Spontaneous transverse field in impurity graphene

The periodic Anderson model and the average electron method are used to show that an electric field normal to an applied dc electric field can spontaneously appear in impurity-containing graphene. This effect can be related to a nonequilibrium electron subsystem in graphene. The characteristics of the spontaneous field are revealed as a function of the problem parameters.     

Domain structure of graphene with Hubbard interaction under conditions of emergence of a spontaneous transverse field

It is demonstrated that, when an external static electric field is applied to graphene with Hubbard interaction between electrons, an electric field perpendicular to the applied field can spontaneously arise. The characteristics of the spontaneous field, depending on the parameters of the problem, are determined. Based on the proposed synergetic potential, the domain structure is examined.     

Electric field effect in ultrathin zigzag graphene nanoribbons

The electric field effect in ultrathin zigzag graphene nanoribbons containing only three or four zigzag carbon chains is studied by first-principles calculations, and the change of conducting mechanism is observed with increasing in-plane electric field perpendicular to the ribbon. Wider zigzag graphene nanoribbons have been predicted to be spin-splitted for both valence band maximum(VBM) and conduction band minimum(CBM) with an applied electric field and become half-metal due to the vanishing band gap of one spin with increasing applied field. The change of VBM for the ultrathin zigzag graphene nanoribbons is similar to that for the wider ones when an electric field is applied. However, in the ultrathin zigzag graphene nanoribbons, there are two kinds of CBMs, one is spin-degenerate and the other is spin-splitted, and both are tunable by the electric field. Moreover, the two CBMs are spatially separated in momentum space. The conducting mechanism changes from spin-degenerate CBM to spin-splitted CBM with increasing applied electric field. Our results are confirmed by density functional calculations with both LDA and GGA functionals, in which the LDA always underestimates the band gap while the GGA normally produces a bigger band gap than the LDA.     

Recent progress on integrating two-dimensional materials with ferroelectrics for memory devices and photodetectors

Two-dimensional(2D) materials, such as graphene and Mo S2 related transition metal dichalcogenides(TMDC), have attracted much attention for their potential applications. Ferroelectrics, one of the special and traditional dielectric materials,possess a spontaneous electric polarization that can be reversed by the application of an external electric field. In recent years, a new type of device, combining 2D materials with ferroelectrics, has been fabricated. Many novel devices have been fabricated, such as low power consumption memory devices, highly sensitive photo-transistors, etc. using this technique of hybrid systems incorporating ferroelectrics and 2D materials. This paper reviews two types of devices based on field effect transistor(FET) structures with ferroelectric gate dielectric construction(termed Fe FET). One type of device is for logic applications, such as a graphene and TMDC Fe FET for fabricating memory units. Another device is for optoelectric applications, such as high performance phototransistors using a graphene p-n junction. Finally, we discuss the prospects for future applications of 2D material Fe FET.     

Radiation from electrons in graphene in strong electric field

We study the interaction of electrons in graphene with the quantized electromagnetic field in the presence of an applied uniform electric field using the Dirac model of graphene. Electronic states are represented by exact solutions of the Dirac equation in the electric background, and amplitudes of first-order Feynman diagrams describing the interaction with the photon field are calculated for massive Dirac particles in both valleys. Photon emission probabilities from a single electron and from a many-electron system at the charge neutrality point are derived, including the angular and frequency dependence, and several limiting cases are analyzed. The pattern of photon emission at the Dirac point in a strong field is determined by an interplay between the nonperturbative creation of electron–hole pairs and spontaneous emission, allowing for the possibility of observing the Schwinger effect in measurements of the radiation emitted by pristine graphene under DC voltage.     

垂直电场下扭转双层石墨烯光学吸收性质的理论研究

层间扭转角度是对石墨烯物理性质宽波段可调谐的一个新参量.本文采用2°<θ<15°扭转角度下的连续近似模型,获得了不同扭转角度双层石墨烯分别在有、无电场下的能带结构,通过电子-光子相互作用跃迁速率,计算模拟了范霍夫奇点附近电子带内跃迁和带间跃迁所引起的光学吸收谱.结果表明,在无外加电场时,带间跃迁吸收峰的位置随着扭转角度的增大而发生从红外到可见光波段的蓝移,且吸收系数增大,带内跃迁的光学吸收系数相对于带间跃迁高出2个数量级;而存在外加电场时,两个范霍夫奇点在波矢空间的位置发生偏移,带间跃迁吸收峰发生分裂,且两个分裂的吸收峰位置随着电场强度的不断增大而反向行进.上述研究结果对石墨烯材料在光电器件方面的应用有一定指导作用.     

Analysis of the Zeeman effect on the energy spectrum in graphenes

An analysis of the Zeeman effect with a strong external magnetic field on the energy spectrum in graphene is presented. In general, the Hamiltonian of graphene in applied electric and magnetic fields is not of SO(1, 2) invariance even in the nearest-neighbor approximation because of the Zeeman coupling. But an approximate SO(1, 2) invariance can be obtained when the applied magnetic field is very strong. This approximate invariance can be used to relate the energy structure of graphene in the presence of both electric and magnetic fields to that when there is only magnetic field. Therefore, it can help explain the recently found quantum Hall conductance (4q ²/h)L for L = 0.1.     

Magnetoelectric oxide films for spin manipulation in graphene

The challenge of creating a graphene spin field effect transistor (spin‐FET) demands a magnetic gate dielectric material whose magnetization can be switched electrically. We have grown films of Cr₂O₃ on top of graphite and graphene by pulsed laser deposition that shows this crucial functionality. We demonstrate that the Cr₂O₃ films are magnetoelectric by poling them in combined electric and magnetic fields and then using magnetic force microscopy to observe spontaneous surface domain structure as a function of poling field. In addition, we show that the electric field created by a conducting AFM tip can be used to write magnetic patterns in the film that demonstrate the kind of continuous magnetoelectric control needed for a prototype spin‐FET. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Interface contact and modulated electronic properties by external vertical strains and electric fields in graphene/MoS2 heterostructure

Based to the first-principles calculations, we study the electronic properties of graphene/MoS₂ heterostructure by modulating the vertical strains and applying external electric field. Graphene/MoS₂ heterostructure is a van der Waals heterostructure (vdWH) with the interlayer spacing is 3.2 Å for the equilibrium state, and the contact property of the interface is n-type Schottky contact. The Schottky barrier height (SBH) changes with vertical strains which induces a change of charge transfer between graphene and MoS₂ layer. In addition, with strain or without strain, the applied positive electric field can effectively promote the charge transfer from graphene to MoS₂, while the negative electric field has the opposite effect. These findings support for the design of field effect transistors based on graphene vdWHs.     

Tunable terahertz acoustic-phonon emission from monolayer molybdenum disulfide

The acoustic-phonon emission from monolayer molybdenum disulfide (ML-MoS₂) driven by a direct-current electric field is studied theoretically using the Boltzmann equation method. It is found that the Cerenkov emission of terahertz acoustic-phonons can be generated when a very weak electric field is applied to ML-MoS₂. The physical mechanisms of acoustic-phonon emission are analyzed from the perspective of condensed matter physics. The acoustic-phonon emission from ML-MoS₂ is also compared with those from graphene and GaAs. The results reveal that the frequencies of acoustic-phonons generated by ML-MoS₂ are between the frequencies of those generated from GaAs and graphene. The results of this work suggest that the ML-MoS₂ can make up for graphene and GaAs in respect of acoustic-phonon emission and be used in tunable hypersonic devices such as terahertz sound sources.     

Negative differential conductivity of bigraphene controlled by an external voltage in a magnetic field

The current-voltage characteristic of a graphene bilayer has been calculated using the average electron method in the case of applied strong electric and magnetic fields. In the relaxation time approximation, it has been shown that, in bigraphene in the presence of a constant magnetic field applied in the direction perpendicular to the bigraphene layers, these can arise states with a negative differential conductivity. In addition, the generation of terahertz pulses can be performed in this system over a rather wide range of interlayer voltages.     

Alternating field-induced phase transition in zigzag carbon nanotubes

We predict the possibility of spontaneous emergence of a constant electric field in carbon nanotubes if a high-frequency electric field is applied to them. The effect can be related to the nonequilibrium state of the electron subsystem of carbon nanotubes. The characteristics of the spontaneous field with respect to the parameters of the problem under consideration are revealed.     

Nernst-Ettingshausen effect in graphene

The Nernst-Ettingshausen effect corresponds to the regime of crossed magnetic and electric fields. In the current theoretical studies of this effect in graphene, the dependence of the Landau levels on the applied electric field is neglected. This dependence takes place in the case of the nonquadratic energy spectrum of the charge carriers. In this work, oscillations of the Nernst coefficient in graphene with a zero and nonzero band gap have been studied taking into account such dependence. The effect of the Coulomb interaction on these oscillations is considered.     

On the theory of Nernst–Ettingshausen oscillations in monolayer and bilayer graphene

The Landau levels in graphene in crossed magnetic and electric fields are dependent on the electric field. However, this effect is not taken into account in many theoretical studies of graphene in crossed fields. In particular, it is not considered in the Nernst–Ettingshausen effect, in which the regime of crossed fields is realized. In this Letter, we considered the Nernst–Ettingshausen effect in monolayer and bilayer graphene, taking into account the dependence of Landau levels on the electric field. We showed that the magnitude and period of the Nernst coefficient oscillations depend on the electric field. This fact is important for the fundamental theory of Nernst–Ettingshausen effect in graphene and gives the new possibility for control of this effect using an applied electric field. The latter is very interesting for practical applications.     

变形及电场作用对石墨烯电学特性影响的第一性原理计算

利用基于密度泛函理论的第一性原理方法,系统研究了变形、电场及共同作用对石墨烯电学特性影响的电子机理.研究表明,本征石墨烯的能隙及态密度值在费米能级处均为0,呈现出半金属特性;在一定的变形量下对石墨烯施加剪切、拉伸、扭转及弯曲变形作用,发现剪切和扭转变形对打开石墨烯能隙的作用明显;对本征石墨烯施加不同方向的电场,可知010电场方向对打开石墨烯能隙的作用效果最强.这是因为该电场方向下石墨烯C-C原子间的布居数正值较大,成键键能较高,而负值数值较小,反键键能较低;线性增加电场强度,石墨烯的能隙呈线性增长势;变形及电场共同作用下,外加电场提高了变形对打开石墨烯能隙的作用效果,但不及两种外场叠加的作用效果.     

Electric-field control of the activity of the graphene-based catalyst

Effects of an electric field on CO oxidation of Au-embedded graphene are investigated using first-principles method. Results of our calculations show that the initial step of the reaction is more likely to proceed via the Langmuir-Hinshelwood mechanism in the presence of the field, and the reaction barrier can be tuned continuously by the electric field. However, the applied electric field makes it more difficult for the product of the reaction, CO2, to desorb from the reaction site. These two competing eff...     

Tunable Fano resonances in the ballistic transmission and tunneling lifetime in a biased bilayer graphene nanostructure

Dynamics of the Dirac fermions, in particular the transmission coefficient and the resonant tunneling lifetime are studied across a bilayer graphene electrostatic double barrier structure modulated by an in plane homogeneous electric field. Asymmetric Fano type resonances are noted for the first time in the transmission spectrum of the bilayer graphene nanostructures and are found to be highly sensitive to the direction of incidence of the charge carriers and the applied homogeneous electric field. The effect of the external field on the extended and the evanescent modes is also analysed. Resonant tunneling lifetime is found to be highly anisotropic in nature. The chiral carriers are either accelerated or decelerated by the electric field depending on the energy of the quasi-bound states, the angle of incidence and the magnitude of the applied field. Effects of the external field on the localization of the chiral carriers are also discussed.     

Tuning the Schottky barrier in the arsenene/graphene van der Waals heterostructures by electric field

Using density functional theory calculations, we investigate the electronic properties of arsenene/graphene van der Waals (vdW) heterostructures by applying external electric field perpendicular to the layers. It is demonstrated that weak vdW interactions dominate between arsenene and graphene with their intrinsic electronic properties preserved. We find that an n-type Schottky contact is formed at the arsenene/graphene interface with a Schottky barrier of 0.54 eV. Moreover, the vertical electric field can not only control the Schottky barrier height but also the Schottky contacts (n-type and p-type) and Ohmic contacts (n-type) at the interface. Tunable p-type doping in graphene is achieved under the negative electric field because electrons can transfer from the Dirac point of graphene to the conduction band of arsenene. The present study would open a new avenue for application of ultrathin arsenene/graphene heterostructures in future nano- and optoelectronics.     

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.