We study the possibility of spin injection from Fe into Si(001), using the Schottky barrier at the Fe/Si contact as tunneling barrier. Our calculations are based on density-functional theory for the description of the electronic structure and on a Landauer-Buttiker approach for the current. The current-carrying states correspond to the six conduction band minima of Si, which, when projected on the (001) surface Brillouin zone (SBZ), form five conductance hot spots: one at the SBZ center and four symmetric satellites. The satellites yield a current polarization of about 50%, while the SBZ center can, under very low gate voltage, yield up to almost 100%, showing a zero-gate anomaly. This extremely high polarization is traced back to the symmetry mismatch of the minority-spin Fe wavefunctions to the conduction band wavefunctions of Si at the SBZ center. The tunneling current is determined by the complex band structure of Si in the [001] direction, which shows qualitative differences compared to that of direct-gap semiconductors. Depending on the Fermi level position and Schottky barrier thickness, the complex band structure can cause the contribution of the satellites to be orders of magnitude higher or lower than the central contribution. Thus, by appropriate tuning of the interface properties, there is a possibility to cut off the satellite contribution and to reach high injection efficiency. Also, we find that a moderate strain of 0.5% along the [001] direction is sufficient to lift the degeneracy of the pockets so that only states at the zone center can carry current.
We develop a phase-space ab-initio formalism to compute Ballistic Electron Emission Spectroscopy current-voltage I(V)'s in a metal-semiconductor interface. We consider injection of electrons into the conduction band for direct bias ($V>0$) and injection of holes into the valence band or injection of secondary Auger electrons into the conduction band for reverse bias ($V<0$). Here, an ab-initio description of the semiconductor inversion layer (spanning hundreds of Angstroms) is needed. Such formalism is helpful to get parameter-free best-fit values for the Schottky barrier, a key technological …
Spin injection across interfaces driven by ultrashort optical pulses on femtosecond timescales constitutes a new way to design spintronics applications. Targeted utilization of this phenomenon requires knowledge of the efficiency of non-equilibrium spin injection. From a quantitative comparison of ab-initio time-dependent density functional theory and interface-sensitive, time-resolved non-linear optical experiment, we determine the spin injection efficiencies (SIE) across ferromagnetic/metal interfaces and discuss their microscopic origin, i.e. the influence of spin-orbit coupling and the int…
Improving Li$_2$O$_2$ conductance via polaron preemption: an \textit{ab initio} study of Si doping
本文报道了Li $_2$ O $_2$的\textit{ab initio}电子结构模拟，其中1.6%的锂原子被硅取代。结果表明，这导致了Li $_2$ O $_2$带隙中导电杂质态的形成.我们表明，这些国家起源于反键轨道的氧对，是非常稳定的对可能的极化子形成（电子注入后）。通过这种极化子抢占机制，预期所提出的化合物显示出比化学计量的Li $_2$ O $_2$显著更高的电子迁移率，这可能在锂-空气电池中具有重要的应用。
Moving beyond traditional 2D materials is now desirable to have switching capabilities (e.g., transistors). Here we propose using borophene because, as we will show in this letter, obtaining regions of the electronic bandstructure which act as valence and conduction bands, with an apparent bandgap, may be obtainable in the foreseeable future. Here for particular allotropes of borophene, density of states (DOS) and electronic bandstructure diagrams with E(k) vs k are found with much improved accuracy by ab initio quantum calculations using hybrid functionals of several types. This procedure sho…
Defects creation and annihilation is a fundamental concept in device fabrication. This report studies the optical bandgap modification in MgO by MeV Ni ion irradiation-induced defect states between valance and conduction band. Ion implantation on MgO single crystal produces substitutional defect states along with F (anionic vacancy center), $F_2$, other oxygen vacancy center and V (cationic vacancy center) centers confirmed from absorption and photoluminescence spectra that can be applied as filament in valance charge memory based resistive random access memories. The variation of optical band…
Valence Bond Order and Antiferromagnetism in Silicene - ab initio Results
硅烯和石墨烯相似，具有$π$ - $π^*$带。然而，硅烯的带宽仅为石墨烯的三分之一。这导致Hubbard U与带宽W的比率显著增加，石墨烯中的U/W $\sim$ 0.5比硅烯中的$\sim 1$。这种增强、二维性和唯象性表明硅烯（G. Baskaran，arXiv：1309.2242）.我们通过从头计算表明，与石墨烯不同，硅烯具有两种不稳定性：i）价键（凯库勒）二聚化和ii）弱的两个亚晶格反铁磁顺序，从而支持上述提议。这些不稳定性的存在，在费米面嵌套的情况下，指向莫特本地化，\textit{within the frame work of ab-initio scheme}。基底依赖的结构重建实验中看到的硅烯被解释为广义凯库勒键序。
Unmasking charge transfer in the Misfits: ARPES and ab initio prediction of electronic structure in layered incommensurate systems without artificial strain
Efficient ab-initio method for the calculation of frequency dependent non-linear optical response in semiconductors: application to second harmonic generation
Ab-initio study of structural, elastic, electronic, optical and thermodynamic properties of MgV2O6
采用基于密度泛函理论（DFT）的平面波紫外赝势方法，对正交MgV 2 O 6的结构、力学、电子、光学和热力学性质进行了从头计算.计算的晶格参数与实验数据符合得很好。计算了二阶弹性常数和其它相关量，如杨氏模量、剪切模量、泊松比、压缩系数、各向异性因子、声速和德拜温度。在分析计算的弹性常数后，表明所研究的化合物是机械稳定的。电子能带结构分析表明，该化合物具有半导体性质，带隙为2.195 eV，主要来自O-2s态。
Heterovalent interlayers and interface states: an ab initio study of GaAs/Si/GaAs (110) and (100) heterostructures
我们从第一性原理出发，研究了GaAs/Si/GaAs（110）和（100）取向突变异质结构中的局域态和共振态的存在，以及充分发展的Si/GaAs（110）异质结中的局域态和共振态。在（100）取向的结构中，我们发现了位于Si/GaAs界面的价带和导带相关的近带边态。相反，在（110）系统中，界面态出现在价带的更深处;最高的价相关共振比GaAs价带最大值低约1 eV。利用它们特有的成键性质和原子特性，我们能够跟踪从充分发展的Si/GaAs二元结到包含2或1 ML Si的三元GaAs/Si/GaAs（110）系统的局域态和共振的演化。这种方法也使我们能够显示（110）和（100）系统的界面状态之间的联系。最后，基于Koster-Slater模型讨论了Si/GaAs（110）界面局域态存在的条件。
The electronic properties of the twisted InSe bilayer are studied by large-scale density functional theory. Spectral Function Unfolding reveals that the electronic structure of the twisted system can be described in terms of a combination of features of the bandstructures of the aligned InSe bilayer with different stacking configurations, enabling predictions of the band gap and the effective mass for holes. The effective mass for holes in the twisted InSe bilayer is shown to be influenced primarily by the interlayer distance. The intralayer and interlayer exciton binding energies are thus cal…
The change in the composition of Pb$_{1-x}$Sn$_x$Te IV-VI semiconductor or in its lattice parameter can drive a transition from the topologically trivial to the topological crystalline insulator (TCI), crossing a region where the alloy is in the Weyl semimetal phase. Incorporation of the magnetic Mn ions induces strong perturbations of the electronic structure, which act on both orbital and spin variables. Our first principles calculations show that the presence of Mn shifts the TCI and the Weyl region towards higher Sn contents in Pb$_{1-x}$Sn$_x$Te. When the Mn spin polarization is finite, t…
Density functional theory (DFT) and thermal DFT (thDFT) calculations were used to evaluate the energy band structure, bandgap, and the total energy of various graphene quantum dots (GQDs). The DFT calculations were performed using local density approximation for the exchange-correlation functional and norm-conserving pseudopotentials. We consider the triangular and hexagonal GQDs with zigzag and armchair edges and 1-3 nm dimensions with many hundred atoms. The simulation results show that all of these GQDs are direct bandgap semiconductors with a flat band structure, and they are suitable for …
Ultra-wide band gap (UWBG) materials are poised to play an important role in the future of power electronics. Devices made from UWBG materials are expected to operate at higher voltages, frequencies, and temperatures than current silicon and silicon carbide based devices; and can even lead to significant miniaturization of such devices. In the UWBG field, aluminum nitride and boron nitride have attracted great interest, however, the B$_x$Al$_{1-x}$N alloys are much less studied. In this article, using first-principles simulations combining density-functional theory and cluster expansion method…
