第一原理計算研究掌性外爾半金屬的自旋傳導及非線性光學性質

Abstract

外爾半金屬因為能帶中有被視為貝里曲率磁單極的特殊交錯而在近幾年受到注意。過渡金屬矽化物，包括一矽化鈷、一矽化銠、一鍺化鈷和一鍺化銠，是一組缺少鏡面對稱性質的掌性外爾半金屬。因此，這些材料的外爾結點坐落於不同的能階上。多重簡併並且擁有較大陳數的外爾結點被預言存在於這些材料中。因此，這些材料成為一個研究外爾結點跟物理性質關係的好平台。 在這個論文中，我們系統性地透過第一原理計算研究以上材料的自旋霍爾效應，自旋能斯特效應還有非線性光學性質。在我們的研究中，我們展示了一鍺化銠在所有材料中擁有最高的自旋霍爾電導率-139 (ħ/e)(S/cm)，並且在室溫下一矽化鈷和一鍺化鈷擁有大的自旋能斯特電導率，分別為-1.00 (ħ/e)(A/m K) 和 -1.25 (ħ /e) (A/m K)。非線性光學的部分，我們計算了二次階波產生還有體光伏效應。我們發現偏移電流會在低能量的區域產生峰值，而這個峰值並沒有出現在線性光學上。並且，透過我們的計算我們揭露透過調整費米能量，有可能得到被預測會發生在這個材料上的量子體光伏效應。

Weyl semimetals have received a lot of attention in recent years because of the unique band crossing which can be seen as a monopole of Berry curvature. Among them, transition metal silicides, including CoSi, RhSi, CoGe, and RhGe, form a group of chiral Weyl semimetals with no mirror symmetry. Hence, the pairs of Weyl nodes in these materials are located at different energy levels. Also, multifold Weyl nodes with a large Chern number were predicted in these materials. Therefore, these materials should be a good platform to study the relation between these Weyl nodes and novel physical phenomena. In this thesis, we systematically study the spin Hall effect (SHE), spin Nernst effect (SNE), and nonlinear optical (NLO) effects for the above-mentioned materials by first-principles calculation. We show that the highest spin Hall conductivity (SHC) value among these materials is RhGe with -139 (ħ /e)(S/cm). Also, spin Nernst conductivity (SNC) at room temperature is large for both CoSi and CoGe, with the value of -1.00 (ħ /e)(A/m K and -1.25 (ħ /e)(A/m K), respectively. For NLO, second harmonic generation (SHG) and bulk photovoltaic effect (BPVE) are calculated. We find that linear shift current shows a peak in the low energy region which doesn't appear in the optical conductivity. Also, through our calculation, we reveal that by tuning Fermi energy, it is possible to get quantized circular injection current which was predicted to happen in chiral Weyl semimetals.