應用鈮/碲化銻雙層膜人工釘扎於陽極氧化鋁基板之磁通匹配效應研究

Abstract

本實驗大致上可以分成兩個部分，首先為使用磁控濺鍍的方法製備Nb以及Nb/$\\mathrm{Sb}_2\\mathrm{Te}_3$兩種薄膜樣品，傳統超導與拓墣材料的薄膜介面因鄰近效應成為非傳統超導材料--拓墣超導。在前人的努力下，我們已經掌握這兩種樣品在矽基板上的高品質鍍膜條件；Si(100)基板上，Nb在攝氏540度時有最好的品質，$\\mathrm{Sb}_2\\mathrm{Te}_3$ 則是需在攝氏123度。 但本實驗最重要的目的並非鎖定在製造拓墣超導，而是比較傳統超導以及拓墣超導產生的磁通量子匹配差異，與傳統超導的整數倍量子磁通不同，非傳統超導的量子磁通可能為半整數的性質，以鄰近效應造成之拓墣超導為例，自旋三重態會導致其量子磁通為半整數倍；透過將薄膜鍍在表面充滿高密度深孔的陽極氧化鋁基板，當樣品溫度低於超導溫度時，排磁性會使得每個孔洞成為一個人工的釘扎點，如此一來便可以將量子效應放大，穿過人工釘扎點的磁場強度基本單位必須為一個量子磁通量，其性質能容易被儀器所測量。 我們預期Nb薄膜樣品之量子磁通遵循 $H_n = nH_1$, 以及Nb/$\\mathrm{Sb}_2\\mathrm{Te}_3$ 雙層膜之量子磁通遵循 $H_n =\\qty(\\frac{1}{2}+n)H_1$, 其中因基板的釘扎結構使得 $H_1\\approx 100$ Oe。此週期性磁場可由統計方法運算並得出精確值。

This experiment can be roughly divided into two parts. First, using the method of magnetron sputtering, we prepared two different thin film samples, Nb and Nb/$\\mathrm{Sb}_2\\mathrm{Te}_3$. Due to the proximity effect, the interface between conventional superconductor and topological material becomes unconventional superconductor, known as topological superconductor. Thanks to previous efforts, our senior have already optimized the deposition conditions for high-quality films of both materials on silicon substrate. On Si(100) substrates, Nb film shows high quality when deposited at 540$^\\circ $C, while $\\mathrm{Sb}_2\\mathrm{Te}_3$ requires a deposition temperature of 123$^\\circ $C. However, the main goal of this experiment is not simply to fabricate a topological superconductor, but to compare the difference of flux matching effects between conventional and topological superconductors. Unlike conventional superconductors, which exhibit integer multiples of the magnetic flux quantum, unconventional superconductors may exhibit exotic quantization such as half-integer or one-third-integer flux quanta. In the case of topological superconductors induced by the proximity effect, the spin-triplet pairing leads to half-integer flux quantization. By depositing the films onto anodic aluminum oxide (AAO) substrates with a high-order nanopores, each pore becomes an artificial pinning center due to the Meissner effect when the sample is cooled below its superconducting critical temperature. This magnifies the quantum effect: the magnetic field penetrating the pinning sites must be quantized in units of the magnetic flux quantum, allowing for direct measurement using experimental instruments. We expect that the flux quantization for the Nb film follows the relation $H_n = nH_1$,and for the Nb/$\\mathrm{Sb}_2\\mathrm{Te}_3$ bilayer it follows $H_n = \\qty(\\frac{1}{2}+n)H_1$, where the artificial pinning array yields $H_1 \\approx 100\\mathrm{~Oe}$. This periodic magnetic field structure can be computed and validated using statistical analysis methods.