釔鋇銅氧/鉍酸鋇氟參雜雙層薄膜於氧化鎂(001)基板上之 超導特性研究

Abstract

本研究利用射頻磁控濺鍍法，於氧化鎂 MgO (001) 基板上製備厚度為 50 nm 之釔鋇銅氧（YBa₂Cu₃O₇−δ，YBCO）單層薄膜，以及氟摻雜鉍酸鋇（BaBi(O,F)₃，BBOF）厚度分別為 10、15 與 20 nm 之 BBOF/YBCO 雙層薄膜，探討 BBOF 覆蓋層厚度對 YBCO 晶體結構、磁性、超導傳輸及磁通動力學之影響。樣品分別透過 X 光繞射、超導量子干涉儀及低溫四點式電性量測進行分析。 結構分析顯示，覆蓋 BBOF 後，YBCO 層仍主要維持沿 c軸方向之優選取向；然而，隨 BBOF 厚度增加，YBCO 特徵繞射峰之位置與峰形產生變化，顯示異質界面附近可能存在晶格失配、局部應變、結構缺陷及氧空缺重新分布等效應。由於 BBOF 與 YBCO 之晶格常數差異較大，界面應力可能透過局部晶格扭曲與結構鬆弛加以釋放，進而影響 YBCO 層之超導性質。 磁性與電性量測結果顯示，具有較薄 BBOF 覆蓋層之 Y50B10 樣品呈現較完整的超導轉變、較佳的電流連通性，以及較高的有效臨界電流密度與磁通釘扎力。隨 BBOF 厚度由 10 nm 增加至 20 nm，樣品之正常態電阻率與低溫殘餘電阻增加，超導轉變溫度下降且轉變區域逐漸展寬；有效磁通活化能亦呈現 Y50B10、Y50B15 至 Y50B20 依序降低的趨勢，顯示較厚的 BBOF 覆蓋層會削弱 YBCO 層對磁通運動的限制能力。Y50B15 的整體表現大致介於 Y50B10 與 Y50B20 之間，反映雙層薄膜由較佳超導連通性逐步過渡至明顯受抑制狀態。 熱激發磁通流與電阻式上臨界磁場分析顯示，三組樣品皆具有明顯的磁場方向依賴性，反映 YBCO 層狀晶體結構所造成的超導各向異性。其中，磁場平行 ab面時之外插上臨界磁場隨 BBOF 厚度增加而降低，顯示該方向之超導穩定性逐漸減弱；磁場平行 c軸方向的外插結果則未呈現相同的單調趨勢。由於 Y50B20 在低溫下仍具有明顯殘餘電阻，未完全進入理想零電阻狀態，因此其電阻式上臨界磁場與相干長度應視為電阻轉變所對應的表觀特徵參數，主要用於不同樣品間之相對比較。 Berezinskii–Kosterlitz–Thouless（BKT）相變分析顯示，Y50B10 與 Y50B15 在零磁場下皆呈現隨溫度降低而增強的非線性電流－電壓傳輸，其冪次指數約於 82–83 K 通過理論判準 α=3，顯示兩組樣品具有 BKT 型相位有序特徵。施加 10 Oe 垂直磁場後，Y50B10 的特徵交越溫度下降至約 80–81 K，反映外加磁場所引入的自由渦旋會削弱超導相位有序；Y50B15 在目前的溫度間距與擬合解析度下則未觀察到可明確分辨的磁場位移。相較之下，Y50B20 於目前量測溫區內主要呈現近似歐姆傳輸，其冪次指數接近 1，尚未觀察到明確的 BKT 型轉變。整體結果顯示，隨 BBOF 覆蓋層增厚，界面應變、缺陷分布及氧含量不均等效應可能逐步削弱 YBCO 層之超導相位剛性、巨觀電流連通性與磁通釘扎能力。

In this study, 50-nm-thick yttrium barium copper oxide (YBa₂Cu₃O₇−δ, YBCO) single-layer films and BaBi(O,F)₃/YBCO (BBOF/YBCO) bilayer films with BBOF overlayer thicknesses of 10, 15, and 20 nm were deposited on MgO (001) substrates by radio-frequency magnetron sputtering. The effects of the BBOF overlayer thickness on the crystal structure, magnetic properties, superconducting transport, and vortex dynamics of YBCO were investigated using X-ray diffraction, superconducting quantum interference device magnetometry, and low-temperature four-probe electrical measurements. Structural characterization showed that the YBCO layers retained a preferential c-axis orientation after the deposition of BBOF. However, variations in the positions and profiles of the characteristic YBCO diffraction peaks were observed with increasing BBOF thickness, suggesting the possible presence of lattice mismatch, local strain, structural defects, and oxygen-vacancy redistribution near the heterointerface. Owing to the considerable difference in lattice parameters between BBOF and YBCO, the interfacial strain may be accommodated through local lattice distortion and structural relaxation, thereby modifying the superconducting properties of the YBCO layer.Magnetic and electrical measurements showed that the Y50B10 sample with the thinnest BBOF overlayer exhibited a more complete superconducting transition, better current connectivity, and higher effective critical current density and vortex-pinning force. As the BBOF thickness increased from 10 to 20 nm, the normal-state resistivity and low-temperature residual resistance increased, whereas the superconducting transition temperature decreased and the transition became broader. The effective vortex activation energy followed the order Y50B10 >Y50B15 >Y50B20, indicating that a thicker BBOF overlayer progressively weakened the restriction of vortex motion in the YBCO layer. The overall behavior of Y50B15 was intermediate between those of Y50B10 and Y50B20, representing a gradual transition from relatively good superconducting connectivity to a strongly suppressed state. Thermally activated flux-flow and resistive upper-critical-field analyses revealed pronounced magnetic-field-direction dependence in all three samples, reflecting the superconducting anisotropy associated with the layered crystal structure of YBCO. The extrapolated upper critical field for the field applied parallel to the abplane decreased with increasing BBOF thickness, indicating a gradual reduction in superconducting stability along this orientation. In contrast, the extrapolated results for the field applied parallel to the caxis did not exhibit the same monotonic trend. Because Y50B20 retained a substantial residual resistance at low temperatures and did not completely reach an ideal zero-resistance state, its resistive upper critical fields and coherence lengths should be regarded as apparent characteristic parameters derived from the resistive transition and used primarily for relative comparison among the samples. Berezinskii–Kosterlitz–Thouless (BKT) transition analysis showed that Y50B10 and Y50B15 exhibited increasingly nonlinear current–voltage characteristics with decreasing temperature under zero magnetic field. Their power-law exponents crossed the theoretical criterion of α=3at approximately 82–83 K, indicating BKT-like phase-ordering behavior. After a perpendicular magnetic field of 10 Oe was applied, the characteristic crossover temperature of Y50B10 decreased to approximately 80–81 K, suggesting that field-induced free vortices weakened the superconducting phase ordering. For Y50B15, no clearly distinguishable field-induced shift was resolved within the present temperature interval and fitting resolution. In contrast, Y50B20 exhibited predominantly ohmic transport within the measured temperature range, with a power-law exponent close to 1, and no distinct BKT-like transition was observed. These results indicate that increasing the BBOF overlayer thickness progressively suppresses the superconducting phase stiffness, macroscopic current connectivity, and vortex-pinning capability of the YBCO layer, possibly through interfacial strain, modified defect distributions, and inhomogeneous oxygen content.