Bilayer Silicene Research Articles

Электронные свойства пленок силицена, подвергнутых нейтронному легированию

Abstract The radiation doping of single-crystal silicon with phosphorus retains the structure of the sample, reduces internal stresses, and increases the lifetime of minority charge carriers. The study is concerned with the effect of phosphorus additives on the electronic properties of silicene. The electron density-of-states spectra of a phosphorus-doped single layer and 2 × 2 bilayer silicene on a graphite substrate are calculated by the quantum-mechanical method. The carbon substrate imparts semiconductor properties to silicene due to p – p hybridization. Doping with phosphorus can retain or modify the metal properties gained by silicene. The position of phosphorus dopant atoms in silicene influences the semiconductor–conductor transition. The theoretical specific capacity of a phosphorus-doped silicene electrode decreases, and the electrode becomes less efficient for application in lithium-ion batteries. However, the increase in the conductivity is favorable for use of this material in solar cells.

Kinetic test of a doped silicene-graphite anode element in a computer experiment

The stability of the system “bi-layer silicene on the graphite substrate” is studied in the molecular dynamics simulation. Silicene sheets are doped with phosphorus, and graphite sheets are doped with nitrogen. Lithium ion moves along a silicene channel with a gap in the range of 0.6–0.8 nm. The time for the ion to pass the channel and leave it decreases with an increase in the channel gap. There is a tendency of the silicene sheets roughness growth with an increase in the gap between silicene sheets (except, 0.75 nm). Doping phosphorus and nitrogen atoms stabilize the silicene and graphite structure.

Strain induced spin-splitting and half-metallicity in antiferromagnetic bilayer silicene under bending.

Searching for half-metals in low dimensional materials is not only of scientific importance, but also has important implications for the realization of spintronic devices on a small scale. In this work, we show theoretically that simple bending can induce spin-splitting in bilayer silicene. For bilayer silicene with Bernal stacking, the monolayer has a long range ferromagnetic spin order and between the two monolayers, the spin orders are opposite, giving rise to an antiferromagnetic configuration for the ground state of the bilayer silicene. Under bending, the antiferromagnetic spin order is retained but the energetic degeneracy of opposite spin states is lifted. Due to the unusual deformation potentials of the conduction band minimum (CBM) and valence band maximum (VBM) as revealed by density-functional theory calculations and density-functional tight-binding calculations, this spin-splitting is nearly proportional to the degree of bending deformation. Consequently, the spin-splitting can be significant and the desired half-metallic state may emerge when the bending increases, which has been verified by direct simulation of the bent bilayer silicene using the generalized Bloch theorem. Our results hint that bilayer silicene may be an excellent candidate for half-metallicity.

Quantum spin-valley Hall effect in AB-stacked bilayer silicene

Our density functional theory calculations show that while AB-stacked bilayer silicene has a non-quantized spin-valley Chern number, there exist backscattering-free gapless edge states within the bulk gap, leading to a quantum spin-valley Hall effect. Using a tight-binding model for a honeycomb bilayer, we found that the interlayer potential difference and the staggered AB-sublattice potential lead to abrupt and gradual change of the valley Chern number from a quantized value to zero, respectively, while maintaining backscattering-free gapless edge states if the valley Chern number is not too close to zero. Under an inversion symmetry-breaking potential in the form of the staggered AB-sublattice potential, such as an antiferromagnetic order and a hexagonal diatomic sheet, a finite but non-quantized (spin-)valley Chern number can correspond to a quantum (spin-)valley Hall insulator.

Multilayer silicene: Structure, electronics, and mechanical property

Abstract Herein, we performed first-principle calculations and classical molecular dynamics simulations to study geometry optimizations, band structures, and mechanical properties of differently stacked multilayer silicenes. Several local energy minima have been identified as metastable conformations with different stacking modes and layer numbers. Bandstructure of the low buckled AA bilayer silicene optimized with SCAN + rvv10 presents semiconducting behavior with a bandgap of 0.4419 eV. Young’s modulus of multilayer silicene shows low dependence on layer number or stacking mode. Whereas, fracture stress and strain are sensitive to the number of layers, specific stacking modes, and chiralities. Furthermore, bending moduli of multilayer silicenes (e.g., 0.44 eV for monolayer silicene) are even lower than that of graphene, which may attribute to the flexibility of bond angle.

Computational Study of Lithium Intercalation in Silicene Channels on a Carbon Substrate after Nuclear Transmutation Doping

Silicene is considered to be the most promising anode material for lithium-ion batteries. In this work, we show that transmutation doping makes silicene substantially more suitable for use as an anode material. Pristine and modified bilayer silicene was simulated on a graphite substrate using the classical molecular dynamics method. The parameters of Morse potentials for alloying elements were determined using quantum mechanical calculations. The main advantage of modified silicene is its low deformability during lithium intercalation and its possibility of obtaining a significantly higher battery charge capacity. Horizontal and vertical profiles of the density of lithium as well as distributions of the most significant stresses in the walls of the channels were calculated both in undoped and doped systems with different gaps in silicene channels. The energies of lithium adsorption on silicene, including phosphorus-doped silicene, were determined. High values of the self-diffusion coefficient of lithium atoms in the silicene channels were obtained, which ensured a high cycling rate. The calculations showed that such doping increased the normal stress on the walls of the channel filled with lithium to 67% but did not provoke a loss of mechanical strength. In addition, doping achieved a greater battery capacity and higher charging/discharging rates.

Strain and doping effects on the antiferromagnetism of AB-stacked bilayer silicene

We have investigated the effects of biaxial strain and charge doping on the antiferromagnetic order of AB-stacked bilayer silicene by using the density functional theory calculations. The antiferromagnetic order, which decreases and disappears with compressive strain and charge doping, increases with tensile strain and undergoes a metal–insulator–metal transition. Increased lattice constant increases the overlap between the conduction and valence bands and the density of states at the Fermi level, which are responsible for the antiferromagnetic order.

Rich Magnetic Quantization Phenomena in AA Bilayer Silicene

The rich magneto-electronic properties of AA-bottom-top (bt) bilayer silicene are investigated using a generalized tight-binding model. The electronic structure exhibits two pairs of oscillatory energy bands for which the lowest conduction and highest valence states of the low-lying pair are shifted away from the K point. The quantized Landau levels (LLs) are classified into various separated groups by the localization behaviors of their spatial distributions. The LLs in the vicinity of the Fermi energy do not present simple wave function modes. This behavior is quite different from other two-dimensional systems. The geometry symmetry, intralayer and interlayer atomic interactions, and the effect of a perpendicular magnetic field are responsible for the peculiar LL energy spectra in AA-bt bilayer silicene. This work provides a better understanding of the diverse magnetic quantization phenomena in 2D condensed-matter materials.

Valley- and spin-dependent quantum Hall states in bilayer silicene

The Hall conductivity $\sigma_{xy}$ of many condensed matter systems presents a step structure when a uniform perpendicular magnetic field is applied. We report the quantum Hall effect in buckled AB-bottom-top bilayer silicene and its robust dependence on the electronic valley and spin-orbit coupling. With the unique multi-valley electronic structure and the lack of spin degeneracy, the quantization of the Hall conductivity in this system is unlike the conventional sequence as reported for graphene. Furthermore, the conductivity plateaux take different step values for conduction ($2e^2/h$) and valence ($6e^2/h$) bands since their originating valleys present inequivalent degeneracy. We also report the emergence of fractions under significant effect of a uniform external electric field on the quantum Hall step structure by the separation of orbital distributions and the mixing of Landau levels from distinct valleys. The valley- and spin-dependent quantum Hall conductivity arises from the interplay of lattice geometry, atomic interaction, spin-orbit coupling, and external magnetic and electric fields.

Electric-field-diversified optical properties of bilayer silicene.

The rich optical properties of AA-bottom-top (bt) bilayer silicene (BS) in a uniform perpendicular electric field (E) are investigated through the use of the tight-binding model. The distinctive multivalley band structure presents a semimetallic behavior but with a sizeable intraband gap. The main features of the energy dispersion appear in the optical absorption spectrum through transitions between band-edge states obeying specific selection rules. The E field clearly modifies the energy dispersion, especially opening a band gap, leading to a substantial influence on the optical properties. This field creates additional excitation channels and increases the frequency and intensity of the spectral structures. The inclusion of electron-hole interaction gives rise to a slight difference in the spectral intensity and strongly affects the fluctuation in the threshold intensity as the field strength is varied. The interplay between the special lattice structure, atomic interactions, electron-hole interaction, and a E field robustly diversifies the absorption spectra.

Tunable Cloaking of Mexican-hat Confined States in Bilayer Silicene

We present the ballistic quantum transport of a p-n-p bilayer silicene junction in the presence of spin-orbit coupling and electric field using a four-band model in combination with the transfer-matrix approach. A Mexican-hat shape of low-energy spectrum is observed similarly to bilayer graphene under an interalayer bias. We show that while bilayer silicene shares some physics with bilayer graphene, it has many intriguing phenomena that have not been reported for the latter. First, the confined state producing a significantly non-zero transmission in Mexican hat. Second, the cloaking of the Mexican-hat confined state is found. Third, we observe that the Mexican-hat cloaking results in a strong oscillation of conductance when the incident energy is below the potential height. Finally, unlike monolayer silicene, the conductance at large interlayer distances increases with the rise of electric field when the incident energy is above the potential height.

DFT study of the electronic and vibrational properties of silicene/stanene heterobilayer

In the recent years, many studies have been showed that bilayer and heterobilayer silicene (or stanene) might offer properties superior to those of monolayer. Nevertheless, only very few have been synthesized. Using first-principles calculations, we study the structural, vibrational, and electronic properties of silicene/stanene heterobilayer. We show that the structure of silicene/stanene can be stable, where the dynamical stability is demonstrated by the phonon dispersion curve. The interlayer binding energy is found to be typical of van der Waals bonding. Our LDA, GGA, and vdW results show that the band-gaps are direct in a range between 70 and 160 meV, which are promising candidates for diverse device applications.

Peculiar optical properties of bilayer silicene under the influence of external electric and magnetic fields

We conduct a comprehensive investigation of the effect of an applied electric field on the optical and magneto-optical absorption spectra for AB-bt (bottom-top) bilayer silicene. The generalized tight-binding model in conjunction with the Kubo formula is efficiently employed in the numerical calculations. The electronic and optical properties are greatly diversified by the buckled lattice structure, stacking configuration, intralayer and interlayer hopping interactions, spin-orbital couplings, as well as the electric and magnetic fields ({E}_{z}hat{z}& {B}_{z}hat{z}). An electric field induces spin-split electronic states, a semiconductor-metal phase transitions and the Dirac cone formations in different valleys, leading to the special absorption features. The Ez-dependent low-lying Landau levels possess lower degeneracy, valley-created localization centers, peculiar distributions of quantum numbers, well-behaved and abnormal energy spectra in Bz-dependencies, and the absence of anti-crossing behavior. Consequently, the specific magneto-optical selection rules exist for diverse excitation categories under certain critical electric fields. The optical gaps are reduced as Ez is increased, but enhanced by Bz, in which the threshold channel might dramatically change in the former case. These characteristics are in sharp contrast with those for layered graphene.

Interband and intraband transition, dynamical polarization and screening of the monolayer and bilayer silicene in low-energy tight-binding model

We investigate the interband and intraband transition of the monolayer and AB-stacked bilayer silicene in low-energy tight-binding model under the electric field, where we focus on the dynamical polarization function, screening due to the charged impurity, and the plasmon dispersion. We obtain the logarithmically divergen polarization function within the random-phase-approximation (RPA) whose logarithmic singularities corresponds to the discontinuities of the first derivative which is at the momentum ${\bf q}=2{\bf k}_{F}$ in static case and indicate the topological phase transition point from the gapless semimetal to the gapped band insulator. We also obtain the power-law-dependent Friedel oscillation which can be enhanced by increasing the Rashba-coupling, that can contribute to the screened potential of the charged impurity which scale as $\sim r^{-1/2}$ in the short distance from the impurity and scale as $\sim r^{-1/3}$ in the long distance from the impurity. In the single-particle excitation regime with the electron-hole continuum, the interband and intraband transition happen, and the plasmon dispersion, which we mainly focus on the optical plasmon (which $\sim\sqrt{{\bf q}}$ in long-wavelength limit) in this paper, start to damped into the electron-hole pairs due to the nonzero imaginary part of the polarization function.

Electronic transport and dynamical polarization in bilayer silicene-like systems

Abstract We investigate the semiclassical electronic transport properties of the bilayer silicene-like systems in the presence of charged impurity. The trigonal warping due to the interlayer hopping, and its effect to the band structure of bilayer silicene is discussed. Besides the trigonal warping, the external field also gives rise to the anisotropic effect of the mobility (at finite temperature) which can be explored by the Boltzmann theory. The dynamical polarization as well as the scattering wave vector-dependent screening within random phase approximation are very important in determining the scattering behavior as well as the self-consistent transport. We detailedly discuss the transport behavior under the short- or long-range potential. The phonon scattering with the acoustic phonon mode which dominants at high temperature is also studied within the density functional theory (DFT). Our results are also valid for the bilayer graphene or bilayer MoS2, and other bilayer systems which with strong interlayer coupling.

Interplay of Floquet Lifshitz transitions and topological transitions in bilayer Dirac materials

We show how transitions between different Lifshitz phases in bilayer Dirac materials with and without spin-orbit coupling can be studied by driving the system. The periodic driving is induced by a laser and the resultant phase diagram is studied in the high frequency limit using the Brillouin-Wigner perturbation approach to leading order. The examples of such materials include bilayer graphene and spin-orbit coupled materials such as bilayer silicene. The phase diagrams of the effective static models are analyzed to understand the interplay of topological phase transitions, with changes in the Chern number and topological Lifshitz transitions, with the ensuing changes in the Fermi surface. Both the topological transitions and the Lifshitz transitions are tuned by the amplitude of the drive.

Topological phases in nodeless tetragonal superconductors

We compute the topological phase diagram of 2D tetragonal superconductors for the only possible nodeless pairing channels compatible with that crystal symmetry. Subject to a Zeeman field and spin–orbit coupling, we demonstrate that these superconductors show surprising topological features: non-trivial high Chern numbers, massive edge states, and zero-energy modes out of high symmetry points, even though the edge states remain topologically protected. Interestingly, one of these pairing symmetries, , has been proposed to describe materials such as water-intercalated sodium cobaltates, bilayer silicene or highly doped monolayer graphene, which opens the way for further applications of our results.

Fully spin-polarized current in gated bilayer silicene

By applying density functional theory calculations, we predict that the groundstate of bilayer silicene at certain interlayer distances can be antiferromagnetic. At small electron or hole doping, it becomes half metallic under applied out-of-plane electric field, which can be used to produce fully spin-polarized field-effect-driven current even in the absence of external magnetic field, ferromagnetic substrates, doped magnetic ions, or spin-orbital coupling. Our finding points out a new route to overcome the major challenge of spintronics.

Graphene and Graphite Supports for Silicene Stabilization: A Computation Study

The methods of molecular dynamics were used to study structural changes observed in bilayer silicene and thin slices of an ordinary Si crystal when transferred to the graphite substrate. The shape of the radial distribution function indicates noticeable distortion of the long-range order in the Si crystalline film. The first high peak of this function indicates honeycomb lattice in silicene. The motion the lithium ion in the silicene-graphene channel has been studied. The external strengthening of the silicene channel with graphene sheets not only increases its firmness but also improves the passage of intercalated lithium ions through this channel. The stresses in the channel walls are substantially decreased due to vacancy defects. Defects in the form of hexa-vacancies preserve their shape better than others defects when lithium ions pass through the channel.

Spin-valley and layer polarizations induced by topological phase transitions in bilayer silicene

We study spin-valley transport property in a single barrier bilayer silicene junction, where the barrier is under topological phase and controllable by electric field, staggered exchange field and off-resonant circularly polarized light. The tight binding model is adopted to describe the Hamiltonian. The ballistic transport through a topological barrier is investigated using the standard Landauer's formalism. The topological phases in bilayer silicene may be classified by spin-valley Chern numbers which is twice that in monolayer silicene. It is found that the four species of electron, two valley and two spin degrees of freedom, may be perfectly filtered. Electron having zero Chern number may allow to tunnel through the barrier, while the others are completely suppressed. This perfect spin-valley filter is due directly to topological phase transitions when staggered exchange field or off-resonant circularly polarized light is applied into the barrier. The perfect switching of layer (or pseudospin) polarization is predicted, allowed to control 100% carrier flowing only in top (or bottom) layer by electric field. The perfect switching from 100% to −100% of spin and valley polarizations is also found due to topological phase transition in the barrier. The result is quite different form transport property in bilayer graphene which lacks spin-orbit interaction and is planar atomic structure. This work reveals the potential of bilayer silicene as a topological material for spin-valleytronics.