Based on the first-principle calculations,we propose that the monolayer VCl3 and VBr3 are quantum anomalous Hall insulators with in-plane magnetization without considering the correlation effect of the 3d electron-electron interaction.The band gap is predicted to be about 3.4 meV for VCl3,but no global gap for VBr3.It is interesting to note that VCl3 (VBr3) possesses a Chern number of C =3 (C =1) with three (one) chiral edge states.After considering correlation effect,we obtain Mott insulator if U＞0.45 (U＞0.35) eV for VCl3(VBr3).

Based on the first-principle calculations, we propose that the monolayer VCl3 and VBr3 are quantum anomalous Hall insulators with in-plane magnetization without considering the correlation effect of the 3d electron-electron interaction. The band gap is predicted to be about 3.4 meV for VCl3, but no global gap for VBr3. It is interesting to note that VCl3 (VBr3) possesses a Chern number of C=3 (C=1) with three (one) chiral edge states. After considering correlation effect, we obtain Mott insulator if U>0.45 (U>0.35) eV for VCl3 (VBr3).

Hybrid organic-inorganic perovskite solar cells (HOIPs), especially CH3NH3PbI3 (MAPbI3), have received tremendous attention due to their excellent power conversion efficiency (25.2%). However, two fundamental hurdles, long-term stability and lead (Pb) toxicity, prevent HOIPs from practical applications in the solar industry. To overcome these issues, compositional engineering has been used to modify cations at A- and B-sites and anions at the X-site in the general form ABX3. In this work, we used the density functional theory (DFT) to incorporate Rb, Cs, and FA at the A-site to minimize the volatile nature of MA, while the highly stable Ca2+ and Sr2+ were mixed with the less stable Ge2+ and Sn2+ at the B-site to obtain a Pb-free perovskite. To further enhance the stability, we mixed the X-site anions (I/Br). Through this approach, we introduced 20 new perovskite species to the lead-free perovskite family and 7 to the lead-containing perovskite family. The molecular dynamic (MD) simulations, enthalpy formation, and tolerance and octahedral factor study confirm that all of the perovskite alloys we introduced here are as stable as pristine MAPbI3. All Pb-free perovskites have suitable and direct band gaps (1.42-1.77 eV) at the Γ-point, which are highly desirable for solar cell applications. Most of our Pb-free perovskites have smaller effective masses and exciton binding energies. Finally, we show that the introduced perovskites have high absorption coefficients (105 cm-1) and strong absorption efficiencies (above 90%) in a wide spectral range (300-1200 nm), reinforcing their significant potential applications. This study provides a new way of searching for stable lead-free perovskites for sustainable and green energy applications.

基于非平衡格林函数结合密度泛函理论的第一性原理量子输运方法,预言一种完全自旋极化的电子器件。该器件是由半金属Ti Cl3做电极,半导体Rh Cl3做中心区搭建的磁性隧道结。分别计算小偏压范围（0～20 m V）和大偏压范围（0～0. 6 V）下的自旋极化电流曲线。发现在小偏压范围内,器件的隧穿磁电阻（tunneling magnetoresistance,TMR）高达100%,并在这一偏压范围内维持稳定;在电极极化方向平行构型（parallel configuration,PC）下的自旋注入效率也高达100%,具有很强的稳定性。在大偏压范围内,随着偏压的增加TMR逐渐减小,PC构型下的自旋注入效率一直保持100%不变。最后,通过对器件投影态密度图的分析,解释上述物理现象。

Based on the spinor Boltzmann equation, we obtain the continuity equations of charge and spin accumulation in two-dimensional ferromagnets with Rashba or Dresselhaus spin-orbit coupling, in which the spin diffusion equation (the continuity equation of spin accumulation) gives the spin-orbit torques that have an extra term of charge density gradient that is paid little attention in previous studies. Numerical results indicate this term plays an important role in inhomogeneous systems. In order to study the magnetization dynamics of ferromagnets, we solve the Landau-Lifshitz-Gilbert equation together with the spin diffusion equation. Our results indicate that the magnetization in systems with Rashba SOI is easier to be switched by an external electric field than the system with Dresselhaus SOI, and for anisotropic ferromagnets, we study the critical external electric field that manipulate the reversal of magnetization.

Inspired by the successful synthesis of Fe/Cu-5,5 '-bis(4-pyridyl)(2,2 '-bipirimidine) (PBP), a family of two-dimensional (2D) metal-organic frameworks (MOFs) with the Shastry-Sutherland lattice, i.e., transition metal (TM)-PBP (TM = Cr, Mn, Fe, Co, Ni, Cu, Zn) has been systematically investigated by means of first-principles density functional theory calculations and Monte Carlo simulations. Mn-PBP is discovered to be the first ferromagnetic 2D MOF with the Shastry-Sutherland lattice and the Curie temperature is predicted to be about 105 K, while Fe-PBP, TM-PBP (TM = Cr, Co, Ni) and TM-PBP (TM = Cu, Zn) are found to be stripe-order antiferromagnetic, magnetic-dimerized and nonmagnetic, respectively. The electronic structure calculations reveal that TM-PBP MOFs are semiconductors with band gaps ranging from 0.12 eV to 0.85 eV, which could be easily modulated by various methods. Particularly, Mn-PBP would exhibit half-metallic behavior under compressive strain or appropriate electron/hole doping and a Mn-PBP based spintronic device has been proposed. This study not only improves the understanding of the geometric, electronic and magnetic properties of the 2D TM-PBP MOF family, but also provides a novel spin lattice playground for the research of 2D magnetic systems, which has diverse modulating possibilities and rich potential applications.

近年来,新的二维材料不断被发现和合成,探索新型二维材料成为研究热点.采用第一性原理计算对不同二维材料进行筛选,发现二维三氯化锆(ZrCl3)具有半金属性质.系统研究二维三氯化锆的几何结构、电子结构和磁结构等性质,通过声子谱计算,发现其结构是动力学稳定的;通过电子结构计算,发现费米能级附近只存在自旋向上的电子态密度,显示二维三氯化锆具有半金属特性.通过比较其铁磁态和反铁磁态的能量,发现铁磁基态能量更低,每个金属Zr原子具有1个玻尔磁子的磁矩.估算出磁交换常数J=7.58 meV.结合蒙特卡洛模拟,估计出该体系的居里温度约为130 K.

Organic inorganic halide perovskites are quite promising in applications of large scale photovoltaic technology. However, toxicity is one of the crucial issues in these materials, and searching for environmentally friendly perovskite materials for green energy applications is in high demand. Here we present a systematic ab initio study on the replacement of toxic Pb in the perovskite CH3NH3PbI3 (MAPbI(3)) with possible mono- and a few binary replacements. In the mono-replacements study, Ge and Sn are the best alternatives to Pb. In the binary replacements, we replace Pb by mixing Ca/Si and Zn/Si. In case of Ca/Si, a monotonic decrease in band gaps with a monotonic increase in the optical absorption was observed with increasing the Ca concentration. It is observed for the first time that the substitution of Ca/Si (or Zn/Si) at the B-site with various ratios would lead to remarkably high device absorption efficiencies. The band gaps of the studied mixed replacements are in the ideal ranges for single-junction solar cell and one cell in tandem architecture. As a result of the smaller effective masses, the mixed replacements could have better carrier mobility. An ab initio molecular dynamic simulation demonstrates the stability of the mixed replacements. More importantly, the mixed substituting elements are highly abundant in the earth. This work is helpful to gain further insights into developing green solar cells with low cost and high performance and would lead to wide applications in the future.

The properties of Li-ion adsorption and migration in layered SnSe2 are systematically investigated using the first principle calculations.It is found that the Li atoms are adsorbed strongly on substrate SnSe2,and the binding energy (＞ 3 eV) is significantly higher than those on graphene,phosphorene,MoS2,and some other two-dimensional (2D) layered materials.Bader charge analysis reveals that almost the whole charge of 2s electron of the Li atom transfers to substrate SnSe2 and Li exists in the cationic state.The Li-ion migration energy barrier for monolayer SnSe2 is 0.197 eV,which is significantly lower than those for graphene,MoS2,and other 2D materials.The average open-circuit voltage of 3.05 V is predicted in the monolayer SnSe2-based Li-ion battery.The Li intercalation also leads to a transition from semiconductor to metallic state and gives rise to a good electrical conductivity.These findings provide insights into the Li-ion adsorption properties and migration mechanism in layered transition-metal dichalcogenide.

Novel stable perovskites: Pb in CH3NH3PbI3 is replaced by Ca/(Ge, Sn) or Sr/(Ge, Sn) and exhibits high optical absorption.

硒化锡(SnSe)和硫化锡(SnS)是具有广泛应用前景的热电材料和光电材料.通过应用第一性原理方法,系统地研究硒化锡和硫化锡的力学性质和能带结构,探讨应变对其性质的影响.发现这些性质是各向异性的.计算应变-应变曲线、杨氏模量、声子谱、声速和应变下的能带结构.发现硒化锡和硫化锡是具有负泊松率的拉胀材料,它们可用于传感器和生物医学等领域;在Z方向施加单轴-5％到5％的应变时,硒化锡的带隙从0.7 eV变到1.03 eV,硫化锡的带隙从0.85 eV变到1.41 eV,表明应变是调控硒化锡和硫化锡太阳能转换效率的一种有效方法.

The stability, geometric structure and electronic properties of a novel body-centered tetragonal carbon allotrope Bct C-8 are systematically investigated. Bct C-8 are formed by sp(3)-bonded carbon atoms and can be regarded as a compressed bundle of carbon nanotubes (CNT). The transition path from CNT to Bct C-8 has been simulated, indicating a possible approach to synthesize Bct C-8. The X-Ray diffraction, phonon and Raman spectroscopies are presented as reference for experimental studies. Besides, Bct C-8 is a semiconductor with an indirect gap of 1.66 eV and will transform to semimetal when doping with boron and nitride atoms. Furthermore, considering its porous structural character, the lithium storage capability of Bct C-8 are also discussed, a directional fast lithium-ion mobility with a rather low 0.04 eV barrier is revealed. Therefore, the Bct C-8, once synthesized experimentally, would have fruitful applications in carbon-based electronics and energy storage. (C) 2017 Elsevier Ltd. All rights reserved.

By means of extensive ab initio calculations, a new two-dimensional (2D) atomic material tin selenide monolayer (coined as tinselenidene) is predicted to be a semiconductor with an indirect gap (~1.45 eV) and a high hole mobility (of order 10000 cm 2 V −1 S −1 ) and will bear an indirect-direct gap transition under a rather low strain (<0.5 GPa). Tinselenidene has a very small Young’s modulus (20–40 GPa) and an ultralow lattice thermal conductivity (<3 Wm −1 K −1 at 300 K), making it probably the most flexible and most heat-insulating material in known 2D atomic materials. In addition, tinseleniden has a large negative Poisson’s ratio of −0.17, thus could act as a 2D auxetic material. With these intriguing properties, tinselenidene could have wide potential applications in thermoelectrics, nanomechanics and optoelectronics.

By means of first-principles calculations, we explore systematically the geometric, electronic and piezoelectric properties of multilayer SnSe. We find that these properties are layer-dependent, indicating that the interlayer interaction plays an important role. With increasing the number of SnSe layers from 1 to 6, we observe that the lattice constant decreases from 4.27 $\mathring{A}$ to 4.22 $\mathring{A}$ along zigzag direction, and increases from 4.41 $\mathring{A}$ to 4.51 $\mathring{A}$ along armchair direction close to the bulk limit (4.21 $\mathring{A}$ and 4.52 $\mathring{A}$, respectively); the band gap decreases from 1.45 eV to 1.08 eV, approaching the bulk gap 0.95 eV. Although the monolayer SnSe exhibits almost symmetric geometric and electronic structures along zigzag and armchair directions, bulk SnSe is obviously anisotropic, showing that the stacking of layers enhances the anisotropic character of SnSe. As bulk and even-layer SnSe have inversion centers, they cannot exhibit piezoelectric responses. However, we show that the odd-layer SnSe have piezoelectric coefficients much higher than those of the known piezoelectric materials, suggesting that the odd-layer SnSe is a good piezoelectric material.

The anisotropic antiferromagnetic Ising model on the fractal Sierpiński gasket is intensively studied, and a number of exotic properties are disclosed. The ground state phase diagram in the plane of magnetic field-interaction of the system is obtained. The thermodynamic properties of the three plateau phases are probed by exploring the temperature-dependence of magnetization, specific heat, susceptibility and spin-spin correlations. No phase transitions are observed in this model. In the absence of a magnetic field, the unusual temperature dependence of the spin correlation length is obtained with 0 ≤ J b / J a < 1, and an interesting crossover behavior between different phases at J b / J a = 1 is unveiled, whose dynamics can be described by the J b / J a -dependence of the specific heat, susceptibility and spin correlation functions. The exotic spin-spin correlation patterns that share the same special rotational symmetry as that of the Sierpiński gasket are obtained in both the 1 / 3 plateau disordered phase and the 5/9 plateau partially ordered ferrimagnetic phase. Moreover, a quantum scheme is formulated to study the thermodynamics of the fractal Sierpiński gasket with Heisenberg interactions. We find that the unusual temperature dependence of the correlation length remains intact in a small quantum fluctuation.

The current-induced magnetic switching of a single-molecule magnet (SMM) attached on the central region of a spin valve is explored, and the condition for the switching current is derived. Electrons flowing through the spin valve will interact with the SMM via the s–d exchange interaction, producing the spin accumulation that satisfies the spin diffusion equation. We further describe the spin motion of the SMM by a Heisenberg-like equation. Based on the linear stability analysis, we obtain the critical current from two coupled equations. The results of the critical current versus the external magnetic field indicate that one can manipulate the magnetic state of the SMM by an external magnetic field.

Base on Hancheng Mine coal samples ,Weibei coalfield was elaborated by means of mercury porosimetry combined with small angle X-ray scattering (SAXS) based upon the fractal theory .The results show that the variation characteristics of pore structure and pore heterogeneity of tectonically deformed coals (TDC) can be represented by the fractal dimension D .Strong deformed coals have higher fractal dimension DHg ,higher heterogeneity and more complicated pore structure/surface ,and lower permeability ,indicating that the complicated pore structure resulted from strong tectonic deformation contributes to low permeable coalbed methane reservoir .With the stress strengthening ,fractal dimension DSAXS shows an increasing trend , suggesting that tectonic deformation leads to the complication of pore surface structure .Thus ,the fractal dimension can be used as an indicator of the deformation degree of pore structure .

The properties of diamond polytypes under high pressure have been investigated by means of the first-principles calculations. They are unveiled to be thermodynamically and kinetically more stable than hexagonal diamond (h-diamond) even under pressure. The electronic and elastic properties of the diamond polytypes are also explored, revealing very close cohesive energy, band gap and equilibrium density to those of cubic diamond (c-diamond). The Vickers hardness is uncovered to be comparable or even larger than those of the c- and h-diamond. Furthermore, they experience lower energy barriers than the h-diamond during the transforming process from graphite under pressure. Especially, we explored the characterized Raman modes under pressure from 0 to 40GPa for each allotrope, which can be used to identify the diamond polytypes from the c-diamond T2g background mode, and the result may serve as a useful guide for future experimental studies. In addition, the simulated X-ray diffraction spectra under pressure is also presented.