非中心對稱超導體二硒鉭鉛之超導拓樸表面態之研究

Superconducting topological surface states in the noncentrosymmetric bulk superconductor PbTaSe2

關旭佑(Syu-You Guan)

指導教授 ： 張嘉升

國立臺灣大學/理學院/物理學研究所/博士(2017年)

https://doi.org/10.6342/NTU201700225

摘要

在本論文中，我們首先介紹了拓樸絕緣體和拓樸超導體的先前研究。拓樸絕緣體的發現是最近物理學的突破之一。其中的拓樸保護表面可以禁止電子的後向散射，給出了材料的新的傳輸性質。將超導體與拓樸性質結合可能會生成一種稱為拓樸超導體的新材料。尋找拓樸超導體是凝態系統中最重要的問題之一。拓樸超導體的特徵在於體內的完全超導間隙和拓樸保護的無能隙表面（或邊緣）態。在拓樸超導體的每個渦流核心內，可能存在零能量馬約拉那費米子束缚態，其被預測為顯示非阿貝爾統計並且形成可容錯量子計算的基礎。到目前為止，尚沒有符合化學計量比的材料在費米面處具有拓樸表面態與完全能隙的超導性。
論文的第二章介紹超低溫、高磁場、超高真空掃描穿隧顯微鏡的建造。低於1克耳文的溫度和強力磁場可以提高儀器的研究能力。而超高真空的環境可保持研究過程中樣品的清潔度。使這台顯微鏡在三種極端環境下工作需要先進的設計和仔細的操作。測試結果表明這台顯微鏡在能量和空間上都具有很高的分辨率。
第三章談到了原子級解析非中心對稱完全能隙超導體二硒鉭鉛的拓樸表面態。使用準粒子散射干涉成像時，我們發現狄拉克點在E≅1.0 eV的兩個拓樸表面態，其中內圈和部分外圈拓樸表面態在鉛表面上穿過費米面。利用超低溫高磁場超高真空掃描穿隧顯微鏡中亞克耳文的能量分辨率，清楚的解析了二硒鉭鉛的完全超導能隙。這表明拓樸表面態在費米面處被打開能隙。穿隧電導圖顯示在磁場下存在渦流，並在渦流核心處觀察到零能量電導率峰值。這一發現顯示了二硒鉭鉛可能是一拓樸超導體。論文的最後介紹了儀器的未來改進計劃和二硒鉭鉛的進一步研究。增加在低溫的保持時間和研究超導配對機制將是首要的工作。

關鍵字

拓樸超導體 ； 拓樸絕緣體 ； 馬約拉那費米子 ； 掃描穿隧電子顯微鏡 ； 二硒鉭鉛

並列摘要

In this thesis, we first introduce the previous study of topological insulator and the topological superconductor. The discovery of topological insulator (TI) is a recent breakthrough of physics. The topological protect surface in TI, forbidden the backscattering of electrons, gives new transport properties of the material. Combining the superconductor with the topological properties may host a type of new material called topological superconductor (TSC). The search for TSCs is one of the most urgent contemporary problems in condensed matter systems. TSCs are characterized by a full superconducting gap in the bulk and topologically protected gapless surface (or edge) states. Within each vortex core of TSCs, there exist the zero-energy Majorana bound states, which are predicted to exhibit non-Abelian statistics and to form the basis of the fault-tolerant quantum computation. To date, no stoichiometric bulk material exhibits the required topological surface states (TSSs) at the Fermi level (EF) combined with fully gapped bulk superconductivity.
In the second chapter, we introduce the construction of an ultra-low-temperature (ULT) high-magnetic-field (HF) ultra-high-vacuum (UHV) scanning tunneling microscope. Sub-Kelvin temperature and strong field advance the ability of instrument in research. UHV environment keeps cleanness of the sample during the study. Such instrument working in three extreme environments needs to state-of –the-art design with the careful operation. The test result shows the STM has high resolution in energy and space.
In the third chapter, we report atomic-scale visualization of the TSSs of the noncentrosymmetric fully gapped superconductor PbTaSe2. Using quasi-particle scattering interference imaging, we find two TSSs with a Dirac point at E ≅ 1.0 eV, of which the inner TSS and the partial outer TSS cross EF on the Pb-terminated surface.
With sub-Kelvin energy resolution achieved in the ULT-HF-UHV STM, the fully superconducting gap of PbTaSe2 is clearly resolved, which suggests the TSS gapped out at EF. The tunneling conductance map shows the vortex is presented under the magnetic field, and zero energy conductance peak is observed at vortex core. This discovery reveals PbTaSe2 as a promising candidate for TSC. Lastly, the future improvement of the instrument and further study for PbTaSe2 are introduced. Increasing the holding time at 4 K and investigate the pairing mechanism are priorities.

Keyword: Topological superconductor, topological insulator, Majorana fermion, scanning tunneling microscope, PbTaSe2

並列關鍵字

Topological superconductor ； topological insulator ； Majorana fermion ； scanning tunneling microscope ； PbTaSe2

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