第一原理計算研究拓樸半金屬的自旋傳輸、多鐵性鐵酸鉍的非線性光學效應與光伏材料鈣鈦礦的電子結構模擬及電荷傳輸

Abstract

自旋電子學已經成為凝聚態物理學中一個引人入勝的領域，展示了其在高效利用能量存儲數據和能量收集方面的潛力。最近的理論研究揭示了自然界中近三分之一的非磁性無機化合物的拓撲特性，其中約15-18％表現出拓撲半金屬的特徵。 在此背景下，拓撲半金屬為探索自旋霍爾效應和自旋能斯特效應提供了一個有趣的途徑。而理解能帶拓撲、自旋霍爾效應和自旋能斯特效應之間的複雜相互作用也非常重要。因此，在本文中，我們採用第 一原理密度泛函理論系統地探討了 XCuYAs（X = Zr, Hf；Y = Si, Ge）化合物的電子結構、自旋霍爾效應和自旋能斯特效應。 我們的研究表明，XCuYAs 化合物是狄拉克線節點半金屬，在布里淵區的 A-M 和 X-R 方向上表現出受非對稱保護的節點線。此外，某些化合物的計算本徵自旋霍爾和能斯特電導率顯示出顯著的大小，並可以通過化學摻雜或電門控進行調節。對能帶劈裂和總自旋貝里曲率的細緻分析將這些顯著的電導率和可調性主要歸因於費米能級附近由於自旋軌道耦合而產生的狄拉克點和無間隙的狄拉克節點線。 擴展我們的探索，我們還研究了手性拓撲半金屬 CoSi 家族中的這些效應，建立了自旋霍爾電導率、結構螺旋性和手性費米子手性之間的獨特關係。我們的研究認為這些化合物是推進自旋電子學和自旋熱電子設備的有前途的材料。 與探索拓撲半金屬中的自旋霍爾效應和納斯特效應同時進行的，我們的論文還深入研究了著名的室溫多鐵性化合物 BiFeO3 中的非線性光學（NLO）響應。我們研究的重點是磁性引起的非線性光學響應，作為控制光物質相互作用的新工具。 我們的研究發現顯著的磁性引起的二次諧波產生（SHG）響應。此外，SHG 強度可以通過反轉磁化方向或 Néel 矢量進行調節。值得注意的是，在 4.82 eV 的 SHG 光子能量下，我們觀察到了高達 440％的 SHG 信號的顯著磁性對比度。計算的體光伏效應（BPVE）響應也很突出，我們對這些響應進行了與相關量子幾何量的深入分析。 本研究強調了 BiFeO3 中磁性引起的 NLO 響應的重要性、各向異性和可調性，表明其在基於多鐵性的光伏設備中的潛在應用。此外，我們的研究確立了量子幾何在理解磁性引起的 NLO 響應中的關鍵作用。 最後，我們的論文揭示了 1T-TaS2 和 CH3NH3PbI3 界面的電荷轉移。我們的理論研究揭示了 1T-TaS2 和鈣鈦礦界面的不同電子行為。1T-TaS2 表現出金屬性，而鈣鈦礦顯示出半導體性。此外，界面處的態密度（DOS）主要由 Ta 和 S 原子控制。我們還注意到鈣鈦礦向 1T-TaS2 的電荷轉移，估計約為每個鈣鈦礦薄膜片約 0.12 個電子。這種界面電荷轉移引發了界面電偶極，由分離的正電荷與負電荷定義，界面電偶極可以修改 TaS2 的功函數，並調整電荷聚集的障壁。此外，我們研究了 Rb 取代對（MAFA）PbI3 的影響，其中 MA 代表甲基銨（CH3NH3），FA 代表甲脒 [CH(NH2)2]。我們的研究發現 Rb 4p 軌道在-11 eV 附近具有明顯的 DOS 尖取代我們還發現 Rb 4p 和 5s 軌道在不同取代水平下在能隙附近有微小貢獻，能隙中沒有 Rb 態。這意味著由於不存在能隙態，電子在電荷傳遞過程中可能不會被捕獲，潛在地提升裝置的性能。

Spintronics has emerged as a captivating domain within condensed matter physics, showcasing its potential in energy-efficient data storage and energy harvesting. Recent theoretical investigations have unveiled the topological nature of nearly one-third of nonmagnetic inorganic compounds in nature, with approximately 15-18% exhibiting characteristics of topological semimetals. In this context, topological semimetals present an intriguing avenue for the exploration of spin Hall and spin Nernst effects. Also, it is important to understand the intricate interplay between band topology, spin Hall, and spin Nernst effects. Therefore, in this dissertation, we systematically delve into the electronic structure, spin Hall, and spin Nernst effects within the XCuYAs (X = Zr, Hf; Y = Si, Ge) compounds, employing ab initio density functional theory calculations. Our investigations reveal that the XCuYAs compounds are Dirac line-node semimetals, exhibiting nonsymmorphic symmetry-protected nodal lines along the A-M and X-R directions in the Brillouin zone. Moreover, the calculated intrinsic spin Hall and Nernst conductivities in some of these compounds show substantial magnitudes and tunability through chemical doping or electric gating. A meticulous analysis of band-decomposed and total spin Berry curvatures attributes these considerable conductivities and tunabilities primarily to spin-orbit coupling gapped Dirac points near the Fermi level and gapless Dirac nodal lines. Extending our exploration, we investigate these effects in the chiral topological semimetal, the CoSi family, establishing a unique relationship between spin Hall conductivity, structural helicity, and chiral fermion chirality. Our study posits these compounds as promising materials for advancing spintronic and spin caloritronic devices. In tandem with the exploration of spin Hall and Nernst effects in topological semimetals, our dissertation also delves into nonlinear optical (NLO) responses within the well-known room temperature multiferroic compound, BiFeO3. The focal point of our study is the investigation of magnetism-induced nonlinear optical responses, serving as a novel tool for controlling light-matter interactions. Our findings reveal substantial magnetism-induced second harmonic generation (SHG) susceptibilities. Also, the SHG intensity is tunable with the reversal of magnetization direction or Néel vector. Remarkably, we notice a significant magnetic contrast of the SHG signal, reaching as high as 440%, at an SHG photon energy of 4.82 eV. Calculated bulk photovoltaic (BPVE) responses are also prominent, and we provide an in-depth analysis of these responses in terms of associated quantum geometric quantities. This study highlights the significance, anisotropy, and tunability of magnetism-induced NLO responses in BiFeO3, suggesting their potential utilization in multiferroic-based photovoltaic devices. Furthermore, our investigation establishes the pivotal role of quantum geometry in comprehending magnetism-induced NLO responses. Finally, our dissertation sheds light on the charge transfer at the interface of 1T-TaS2 and CH3NH3PbI3. Our theoretical investigation uncovers distinct electronic behaviors at the interface of 1T-TaS2 and the perovskite. While 1T-TaS2 exhibits metallic properties, the perovskite displays semiconducting behavior. Furthermore, the DOS at the interface is primarily governed by Ta and S atoms. We also notice the charge transfer from the perovskite to 1T-TaS2, estimated at approximately 0.12 electrons per perovskite thin film slab. This interfacial charge transfer induces an interface dipole, defined by the separation of positive and negative charges, which can modify the work function of TaS2 and adjust the barrier for charge collection. Additionally, we investigate the effect of Rb substitution in (MAFA)PbI3, with MA representing methyl ammonium (CH3NH3) and FA representing formamidinium [CH(NH2)2]. Our study reveals distinct DOS peaks attributed to Rb 4p orbitals around -11 eV. We also find minor contributions from Rb 4p and 5s orbitals in the vicinity of the band gap across varying Rb concentrations, with no Rb states in the gap. This implies that, due to the absence of gap states, electrons may not be trapped during charge transport, potentially improving device performance.