force (Fp) versus applied magnetic field (H) relationship was fitted using the Jc -H data. The fitting results suggest that the majority of pinning in YBCO is associated with two-dimensional pinning centers, similar to SrO-YBCO, whereas TiO2-YBCO shows a mixed pinning regime with contributions from both one-dimensional and two-dimensional pinning centers.