Bi/NbSe2界面中的二維超導與反鄰近效應

Abstract

拓樸超導體作為研究Majorana費米子的平台，無論是在量子計算或是量子存儲等領域都具有不錯的潛力，使得拓樸超導材料成為熱門的研究對象之一。而拓樸超導體的製程，可藉由拓樸絕緣體與超導體之間的鄰近效應而形成。此研究中使用了拓樸材料鉍(Bi)濺鍍於超導材料二硒化鈮(NbSe2)上，分析該異質結構的超導特性與電性傳輸特性。 在製程過程中，設計了能在維持高真空環境下將NbSe2表面撕除的機關，使Bi 濺鍍於乾淨的NbSe2表面上，以保證Bi/NbSe2界面的純淨度；並且透過X光繞射儀(XRD)，比較了Bi、NbSe2與Bi/NbSe2的結構，用以確認製程出的材料為Bi/NbSe2的可信性。 在此研究中發現了Bi厚度的變化，對Bi/NbSe2異質結構材料產生了幾個有趣的現象：其一是產生了反鄰近效應，隨著Bi厚度增加到50 nm，超導相變溫度(𝑇c)也從 6.94 K 成長到7.2 K，並在Bi厚度來到100 nm時，𝑇c下降到7.09 K。其二是Bi厚度增加，對於上臨界垂直磁場(𝐻𝑐2⊥)具有先抑制後促進的現象產生，在從Bi厚度增加到20 nm時，𝐻𝑐2⊥從6.36 T降到4.28 T的低點，並在Bi厚度繼續增加100 nm時，𝐻𝑐2⊥逐漸增加至7.35 T。 此外，還發現三種Bi/NbSe2可能具有的特性：第一點是發現變化Bi厚度不會對𝐻𝑐2∥太大的改變，且𝐻𝑐2∥與𝐻𝑐3的比值比理論上的小，推測Bi/NbSe2保有了NbSe2本身具有的Ising超導特性，這也顯示Bi並無抑制Ising超導。第二點是發現Bi/NbSe2在低溫有BKT相變的現象，以及釘扎位能與取對數後磁場呈現線性關係，這兩點都是二維材料性質的展現。第三點是量測磁阻時發現有WAL效應的發生，而WAL效應正是在拓樸材料中很常見的現象。因此結合以上三點，Bi/NbSe2可能具有Ising 超導與二維超導的性質，且同時具有拓樸材料的潛力。

Topological superconductors can help study Majorana fermions, which have potential in fields such as quantum computing or quantum storage, making topological superconducting materials popular. Topological superconductors can be formed by the proximity effect between topological insulators and superconductors. In this study, the topological material bismuth (Bi) was sputtered on the superconducting material niobium diselenide (NbSe2 ) to analyse the superconducting properties and electrical transport properties of the material. During the process, a mechanism is designed to tear off the NbSe2 surface while maintaining a high vacuum environment, allowing Bi to be sputtered on the clean NbSe2 surface to ensure the purity of the Bi/NbSe2 interface. The samples through X-ray diffractometer (XRD) were used to compare the structures of Bi, NbSe2 and Bi/NbSe2 to confirm the credibility that the material produced through the process is Bi/NbSe2. In this study, the change in Bi thickness produced several interesting phenomena for Bi/NbSe2. First is the inverse proximity effect. As the Bi thickness increases to 50 nm the Tc also grows, but then the Bi thickness reaches 100 nm the 𝑇c decreases. Second is that as the Bi thickness increases, 𝐻𝑐2⊥ has a phenomenon of suppression and then promotion. When the Bi thickness increases to 20 nm the 𝐻𝑐2⊥ drops, and then the Bi thickness increases to 100 nm, 𝐻𝑐2⊥ gradually increases. In addition, three possible properties of Bi/NbSe2 were found. First, Bi/NbSe2 retains the Ising superconducting properties of NbSe2 itself, and it is also shown that Bi does not inhibit Ising superconductivity. Second, Bi/NbSe2 has a BKT phase transition phenomenon at low temperatures, which is the behavior of the properties of two-dimensional materials. Third, Bi/NbSe2 has the occurrence of the WAL effect, and this is a common phenomenon in topological materials. Therefore, combining the above three points, Bi/NbSe2 may have the properties of Ising superconductivity, two-dimensional superconductivity, and the potential of topological materials.