外爾半金屬是一種特別的晶體，其中外爾費米子為準粒子並以表面有費米弧為特徵，一個狄拉克點由兩個相反手徵的外爾點組成，外爾點的行為類似於貝瑞曲率描述的磁單極，根據量子磁場理論，與手徵異常有關的新穎電子傳輸現象是由出現在費米能階旁邊的外爾點所造成，這可以理解為給定手徵粒子數不守恆的原因，此不守恆現象只出現在電場（電流）方向與磁場方向平行時。本論文針對外爾半金屬TaAa的物理性質作深入研究，X光粉末繞射儀的量測結果顯示單晶TaAs的晶面垂直於晶軸c軸方向，以物理性質量測系統（PPMS）測量的電性傳輸結果顯示，由時間反轉對稱保護的外爾半金屬TaAs，在零磁場的環境中，隨溫度變化的電阻率表示TaAs為金屬性，而後，當磁場從0T增加到9T，TaAs的電性也隨之由金屬轉換成絕緣體。此報告中展示與磁場有關的席貝克係數量測結果，席貝克係數在磁場方向平行和垂直溫度梯度方向時有著明顯的差異，一般而言，席貝克係數在磁場下的行為與磁阻相似，在小磁場時，兩者皆有反若局域效應，磁阻及席貝克係數皆隨磁場增加而增加。當磁場方向由平行(B∥∇T)轉為垂直(B⊥∇T)溫度梯度方向時，席貝克係數由- 1.5μV/K增加至- 13μV/K，其中的負號顯示TaAs為p型的外爾半金屬。

A Weyl semimetal is a kind of special crystal which has Weyl fermions and exhibits Fermi arcs on its surface. A Dirac point is composed of two Weyl nodes which have opposite chirality, and act as magnetic monopoles. Based on the quantum field theory, the emergence of Weyl nodes near the Fermi level induces novel transport phenomena related to chiral anomaly, which can be understood as the non-conservation of the particle number with given chirality. The anomaly shows up only when the magnetic field is parallel to the electric field. In this work we studied the physical properties of TaAs Weyl semimetal. The crystallographic c-axis which is perpendicular to the surface of TaAs single crystal is confirmed by the result of XRD. The transport properties of TaAs is measured by the physical properties measurement system (PPMS). The temperature dependence of resistivity demonstrates that TaAs is metallic at zero magnetic field, and it then changes from metal to insulator when magnetic field increases form 0 tesla to 9 tesla. The measurement results present evidences that the negative magneto-resistance is generated by the chiral anomaly in a predicted time reversal symmetry in Weyl semimetal TaAs. In the measurements of magnetic field dependence of Seebeck coefficient, the difference of magneto-Seebeck is observed when magnetic field is parallel and perpendicular to the temperature gradient. In general, the behavior of Seebeck coefficient is similar to that of magneto-resistance. At low magnetic field, the Seebeck coefficient and magneto-resistance show weak anti-localization behavior, both increase as magnetic field rises. When the direction of magnetic field changes from parallel ( B∥T) to perpendicular (B⊥T) with respect to the temperature gradient, the magnitude of Seebeck coefficient increases from -1.5 to -13μV/K, whereas the negative sign represents TaAs is p - type Weyl semimetal.

摘要 i
Abstract ii
致謝 iv
Outline v
Figure Outline vi
Table Outline viii
I. Introduction 1
II. The Weyl Semimetal and the Chiral anomaly 6
2.1 Landau Level 6
2.2 Dirac Fermion 8
2.3 Weyl Fermion 10
2.4 Chiral Anomaly 11
2.5 Fermi Arc 14
2.3 TaAs properties 15
III. Thermoelectric properties 19
IV. Experimental setup 27
4.1 X-ray diffraction (XRD) 27
4.2 Physical properties measurement system (PPMS) 30
4.3 Thermocouple 33
4.4 Experimental setup 35
V. Result 41
5.1 XRD properties of TaAs 41
5.2 Transport properties of TaAs 42
5.3 Thermoelectric properties of TaAs 49
VI. Discussion 53
Reference 55

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