Graphene-based modulation-doped superlattice structures

The electronic transport properties of graphene-based superlattice structures are investigated. A graphene-based modulation-doped superlattice structure geometry is proposed consisting of periodically arranged alternate layers: InAs/graphene/GaAs/graphene/GaSb. The undoped graphene/GaAs/graphene structure displays a relatively high conductance and enhanced mobilities at increased temperatures unlike the modulation-doped superlattice structure, which is more steady and less sensitive to temperature and the robust electrical tunable control on the screening length scale. The thermionic current density exhibits enhanced behavior due to the presence of metallic (graphene) monolayers in the superlattice structure. The proposed superlattice structure might be of great use for new types of wide-band energy gap quantum devices.     

Transport properties in a line defect superlattice of graphene

It was recently reported that a kind of graphene line defect can be fabricated in a controllable experimental way. In the present work we theoretically investigate the band structure and the electronic transport properties of a graphene superlattice formed by embedding periodically line defects in the graphene lattice. Based on the calculated results, we suggest that such a superlattice can be used as a quantum wire array which can carry much larger current than a single graphene nanoribbon. A remarkable advantage of this superlattice over other quantum wires is that the electronic transport in it is insensitive to scattering effects except that the scattering potential range is smaller than the graphene lattice constant. Moreover, we find that the anisotropy of the Dirac cone presented in this superlattice has a nontrivial influence on the universal minimal conductivity and the sub-Poissonian shot noise of graphene.     

Mutual rectification of cnoidal and sinusoidal electromagnetic waves with orthogonal polarization planes in a graphene-based superlattice

The effect of mutual rectification of two electromagnetic waves in a graphene superlattice is studied for a cnoidal wave polarized along the superlattice axis and a sinusoidal wave polarized along the orthogonal direction. An expression for the density of the direct current generated by mixing of these waves is obtained. The direct current is studied as a function of the cnoidal wave amplitude. The possibility of varying both the value and the direction of the direct current with varying the cnoidal wave amplitude is shown.     

Generating high harmonics in a superlattice based on graphene in the presence of static and alternating electric fields

The generation of high harmonics of current density in a superlattice based on graphene on a striped substrate is considered under the influence of static and alternating electric fields polarized along the superlattice axis. The areas of system parameter values are revealed for the first four harmonics, the amplitude of each of which surpasses the other three.     

Effect of the mutual rectification of two electromagnetic waves with perpendicular polarization planes in a superlattice based on graphene

The effect of the mutual rectification of two electromagnetic waves with perpendicular polarization planes in a superlattice based on graphene was investigated. It was shown that, due to the nonadditive nature of the electronic spectrum of a superlattice based on grapheme, a constant term of current arises along its axes. The dependence of the constant of current density on the intensity of an electromagnetic wave is oscillatory in nature.     

Controlling Chaos Probability of a Bose-Einstein Condensate in a Weak Optical Superlattice

The spatial chaos probability of a Bose-Einstein condensate perturbed by a weak optical superlattice is studied. It is demonstrated that the spatial. chaotic solution appears with a certain probability in a given parameter region under a random boundary condition. The effects of the lattice depths and wave vectors on the chaos probability are illustrated, and different regions associated with different chaos probabilities are found. This suggests a feasible scheme for suppressing and strengthening chaos by adjusting the optical superlattice experimentaJly.     

Engineering the electronic structure of graphene superlattices via Fermi velocity modulation

Graphene superlattices have attracted much research interest in the last years, since it is possible to manipulate the electronic properties of graphene in these structures. It has been verified that extra Dirac points appear in the electronic structure of the system. The electronic structure in the vicinity of these points has been studied for a gapless and gapped graphene superlattice and for a graphene superlattice with a spatially modulated energy gap. In each case a different behavior was obtained. In this work we show that via Fermi velocity engineering it is possible to tune the electronic properties of a graphene superlattice to match all the previous cases studied. We also obtained new features of the system never observed before, reveling that the electronic structure of graphene is very sensitive to the modulation of the Fermi velocity. The results obtained here are relevant for the development of novel graphene-based electronic devices.     

Pendulum limit,chaos and phase-locking in the dynamics of ac-driven semiconductor superlattices

We describe a limiting case when nonlinear dynamics of an ac-driven semiconductor superlattice in the miniband transport regime is governed by a periodically forced and damped pendulum equations. We find analytically the conditions for a transition to chaos. With increasing temperature the chaos disappears. We also discuss fractional dc voltage states in a superlattice originating from phase-locked states of the pendulum.     

Mode selective control of flow turbulence

We study spin transport in normal/ferromagnetic/normal/ferromagnetic.../normal graphene superlattices, which can be realized by putting a series of magnetic insulator bars on top of a graphene sheet. Owing to magnetic proximity effect, local exchange splittings will be induced in the graphene sheet, effectively forming a magnetic graphene superlattice. The spin polarization of tunneling conductance and the magneto resistance (MR) exhibit oscillatory behavior with the gate voltage. The superlattice structure leads to an enhanced spin polarization and MR ratio, making the magnetic graphene superlattice become very promising in spintronics applications.     

一维石墨烯超晶格上的氢吸附

通过化学气相沉积法在一种典型的高指数面铜(410)-氧上制备出了一维石墨烯超晶格,并利用拉曼光谱(Raman)、低能电子衍射(LEED)、扫描隧道显微术(STM)等方法研究了氢原子在该超晶格上的吸附.实验结果揭示了由于超晶格的调制作用所引起的无缺陷石墨烯区域的选择性吸附,氢原子优先吸附在石墨烯超晶格一维周期中的相同部位.这一发现为进一步通过一维超晶格调控石墨烯的性质提供了更多可能.在石墨烯上的吸附氢原子除了单个或成对存在之外,实验还首次观察到一种新的三氢原子位型.     

2D chaotic quantum states in superlattices

The semiclassical motion of an electron along the axis of a superlattice may be calculated from the miniband dispersion curve. Under a weak electric field the electron executes Bloch oscillations which confines the motion along the superlattice axis. When a magnetic field, tilted with respect to the superlattice axis, is applied the electron orbits become chaotic. The onset of chaos is characterised by a complex mixed stable-chaotic phase space and an extension of the orbital trajectories along the superlattice axis. This delocalisation of the orbits is also reflected in the quantum eigenstates of the system some of which show well-defined patterns of high probability density whose shapes resemble certain semiclassical orbits. This suggests that the onset of chaos will be manifest in electron transport through a finite superlattice. We also propose that these phenomena may be observable in the motion of trapped ultra-cold atoms in an optically induced superlattice potential and magnetic quadrupole potential whose axis is tilted relative to the superlattice axis.     

Transport in magnetic graphene superlattice with Rashba spin-orbit interaction

Based on the transfer-matrix method, we have investigated the spin-dependent transport properties of magnetic graphene superlattice in the presence of Rashba spin-orbit interaction (RSOI). It is shown that the angular range of the spin transmission probability through magnetic graphene superlattice can be efficiently controlled by the number of barriers. As the number of magnetic barriers increases, the angular range of the transmission through the magnetic superlattice decreases, the gaps in the transmission and conductivity versus energy become wider. It is also found that the spin conductivities oscillate with the Fermi energy and RSOI strength. Specifically, when a magnetic field is present, the spin polarisation can be observed, whereas for the RSOI alone it is zero. Application of such a phenomenon to design a spin polarised electron device based on the graphene material is anticipated.     

Amplification of terahertz radiation in carbon nanotubes

We investigate theoretically the feasibility of amplification of terahertz radiation in aligned achiral carbon nanotubes, a zigzag (12,0) and an armchair (10,10) in comparison with a superlattice using a combination of a constant direct current (dc) and a high-frequency alternate current (ac) electric fields. The electric current density expression is derived using the semiclassical Boltzmann transport equation with a constant relaxation time. The electric field is applied along the nanotube axis. Analysis of the current density versus electric field characteristics reveals a negative differential conductivity behavior at high frequency, as well as photon assisted peaks. The photon assisted peaks are about an order of magnitude higher in the carbon nanotubes compared to the superlattice. These strong phenomena in carbon nanotubes can be used to obtain domainless amplification of terahertz radiation at room temperature.     

Dynamic conductivity of ac–dc-driven graphene superlattice

The dynamic conductivity of graphene superlattice in the presence of ac electric field and dc electric field with longitudinal and transversal components with respect to superlattice axis was calculated. In the case of strong transversal component of dc field conductivity of graphene superlattice was shown to be such as if the electrons had got the effective mass. In the case of weak transversal component of dc field conductivity was shown to change its sign if the frequency of ac field was an integer multiple of half of Bloch frequency.     

Instability of dipole magnetoexcitons in quantum wells' and graphene superlattices

We propose the Bose-Einstein condensation and superfluidity of quasi-two-dimensional spatially indirect magnetobiexcitons in a slab of superlattice with alternating electron and hole layers consisting from the semiconducting quantum wells (QWs) and graphene superlattice in high magnetic field. For this system the instability of the ground state of interacting two-dimensional indirect magnetoexcitons in a slab of superlattice with alternating electron and hole layers in high magnetic field is found. The density of superfluid component ns(T) and the temperature of the Kosterlitz-Thouless phase transition to the superfluid state in the system of two-dimensional indirect magnetobiexcitons, interacting as electrical quadrupoles, are obtained for both QW and graphene realizations.     

准周期激励与应变超晶格的动力学稳定性

假设超晶格锯齿形沟道对粒子的作用等效为形状相似的周期场作用. 在经典力学框架内,引入正弦平方势,把粒子运动方程化为具有阻尼项和双频激励项的摆方程. 用Melnikov方法对单频激励系统的分叉与混沌进行分析;用Lyapunov方法对双频激励系统的稳定性进行讨论. 结果表明：在弱非线性情况下,双频激励系统存在局域不稳定,且这种不稳定将向全局扩展,直至混沌的出现;导致混沌的双频激励强度远小于单频激励强度;外加一个适当的超声场可望将这种敏感钝化,使系统的稳定性得到改善. 关键词： 超晶格 准周期激励 混沌 稳定性     

Superlattice based on graphene on a strip substrate

A graphene-based superlattice formed due to the periodic modulation of the band gap has been investigated. Such a modulation is possible in graphene deposited on a strip substrate made of silicon oxide and hexagonal boron nitride. The advantages and some possible problems in the superlattice under consideration are discussed. A model describing such a superlattice is proposed and the dispersion relation between the energy and momentum of carriers has been obtained using the transfer matrix method within this model.     

非线性Kronig-Penney超晶格的二维实映射分析

通过二维实映射研究非线性Kronig-Penney超晶格中的波输运特性，数值计算得到波矢一定情况下不同非线性系数的映射图、以及非线性系数一定情况下不同波矢的映射图.非线性Kronig-Penney超晶格中的非线性，对超晶格中波函数的Bloch波矢有明显的调节作用.随着非线性系数的增大，映射图由周期函数的有限个分立点变为准周期函数的一条闭合轨道、以及有规则或无规则的点分布. 关键词： 非线性Kronig-Penney超晶格 非线性效应 二维实映射 波矢     

Generation of high harmonics by an alternating electric field in superlattices based on graphene

The conductivity of a superlattice based on graphene in an alternating electric field with a vector of intensity directed along the axis of the superlattice is described. The electronic system is described in terms of the kinetic Boltzmann equation in approximation of a constant time of relaxation. Nonlinear responses to the applied harmonic field are calculated and analyzed.     

Effect of a constant electric field on mutual rectification in a graphene superlattice

The effect of a transverse dc electric field on two-wave mutual rectification in a graphene superlattice (GSL) is investigated. Two field orientations are considered: (i) the polarization plane is parallel to the GSL axis and (ii) the polarization plane is perpendicular to the GSL axis. In both cases, the constant field is perpendicular to the polarization plane. The current density is calculated within a one-miniband model using the Boltzmann equation in the approximation of constant relaxation time.