Intrinsic anomalous Hall effect in nickel: An GGA+U study

The electronic structure and intrinsic anomalous Hall conductivity of nickel have been calculated based on the generalized gradient approximation (GGA) plus on-site Coulomb interaction (GGA+U) scheme. It is found that the intrinsic anomalous Hall conductivity ($\sigma_{xy}^H$) obtained from the GGA+U calculations with $U = 1.9$ eV and $J=1.2$ eV, is in nearly perfect agreement with that measured recently at low temperatures while, in contrast, the $\sigma_{xy}^H$ from the GGA calculations is about 100% larger than the measured one. This indicates that, as for the other spin-orbit interaction (SOI)-induced phenomena in 3$d$ itinerant magnets such as the orbital magnetic magnetization and magnetocrystalline anisotropy, the on-site electron-electron correlation, though moderate only, should be taken into account properly in order to get the correct anomalous Hall conductivity. The intrinsic $\sigma_{xy}^H$ and the number of valence electrons ($N_e$) have also been calculated as a function of the Fermi energy ($E_F$). A sign change is predicted at $E_F = -0.38$ eV ($N_e = 9.57$), and this explain qualitatively why the theoretical and experimental $\sigma_{xy}^H$ values for Fe and Co are positive. It is also predicted that fcc Ni$_{(1-x)}$Co(Fe,Cu)$_x$ alloys with $x$ being small, would also have the negative $\sigma_{xy}^H$ with the magnitude being in the range of $500\sim 1400$ $\Omega^{-1}$cm$^{-1}$. The most pronounced effect of including the on-site Coulomb interaction is that all the $d$-dominant bands are lowered in energy relative to the $E_F$ by about 0.3 eV, and consequently, the small minority spin X$_2$ hole pocket disappears. The presence of the small X$_2$ hole pocket in the GGA calculations is attributed to be responsible for the large discrepancy in the $\sigma_{xy}^H$ between theory and experiment.Comment: 7 pages, 3 figures; Accepted for publication in Physical Review

Ab inito study of electronic properties of strained [110] Si nanowires

近十年來，矽是被用於製造大量電子元件的材料。由於在電子元件介面上不可避免的晶格不匹配，使得應變效應普遍存在於半導體的電子元件中。此應變效應在奈米尺度下對電子結構及其電學性質有顯著的影響。　利用應變效應可以用來有效操控半導體奈米線的物理性質。本篇論文主要透過第一原理計算了解並改變[110]矽奈米線在應變效應下電子結構及電子態性質。本文使用OpenMX (Open source package for Material eXplorer) 計算軟體，這套軟體是建構在密度泛函理論(DFT)之下，所使用的模守恆贗勢(NCPPs)，並使價電子區域波函數局域化，使得在計算過程中達到電腦計算時間與原子數呈線性關係[O(N)]。　計算結果顯示出[110]矽奈米線在未加應變效應下有直接能隙，當直徑越大能隙會越小。在外加足夠的張應變與壓應變下會使得能隙從直接能隙變成間接能隙。電子的有效質量會隨著壓應變而變大，隨著張應變趨近一定值(0.15m0)；電洞的有效質量隨著壓應變而變小，隨著張應變而變大。電子和電洞在能帶邊緣(CBM、VBM)的電荷密度分布，在應變效應下並無顯著的影響，電荷主要分布在奈米線的中心位置。For decades, silicon has been the material of choice for mass fabrication of electronics. Strains always exist in semiconductor electronic devices. Due to unavoidable lattice mismatches at interfaces, effects of strains on the electronic, electrical properties would be particularly significant at nanometer scales. Strain effect can be used to manipulate the physical properties of semiconductor nanowires. Here, we can make use of the strains to enhance the electronic properties through ab initio calculations of strained [110] silicon nanowires. The electronic structure of strained [110] silicon nanowires have been studied with the density functional theory (DFT) using the norm-conserving pseudopotentials (NCPPs). The electronic states are found by O(N) Krylov-subspace method, as implemented by OpenMX (Open source package for Material eXplorer) code. The calculation results show that the band structure of silicon nanowires tends to have an indirect band gap under compressive and tensile strain. Under tensile strain, the electron effective mass remains almost unchangd (∼0.15m0) while the hole effective mass increases slightly. Under compressive strain, the electron effective mass increases significantly, while the hole effective mass decreases slightly. The valance band maximum (VBM) state and conduction band minimum (CBM) state charge density are always distributed inside the silicon nanowires structure with applied strain.1 Introduction ...6 Density functional theory ...7.1 Density functional theory ...7.1.1 The Hohenberg-Kohn theorems...7.1.2 The Kohn-Sham equations ...9 Electronic structure computational methods ...12.1 Calculation method ...12.1.1 About OpenMX ...12.2 Norm-conserving pseudopotentials (NCPPs) ...13.3 Pseudo-atomic localized basis functions ...14.3.1 Localized atom-centered orbitals ...14.3.2 Total energy term in OpenMX ...15.3.3 Two-center integrals ...17.4 Locality and linear scaling O(N) methods ...19 The electronic properties of bulk Si ...22.1 The bulk Si band structure ...22.2 The strained bulk Si ...24 The quantum confinement theory ...27.1 The Si nanowires confinement theory ...27 Atomic and electronic structures of strained [110] Si nanowires ...33.1 The [110] silicon nanowires structure ...33.2 The conduction band strucutre ...36.3 The valence band strucutre ...37.4 The band gap properties ...38.5 Effective mass ...41.6 Nature of the band-edge orbitals ...44 Summary ...46Bibliography ...4

Enhanced Sensitivity of CO on Two-Dimensional, Strained, and Defective GaSe

The toxic gas carbon monoxide (CO) is fatal to human beings and it is hard to detect because of its colorless and odorless properties. Fortunately, the high surface-to-volume ratio of the gas makes two-dimensional (2D) materials good candidates for gas sensing. This article investigates CO sensing efficiency with a two-dimensional monolayer of gallium selenide (GaSe) via the vacancy defect and strain effect. According to the computational results, defective GaSe structures with a Se vacancy have a better performance in CO sensing than pristine ones. Moreover, the adsorption energy gradually increases with the scale of tensile strain in defective structures. The largest adsorption energy reached −1.5 eV and the largest charger transfer was about −0.77 e. Additionally, the CO gas molecule was deeply dragged into the GaSe surface. We conclude that the vacancy defect and strain effect transfer GaSe to a relatively unstable state and, therefore, enhance CO sensitivity. The adsorption rate can be controlled by adjusting the strain scale. This significant discovery makes the monolayer form of GaSe a promising candidate in CO sensing. Furthermore, it reveals the possibility of the application of CO adsorption, transportation, and releasement

Ab Initio Research on a New Type of Half-Metallic Double Perovskites, A2CrMO6 (A = IVA Group Elements; M = Mo, Re and W)

The research based on density functional theory was carried out using generalized gradient approximation (GGA) for full-structural optimization and the addition of the correlation effect (GGA + U (Coulomb parameter)) in a double perovskite structure, A2BB’O6. According to the similar valance electrons between IIA(s2) and IVA(p2), IVA group elements instead of alkaline-earth elements settled on the A-site ion position with fixed BB' combinations as CrM (M = Mo, Re and W). The ferrimagnetic half-metallic (HM-FiM) properties can be attributed to the p-d hybridization between the Crd-Mp and the double exchange. All the compounds can be half-metallic (HM) materials, except Si2CrMoO6, Ge2CrMo and Ge2CrReO6, because the strong-correlation correction should be considered. For M = W, only A = Sn and Pb are possible candidates as HM materials. Nevertheless, an examination of the structural stability is needed, because Si, Ge, Sn and Pb are quite different from Sr. All compounds are stable, except for the Si-based double perovskite structure