Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronicstructure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectivelylocalised Wannier functions, selected columns of the density matrix) and the ability to calculate new properties (shift currents and Berry-curvature dipole, and a new interface to many-body perturbation theory); performance improvements, including parallelisation of the core code; enhancements in functionality (support for spinor-valued Wannier functions, more accurate methods to interpolate quantities in the Brillouin zone); improved usability (improved plotting routines, integration with arXiv:1907.09788v1 [cond-mat.mtrl-sci]
Two-dimensional (2D) van der Waals (vdW) materials provide the possibility of realizing heterostructures with coveted properties. Here, we report a theoretical investigation of the vdW magnetic tunnel junction (MTJ) based on VSe 2 /MoS 2 heterojunction, where the VSe 2 monolayer acts as the ferromagnet with the room-temperature ferromagnetism. We propose the concept of spin-orbit torque (SOT) vdW MTJ with reliable reading and efficient writing operations. The non-equilibrium study reveals a large tunneling magnetoresistance (TMR) of 846 % at 300 Kelvin, identifying significantly its parallel and anti-parallel states. Thanks to the strong spin Hall conductivity of MoS 2 , SOT is promising for the magnetization switching of VSe 2 free layer. arXiv:1904.07499v1 [cond-mat.mtrl-sci] 16 Apr 2019 Quantum-well states come into being and resonances appear in MTJ, suggesting that the voltage control can adjust transport properties effectively. The SOT vdW MTJ based on VSe 2 /MoS 2 provides desirable performance and experimental feasibility, offering new opportunities for 2D spintronics.
Ab-initio calculation of intrinsic spin Hall conductivity (SHC) generally requires a strict convergence criterion and a dense k-point mesh to sample the Brillouin zone, making its convergence challenging and time-consuming. Here we present a scheme for efficiently and accurately calculating SHC based on maximally localized Wannier function (MLWF). The quantities needed by the Kubo formula of SHC are derived in the space of MLWF and it is shown that only the Hamiltonian, the overlap and the spin operator matrices are required from the initial ab-initio calculation. The computation of these matrices and the interpolation of Kubo formula on a dense k-point mesh can be easily achieved. We validate our results by prototypical calculations on fcc Pt and GaAs, which demonstrate that the Wannier interpolation approach is of high accuracy and efficiency. Calculations of α-Ta and β-Ta show that SHC of β-Ta is 2.7 times of α-Ta, while both have the opposite sign relative to fcc Pt and are an order of magnitude smaller than Pt. The calculated spin Hall angle of −0.156, is quite consistent with previous experiment on β-Ta, further suggesting intrinsic contribution may dominate in β-Ta. Our approach could facilitate large-scale SHC calculations, and may benefit the discovery of materials with high intrinsic SHC. arXiv:1810.07637v1 [cond-mat.mtrl-sci] 17 Oct 2018 (0) nk to
We report a comprehensive study on the intrinsic spin Hall conductivity (SHC) in semimetals MoTe2 and WTe2 by the ab initio calculation. Large SHC and desirable spin Hall angles have been discovered, due to the strong spin orbit coupling effect and low charge conductivity in semimetals. Diverse anisotropic SHC values, attributed to the unusual reduced-symmetry crystalline structure, have been revealed. We report an effective method on SHC optimization by electron doping, and exhibit the mechanism on SHC variation with energy shifting by the spin Berry curvature. Our work provides insights into the realization of strong spin Hall effects in 2D systems.
Magnetic tunnel junction (MTJ) based on CoFeB/MgO/CoFeB structures is of great interest due to its application in the spin-transfer-torque magnetic random access memory (STT-MRAM). Large interfacial perpendicular magnetic anisotropy (PMA) is required to achieve high thermal stability. Here we use first-principles calculations to investigate the magnetic anisotropy energy (MAE) of MgO/CoFe/capping layer structures, where the capping materials include 5d metals Hf, Ta, Re, Os, Ir, Pt, Au and 6p metals Tl, Pb, Bi. We demonstrate that it is feasible to enhance PMA by using proper capping materials. Relatively large PMA is found in the structures with capping materials of Hf, Ta, Os, Ir and Pb. More importantly, the MgO/CoFe/Bi structure gives rise to giant PMA (6.09 mJ/m 2 ), which is about three times larger than that of the MgO/CoFe/Ta structure. The origin of the MAE is elucidated by examining the contributions to MAE from each atomic layer and orbital. These findings provide a comprehensive understanding of the PMA and point towards the possibility to achieve advanced-node STT-MRAM with high thermal stability.
This study aims at qualifying the contribution of the urban effect to the total warming recorded by 45 urban or suburban stations in China where rapid and extensive urbanization over the last few decades occurred. Partly due to differences in urbanization and stations' geographic location, the total warming trends for 1955-2012 vary from of −0.10 to 0.49°C and −0.03 to 0.64°C per decade for the annual averaged daily mean and daily minimum temperature, respectively. A principal component analysis of seven factors on the siting and geographical coordinates of the meteorological stations shows three dominant factors (urban size, relative position of meteorological station to city center and geographic location of station) accounting for 87.1 % of the total explained variance. An index quantifying the impact of the first two dominating factors of the urban effect is proposed considering also the dominating wind direction. The positive correlation between the temperature trends and the index is significant (P<0.05), indicating that urbanization has significantly influenced the Climatic Change (2015) 132:631-643
It has been reported in experiments that capping layers which enhance the perpendicular magnetic anisotropy (PMA) of magnetic tunnel junctions (MTJs) induce great impact on the tunnel magnetoresistance (TMR). To explore the essential influence caused by capping layers, we carry out ab initio calculations on TMR in the X(001)|CoFe(001)|MgO(001)|CoFe(001)|X(001) MTJ, where X represents the capping layer material which can be tungsten, tantalum or hafnium. We report TMR in different MTJs and demonstrate that tungsten is an ideal candidate for a giant TMR ratio. The transmission spectrum in Brillouin zone is presented. It can be seen that in the parallel condition of MTJ, sharp transmission peaks appear in the minority-spin channel. This phenomenon is attributed to the resonant tunnel transmission effect and we explained it by the layer-resolved density of states (DOS). In order to explore transport properties in MTJs, the density of scattering states (DOSS) was studied from the point of band symmetry. It has been found that CoFe|tungsten interface blocks scattering states transmission in the anti-parallel condition. This work reports TMR and transport properties in MTJs with different capping layers, and proves that tungsten is a proper capping layer material, which would benefit the design and optimization of MTJs.Since the discovery of tunnel magnetoresistance (TMR) in the Fe|Ge-oxide|Co trilayer structure, 1 the TMR effect has become the significant principle of non-volatile magnetic random access memory (MRAM). 2 As the basic element of MRAM, magnetic tunnel junction (MTJ) was investigated in detail. 3 Thanks to rapid advances in growth technique of ultra-thin ferromagnetic films, CoFeB|MgO MTJs with high TMR ratio have been achieved 4-7 as predicted by ab initio calculations. 8 Recently, based on MTJs with high TMR and the spin transfer torque effect, the spin-transfer-torque MRAM (STT-MRAM) is achieved with merits of high density and low power consumption. 9 _______________________________________ a) Electronic
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