三碲化二銻奈米薄片的拓樸超導現象之研究

Abstract

隨著科技的進步，更加複雜的待解決科學問題也隨著增加，例如：複雜的量子系統，即便是利用傳統超級電腦，也有機會需要花費如天文數字般的時間才能夠完成計算，除了透過精進傳統電腦的製程技術來提升計算速度，亦可使用另一種方法，稱為量子電腦。 量子電腦，被譽為下個世代的運算工具，其利用量子態作為邏輯位元，相較於傳統電腦當中的電晶體，多了更多的自由度表示狀態，進而使其在特定幾種邏輯運算中，有機會超越傳統電腦的運算速度，也因此有機會解決非常多現今傳統超級電腦無法解決的複雜科學問題。量子電腦亦有許多種類別，其中有一類別稱為拓樸量子電腦，亦稱為容錯性量子電腦，利用任意子做為量子位元，其所具有的拓樸保護態，使量子態不容易被外界環境影響產生破壞，進而可以解決量子電腦在計算中有誤差產生的問題，是除了增加總量子位元數進而減少計算誤差之外，另一個實現量子電腦的途徑，而其具有高計算速度、多位元以及並行計算的特性，將有機會解決許多現今超級電腦尚未解決的科學問題並帶來更多不同跨領域的共同發展。根據科學家預測，其中一種任意子，馬約拉納零模態，有機會在拓樸超導體中發現，也因此，對於拓樸超導體的相關研究便顯得重要。 在本研究中，我們透過物理氣相傳導法與化學氣相沈積法成功地合成單晶的Sb2Te3二維材料。透過參數調整以及製程標準化，優化其生長再現性，並且利用微影製程、蝕刻製程以及薄膜製程，成功地製作樣品微米級與奈米級量測電極，並成功地量測其物理性質並利用穿透式電子顯微鏡以及能量散佈分析儀觀察其晶體結構與樣品成份。根據穿透式電子顯微鏡的量測結果我們發現透過物理氣相傳導法所製備出的Sb2Te3樣品，其樣品品質良好，HLN模型對WAL效應的擬合支持樣品表面具有拓樸表面態，而在低溫至約莫2 K時開始體現出零電阻現象，並且在改變外加磁場與外加電流的實驗中，皆觀察到樣品具有典型的超導態對於外加磁場與電流的壓抑行為。根據我們的McMillan理論模型擬合，發現其樣品致超導機制來源分為三重態超導與單重態超導，分別對應到A區塊與B區塊。另外，透過化學氣相沈積法所製備出的Sb2Te3樣品，我們則觀察到其電阻在約莫1.5 K時產生的電阻值的驟降，但並沒有產生零電阻現象，而透過改變外加磁場的實驗，觀察到典型的超導態對外加磁場的壓抑現象。透過HLN模型對WAL效應的擬合與分析並將其與物理氣相傳導法的Sb2Te3樣品比較，我們認為其可能具有較多的無序性與缺陷於樣品中，進而導致其並沒有產生零電阻現象，亦有可能是在製備過程中所產生的不均勻分布，使其沒有產生零電阻現象，而不同直流電流大小對電阻的趨勢支持了這樣的可能性。最後，我們認為要合成出成分均勻、良好品質並且具有超導特性的單晶Sb2Te3二維材料，物理氣相傳導法是相對適合的選擇。

With the advancement of science and technology, more complex scientific problems to be solved have also increased. For example, complex quantum systems, even with traditional supercomputers, may take tremendous amounts of time to complete the calculations, on top of specific traditional computer processes; we can utilize another concept, called a quantum computer, to increase the speed of calculations. Quantum computers are recognized as the computing tools of the next generation. The concept of quantum computers is based on quantum states used as logical bits, called qubits. Compared to the transistors in traditional computers, they have more degrees of freedom to express the state, making it possible to surpass the computing speed of traditional computers under certain kinds of logical calculations. Therefore, it increases the number of opportunities to solve many complex scientific problems that traditional supercomputers nowadays cannot. There are also many types of quantum computers, one of which is called topological quantum computers, also known as fault-tolerant quantum computers. It uses anyons as qubits, and its topologically protected state makes it difficult for quantum states to be damaged by the external environment and can minimize errors in the calculation of quantum computers. In addition to increasing the total number of qubits, quantum computers possess high calculation speeds, characteristics of multi-bit, and parallel computing which have the potential to solve many unsolved scientific problems today's supercomputers are unable to and bring more common development across different fields. According to scientists’ predictions, one of the anyons, Majorana zero-mode, has the opportunity to be found in topological superconductors. Therefore, research on topological superconductors is important. In this study, we successfully synthesized 2D single crystal Sb2Te3 using the physical vapor transport method and chemical vapor deposition method. Through parameter adjustment and process standardization, growth reproducibility is optimized, and the micro-scale and nano-scale measurement electrodes of the sample are successfully fabricated by the lithography process, the etching process, and the thin film process. The physical properties are also successfully measured. The crystal structure and sample composition are checked by transmission electron microscope and energy-dispersive X-ray spectroscopy. According to the measurement results of the transmission electron microscope, we found that the Sb2Te3 sample prepared by the physical vapor transport method possesses high quality. The fitting of the HLN model to the WAL effect supports the existence of topological surface state on the sample surface at normal state. The zero-resistance effect began to appear at about 2 K. In the resistance measurement under externally applied magnetic field and current, it was observed that the superconducting state was suppressed by the applied magnetic field and current which is the typical behavior. According to our McMillan theoretical model fitting, it is found that the origin of the superconducting mechanism of the samples are divided into triplet pairing and singlet pairing, corresponding to batch A and batch B samples respectively. On the other hand, the Sb2Te3 sample prepared by chemical vapor deposition method displayed a sudden drop in the resistance near 1.5 K, but there was no zero-resistance effect. Typical superconducting state suppression by the external magnetic field was also observed under different applied magnetic field. Based on the fitting and analysis of the WAL effect by the HLN model and comparing it with the Sb2Te3 sample of physical vapor transport method, we believe that it may have more disorder and defects in the samples that went through chemical vapor deposition method, which leads to no zero-resistance. The phenomenon may also have been caused by the non-uniform distribution during the preparation process, and the resistance measurements under different DC currents supports this possibility. Finally, we believe that to synthesize 2D single-crystal Sb2Te3 with uniform composition, good quality and superconducting properties, the physical vapor transport method is a relatively suitable choice.