磁性硅烯超晶格中电场调制的谷极化和自旋极化

研究了基于硅烯的静电势超晶格、铁磁超晶格、反铁磁超晶格中谷极化、自旋极化以及赝自旋极化的输运性质,分析了铁磁交换场、反铁磁交换场以及化学势对输运性质的影响,讨论了电场对谷极化、自旋极化以及赝自旋极化的调控作用.结果表明:当3种超晶格的晶格数达到10以上时,在硅烯超晶格中很容易实现100%的谷极化、自旋极化和赝自旋极化,而且通过调节超晶格上的外加电场可以使极化方向发生翻转,从而在硅烯超晶格中实现外电场对谷自由度、自旋自由度以及赝自旋自由度的操控.     

应力对硅烯上锂吸附的影响

本文采用基于密度泛函理论的第一性原理平面波赝势方法研究了双轴应力作用下锂原子吸附硅烯的结构及其稳定性. 计算结果表明,在拉应力和一定的压应力作用下,锂吸附的硅烯体系基本保持原有的结构. 而当更大的压应力作用时,硅烯产生了向锂原子方向凸起的结构变化,所得到的体系的总能也有明显地下降. 本文通过对各种应力下的硅烯声子谱的计算,分析了在压应力作用下锂吸附的硅烯结构不稳定的原因. 关键词： 硅烯 应力 第一性原理 声子谱     

Band gap engineering in silicene: A theoretical study of density functional tight-binding theory

In this work, we performed first principles calculations based on self-consistent charge density functional tight-binding to investigate different mechanisms of band gap tuning of silicene. We optimized structures of silicene sheet, functionalized silicene with H, CH₃ and F groups and nanoribbons with the edge of zigzag and armchair. Then we calculated electronic properties of silicene, functionalized silicene under uniaxial elastic strain, silicene nanoribbons and silicene under external electrical fields. It is found that the bond length and buckling value for relaxed silicene is agreeable with experimental and other theoretical values. Our results show that the band gap opens by functionalization of silicene. Also, we found that the direct band gap at K point for silicene changed to the direct band gap at the gamma point. Also, the functionalized silicene band gap decrease with increasing of the strain. For all sizes of the zigzag silicene nanoribbons, the band gap is near zero, while an oscillating decay occurs for the band gap of the armchair nanoribbons with increasing the nanoribbons width. At finally, it can be seen that the external electric field can open the band gap of silicene. We found that by increasing the electric field magnitude the band gap increases.     

Influence of strain and electric field on the properties of silicane

We investigate the influence of strain and electric field on the properties of a silicane sheet. Some elastic parameters of silicane, such as an in-plane stiffness of 52.55 N/m and a Poisson’s ratio of 0.24, are obtained by calculating the strain energy. Compared with silicene, silicane is softer because of its relatively weaker Si-Si bonds. The band structure of silicane is tunable by a uniform tensile strain, with the increase of which the band gap decreases monotonously. Moreover, silicane undergoes an indirect-direct gap transition under a small strain, and a semiconductor-metal transition under a large strain. The electric field can change the Si-H bond length of silicane significantly. When a strong field is applied, the H atom at the high potential side becomes desorbed, while the H atom at the low potential side keeps bonded. So an external electric field can help to produce single-side hydrogenated silicene from silicane. We believe this study will be helpful for the application of silicane in the future.     

Defect silicene and graphene as applied to the anode of lithium-ion batteries: Numerical experiment

Mechanical properties and stability of two layers of defect silicene supported by graphene sheets, between which the lithium ion passes under an electrostatic field, are studied by the molecular dynamics method. Defects are mono-, di-, tri-, and hexavacansies. Graphene and silicene edges are rigidly fixed. Graphene sheets contacting with silicene take a convex shape, deflecting outward. Mono- and divacancies in silicene tend to a size decrease; larger vacancies exhibit better stability. The ion motion control using an electric field becomes possible only using perfect silicene or silicene with mono- and divacancies. The ion penetrated through larger defects, and its motion in the silicene channel becomes uncontrolled. When the ion moves in the channel, the most strong stress spikes appear in silicene containing monovacancies. In the case of fixed edges, perfect silicene intercalated with a lithium ion is inclined to accumulate larger stresses than silicene containing defects.     

Modulation of electronic properties with external fields in silicene-based nanostructures

This work reviews our recent works about the density functional theory(DFT) calculational aspects of electronic properties in silicene-based nanostructures with the modulation of external fields, such as electric field, strain, etc. For the two-dimensional(2D) silicene-based nonostructures, the magnetic moment of Fe-doped silicene shows a sharp jump at a threshold electric field, which indicates a good switching effect, implying potential applications as a magnetoelectric(ME) diode. With the electric field, the good controllability and sharp switching of the magnetism may offer a potential applications in the ME devices. For the one-dimensional(1D) nanostructures, the silicene nanoribbons with sawtooth edges(SSi NRs) are more stable than the zigzag silicene nanoribbons(ZSiNRs) and show spin-semiconducting features. Under external electric field or uniaxial compressive strain, the gapless spin-semiconductors are gained, which is significant in designing qubits for quantum computing in spintronics. The superlattice structures of silicene-based armchair nanoribbons(ASiSLs) is another example for 1D silicene nanostructures. The band structures of ASi SLs can be modulated by the size and strain of the superlattices. With the stain increased, the related energy gaps of ASi SLs will change, which are significantly different with that of the constituent nanoribbons. The results suggest potential applications in designing quantum wells.     

Valley-dependent electron transport in ferromagnetic/normal/ferromagnetic silicene junction

The electron transport through ferromagnetic/normal/ferromagnetic silicene junction with an induced energy gap is investigated in this work. The energy gap can be tuned by applying electric field or exchange fields due to the buckled structure of silicene. We analyze the local electric field, exchange field, length of normal region-dependence transmission probabilities of four groups and valley conductance. These transmission probabilities and valley conductance can be turned on or off by adjusting the local electric field and exchange field. In particular, a fully valley polarized conductance with 80% transmission is found in this junction, which can be caused by the interplay of valley-dependent massive Dirac electron, the exchange potential and the on-site potential difference of sublattices. Our findings will benefit applications in silicene-based high performance nano-electronics.     

Electrical control of valley and spin polarized current and tunneling magnetoresistance in a silicene-based magnetic tunnel junction

We investigate the electronic transport in a silicene-based ferromagnetic metal/ferromagnetic insulator/ferromagnetic metal tunnel junction. The results show that the valley and spin transports are strongly dependent on local application of a vertical electric field and effective magnetization configurations of the ferromagnetic layers. In particular, it is found that the fully valley and spin polarized currents can be realized by tuning the external electric field. Furthermore, we also demonstrate that the tunneling magnetoresistance ratio in such a full magnetic junction of silicene is very sensitive to the electric field modulation.     

Current-induced pseudospin polarization in silicene

The pseudospin polarization induced by an external electric field in silicene in the presence of weakly spinindependent impurities is considered theoretically in the linear response regime based on Green’s function method. We study the effects of the interplay between the sublattice potential and the intrinsic spin orbit coupling on the pseudospin polarization. We show that the pseudospin polarization perpendicular to the electric field is independent of the impurity parameter, while the pseudospin polarization in the direction of the electric field is sensitive to the impurity parameter. The dependences of the pseudospin polarizations on the chemical potential are studied.     

Hot carrier drift induced EM wave instability in a magnetoactive solid state plasma

The effect of a strong longitudinal static electric field on the propagation and instability of transverse circularly polarised EM waves (left and right handed) in the presence of a static magnetic field along the direction of propagation in an InSb plasma has been studied under hot carrier conditions by a phenomenological approach. The results show the possibility of existence of wave instabilities for a wide range of system parameters. The growth rate decreases with the heating d.c. electric field and increases slightly with the static magnetic field.     

Strain and chirality effects on the mechanical and electronic properties of silicene and silicane under uniaxial tension

We present first principles theory calculations on the mechanical and electronic properties of silicene and silicane structure under uniaxial tensile strain along different directions. Chirality effect is more significant in the mechanical properties of silicene than those of silicane. Different failure mechanisms are identified. A small band gap (up to 0.8 eV) is developed from zero with silicene structure under uniaxial tension and vanishes before the structure reaches its in-plane ultimate strength. However, a pre-existing band gap (2.39 eV) exists with silicane structure and decreases to zero with the increasing tensile strain without chirality effects.     

Electro-optical transitions in a semiconductor cylindrical nanolayer

The single-electron states in a quantized cylindrical layer have been considered in the presence of a moderate homogeneous electric field, when the energy imparted to a charge carrier by the electric field becomes comparable to the energy of rotational motion of this particle. The corresponding energy spectrum and the envelopes of the wave functions of charge carriers in the layer have been obtained in an explicit form. The electro-optical absorption band of a weak electromagnetic wave has been calculated. It has been found that the absorption intensity increases with an increase in the intensity of the electric field. The external electric field leads to an explicit dependence of the absorption intensity on the effective masses of charge carriers. The absorption intensity decreases as the difference between the effective masses of charge carriers increases. There is also an effective broadening of the band gap, which is determined by the geometrical dimensions of the sample and the magnitude of the external field.     

强外电场作用下BF分子结构和激发特性

采用密度泛函(DFT)方法LSDA在6-311++G(d,p)基组水平上优化得到了分子轴方向不同电场(-0.03～0.05a.u.)作用下,BF分子的基态结构参数、电偶极矩μ、电荷分布、HOMO能级、LUMO能级等。在优化构型下,用同样的基组采用杂化CIS-DFT方法(CIS-LSDA)研究了同样外电场条件下对BF分子的激发能和振子强度的影响。结果表明:随着电场的增加,分子结构与外电场有着强烈的依赖关系,且对电场方向的依赖呈现非对称性。分子总能量先增大后减小,电偶极矩μ先增大,后减小,最后不断增大。电场对振子强度的影响比较复杂,有的增大有的减小,表明电子跃迁光谱强度受外场影响。     

Electric-field-induced energy spectra and dispersions of ZnO/MgxZn1−xO MQWs

The energy spectra and dispersion relations of carriers in the presence of an electric field applied along the growth direction in ZnO/Mg_xZn_1−xO multiple quantum wells (MQW) are calculated using the asymptotic transfer method (ATM) on the basis of the quasistationary state approximation. The energy spectra of the carriers induce some quasi-bound levels under electric fields. The dispersion relations for the energy of the ground state and lower excitation states still have parabolic shapes for both the electrons and the heavy holes in the presence of a moderate electric field. Our results also reveal that the number of energy levels increases with increasing number of ZnO quantum wells and that the energies increase with both increasing Mg composition x and electric field strength.     

电场作用下溴甲烷的光谱和解离特性

采用了密度泛函理论（density functional theory,DFT）,在6-311++G（d,p）基组水平上使用B3LYP方法研究外电场（0-0.05a.u.）对于溴甲烷分子的键长、能隙及解离势能面的影响.结果表明：外加电场的方向和大小对于分子结构和解离势能面均有显著的影响.随着负向外电场（Br-C键方向）从0增加到0.05a.u.,C-Br键的键长先减小后增大,C-H键的键长逐渐增加,分子能隙E_G逐渐减小,C-Br键的str振动频率逐渐增加而IR振动频率逐渐减小.进一步计算发现：随着正向外电场（C-Br键方向）从0增加到0.03a.u.,溴甲烷分子的势能曲线有所降低,解离势垒逐渐减小.因此,可以通过外电场来控制CH₃Br分子的降解.     

电场作用下P掺杂硅烯选择性吸附CH_4的第一性原理计算

在煤层气中选择性吸附和捕捉甲烷分子,对提高煤矿安全具有十分重要的意义.本文采用第一性原理计算的方法,研究了外加电场作用下P掺杂硅烯对甲烷分子的选择性吸附性能.结果表明:正电场作用下,P掺杂硅烯与甲烷分子之间产生较强的化学吸附,能够快速捕获甲烷分子.当关闭外加电场时,P掺杂硅烯与甲烷分子之间则为微弱的物理吸附,甲烷分子很容易实现脱附.同时还发现,外加电场作用下,P掺杂硅烯与氮气、氧气及水之间的吸附均属于物理吸附,表明P掺杂硅烯可以在这些混合气体中实现甲烷气体的选择性吸附. P掺杂硅烯有望成为选择性好的甲烷传感、捕获新材料.     

Field-modulated low-energy electronic and optical properties of armchair silicene nanoribbons

The tight-binding model including spin–orbit coupling is used to study electronic and optical properties of armchair silicene nanoribbons (ASiNRs) in electric fields. Perpendicular electric field monotonically increases band-gap, the DOS, and absorption frequency and strength. It does not change spin-degeneracy, edge-states, and optical selection rule. However, parallel electric field strongly modulates energy dispersions resulting in oscillatory band-gaps, shift in edge-states, and destruction of spin-degeneracy. It induces more transition channels and constructs new selection rules that exhibits richer optical spectra. Modulations of electronic and optical properties of ASiNRs have strong dependence on the direction of electric field and nanoribbon's geometry.     

Assessment of bilayer silicene to probe as quantum spin and valley Hall effect

Silicene takes precedence over graphene due to its buckling type structure and strong spin orbit coupling. Motivated by these properties, we study the silicene bilayer in the presence of applied perpendicular electric field and intrinsic spin orbit coupling to probe as quantum spin/valley Hall effect. Using analytical approach, we calculate the spin Chern-number of bilayer silicene and then compare it with monolayer silicene. We reveal that bilayer silicene hosts double spin Chern-number as compared to single layer silicene and therefore accordingly has twice as many edge states in contrast to single layer silicene. In addition, we investigate the combined effect of intrinsic spin orbit coupling and the external electric field, we find that bilayer silicene, likewise single layer silicene, goes through a phase transitions from a quantum spin Hall state to a quantum valley Hall state when the strength of the applied electric field exceeds the intrinsic spin orbit coupling strength. We believe that the results and outcomes obtained for bilayer silicene are experimentally more accessible as compared to bilayer graphene, because of strong SO coupling in bilayer silicene.     

Spin thermoelectricity in dualhydrogenated zigzag silicene nanoribbons with surface adsorptions

The buckled structure of silicene provides a feasible pathway to influence its electric and magnetic properties via surface adsorptions. Here, we investigate the magnetic and spin thermoelectric transport properties of dual-hydrogenated zigzag silicene nanoribbons (ZSiNRs) without/with the hydrogen adsorption. The band gaps for two spin channels in ZSiNRs under the hydrogen adsorption are shifted near the Fermi level, leading to the appearance of spin Seebeck effect. Using a temperature difference, one can derive the carriers with the different spin index to flow in the opposite direction. Moreover, a large rectification ratio close to 10⁵ at room temperature is achieved for the spin current, and the charge current exhibits a remarkable negative differential thermoelectric resistance (NDTR) behavior. The results presented here are fascinating potential applications in the fields of silicon-based spin caloritronic devices.     

Mechanism of Carbon Nanotubes Aligning along Applied Electric Field

The mechanism of single-walled carbon nanotubes （SWCNTs） aligning in the direction of external electric field is studied by quantum mechanics calculations. The rotational torque on the carbon nanotubes is proportional to the difference between the longitudinal and transverse polarizabilities and varies with the angle of SWCNTs to the external electric field. The longitudinal polarizability increases with second power of length, while the transverse polarizability increases linearly with length. A zigzag SWCNT has larger longitudinal and transverse polarizabilities than an armchair SWCNT with the same diameter and the discrepancy becomes larger for longer tubes.