拓樸絕緣體Sb₂Te₃與Nb成長於陽極氧化鋁基板之Matching Effect之研究

Abstract

本研究利用直流磁控濺鍍（DC Magnetron Sputtering）以及射頻磁控濺鍍（RF Magnetion Sputtering）於具有週期性奈米孔洞結構之陽極氧化鋁（Anodic Aluminum Oxide, AAO）基板上製備 Nb 單層膜及 Nb/Sb₂Te₃ 雙層膜，並藉由 X-ray 繞射（XRD、場發射掃描式電子顯微鏡（FE-SEM）、四點電性量測法以及超導量子干涉儀（SQUID）磁性量測，探討 AAO 人工釘扎中心對超導渦旋排列及匹配效應（Matching Effect）之影響。 XRD 結果顯示，所製備之Nb薄膜具有良好的(110)優選取向；R(T)量測結果顯示，Nb與Nb/Sb₂Te₃ 樣品皆具有良好的超導轉變特性。利用不同外加磁場下之R(T)曲線配合 Werthamer–Helfand–Hohenberg（WHH）理論估算上臨界磁場，並透過熱激發磁通流（Thermally Activated Flux Flow, TAFF）模型分析釘扎位能與磁場之關係。 M(H)量測結果顯示，AAO/Nb 200 nm 與 AAO/Nb/Sb₂Te₃ 20 nm 樣品皆可觀察到明顯之匹配效應，其中主要峰值並未出現在理論第一匹配場H₁，而較接近約2H₁，且同時觀察到部分半整數匹配訊號。此外，在接近臨界溫度時，增磁與退磁分支出現交叉反轉現象，經背景扣除後仍然存在，顯示其並非僅由 AAO 基板背景磁矩所造成。 FE-SEM 結果顯示，Nb 並未均勻覆蓋於 AAO 表面，而是在孔洞周圍形成類似花瓣狀（petal-like）結構。根據此形貌，本研究提出有效釘扎位勢（effective pinning potential）模型，認為花瓣狀 Nb 成長可能改變渦旋之有效排列方式，使主要匹配特徵峰偏移至約 2H₁，並可能造成部分半整數匹配特徵。雖然目前仍缺乏直接渦旋影像證據，此模型可定性解釋本研究所觀察到之多項實驗結果，提供 AAO/Nb 超導系統中渦旋排列機制之另一種可能解釋。

In this study, niobium (Nb) single-layer films and Nb/Sb₂Te₃ bilayer films were deposited on anodic aluminum oxide (AAO) substrates with periodic nanoporous structures by direct current (DC) magnetron sputtering. The effects of artificial pinning centers introduced by the AAO template and the Sb₂Te₃ overlayer on the superconducting properties and vortex behavior were systematically investigated by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), four-probe electrical transport measurements, and superconducting quantum interference device (SQUID) magnetometry. The XRD results indicate that the deposited Nb films exhibit a preferred (110) orientation. Electrical transport measurements reveal well-defined superconducting transitions for both Nb and Nb/Sb₂Te₃ samples. The upper critical field, H_c2(0), was estimated from resistance-versus-temperature measurements under different magnetic fields using the Werthamer-Helfand-Hohenberg (WHH) theory. Furthermore, the vortex pinning behavior was analyzed using the thermally activated flux flow (TAFF) model, showing that the activation energy decreases with increasing magnetic field, consistent with the vortex dynamics of type-II superconductors. Magnetization measurements demonstrate pronounced matching effects in both AAONb200 and AAONbST200 samples, confirming that the periodic nanoporous AAO substrate acts as an effective artificial pinning array. Interestingly, the dominant匹配特徵峰are observed near approximately twice the theoretical first matching field (2H_1) rather than at the expected first matching field (H_1). In addition, several fractional matching features are observed at specific temperatures. Compared with the 200 nm Nb films, the 500 nm samples exhibit higher superconducting transition temperatures but less regular matching behavior, suggesting that increasing the Nb thickness weakens the influence of the artificial pinning centers on vortex ordering. Magnetization reversal between the increasing- and decreasing-field branches is also observed near the superconducting transition temperature. After subtracting the magnetic background of the AAO substrate, the reversal behavior remains, indicating that it is not primarily caused by the substrate itself. FE-SEM observations reveal that the Nb film does not uniformly cover the AAO surface but instead forms petal-like structures around the nanopores. Based on the SEM morphology together with the magnetic measurements, an effective pinning potential model is proposed. This model qualitatively explains the shift of the dominant matching peaks toward approximately 2H_1, the appearance of fractional matching features, and the different matching behaviors observed for films with different thicknesses. Although direct imaging of vortex configurations is still required for verification, the proposed model provides a consistent physical interpretation of the experimental observations and offers an alternative perspective on vortex arrangements in AAO-based superconducting systems.