Transport through quantum spin Hall insulator/metal junctions in graphene ribbons

Quantum spin Hall insulator/metal interfaces are formed in graphene ribbons with intrinsic spin-orbit coupling by selectively doping two regions creating a potential step. For a clean graphene ribbon, the transmission of the topological edge states through a n-n or p-p junction is perfect irrespective of the ribbon termination, width, and potential step parameters due to the orthogonality of incoming and outgoing edge channels. This is shown numerically for an arbitrary crystallographic orientation of the ribbon and proven analytically for zigzag and metallic armchair boundary conditions. In disordered ribbons, the orthogonality between left- and right-movers is in general destroyed and backscattering sets in. However, transmission approaches one by increasing the ribbon’s width, even in the presence of strong edge roughness.     

The Rashba effect and non-Abelian phases in quantum wire devices

We present a 2D method for studying quantum transport through arbitrary geometries under the influence of both external magnetic fields and Rashba spin-orbit coupling. We apply this method to two geometries—a straight wire and a ring structure. Our results reveal the precession of spin-polarized modes in the straight wire, and conductance resonances in the ring due to phase shifts from the magnetic field and from the Rashba effect. The conductance resonances arising from the Rashba effect are due to the non-Abelian nature of the phase shifts acquired by electrons propagating around the ring.     

Quantum transport of pseudospin-polarized Dirac fermions in gapped graphene nanostructures

We investigate the unusual features of the quantum transport in gapped monolayer graphene, which is in a pseudospin symmetry-broken state with a net perpendicular pseudomagnetization. Using these pseudoferromagnets (PFs), we propose a perfect pseudospin valve effect that can be used for realizing pseudospintronics in monolayer graphene. The peculiarity of the associated effects of pseudospin injection and pseudospin accumulation are also studied. We further demonstrate the determining effect of the sublattice pseudospin degree of freedom on Andreev reflection and the associated proximity effect in hybrid structures of PFs and a superconductor in S/PF and PF/S/PF geometries. In particular, we find a peculiar Andreev reflection that is associated with an inversion of the z component of the carriers pseudospin vector. Our results show that the gapped normal graphene behaves like a ferromagnetic graphene and the effect of the pseudospin degree of freedom in gapped graphene is as important as the spin in a ferromagnetic graphene.     

量子霍尔效应在电阻标准中的应用综述

量子霍尔效应为电学计量提供了最高电阻标准,近年来随着新材料的发现和新技术的进步,量子化霍尔电阻标准也正在发生新的进展。全面介绍了量子化霍尔电阻标准的发展过程,描述量子霍尔效应的发现及其理论和普适性,阐述基于砷化镓和基于石墨烯的量子电阻器件以及量子电阻阵列的发展过程,分析新型石墨烯量子化霍尔电阻标准的优势及其发展现状,总结直流电流比较仪电桥、低温电流比较仪电桥和常温低频电流比较仪电桥等3种电阻传递装置原理和优缺点,并展望量子化霍尔电阻标准的发展方向。     

Effect of temperature, electric and magnetic field on spin relaxation in bilayer graphene

In spintronics, spin degree of freedom of an electron is used to store and process information and thus can provide numerous advantages over conventional electronics by providing new functionalities. In this paper, we employ the semiclassical Monte Carlo approach to study the spin polarized transport in bilayer graphene. Due to lower spin orbit interaction (SOI) and higher spin relaxation lengths, graphene is considered as suitable material for spintronics application. Spin relaxation in bilayer graphene is caused by D’yakonov–Perel (DP) relaxation and Elliott–Yafet (EY) relaxation. The effect of temperature, magnetic field and driving electric field on spin relaxation length is studied. We have considered injection polarization along z-direction which is perpendicular to the plane of graphene and the magnitude of ensemble averaged spin variation is studied along the x-direction which is the transport direction.     

Modeling of graphene-based field-effect transistors through a 1-D real-space approach

In this work, we present a computationally efficient approach for atomistic simulations of graphene nanoribbon (GNR), bilayer graphene (BLG) and bilayer graphene nanoribbon (BLGNR) field-effect transistors. The simulation scheme, which involves the self-consistent solutions of the non-equilibrium Green function method (NEGF) and 2-D Poisson’s equation, is based on the tight binding Hamiltonian in a 1-D real-space basis. We show that the Hamiltonian matrix for smooth edge GNRs and graphene can be expressed by 1 \(\times \) 1 size coupling matrices, which provides easy solutions for NEGF equations and largely reduces the computational time for simulation. The BLG and BLGNR can be described by the two coupled single-layer GNR Hamiltonian matrices, which allows the modeling of these devices by the same transport equations as GNR-FET with small modifications. Furthermore, the developed transport models are verified with the previously reported simulation and theoretical results.     

Atomistic non-equilibrium Green’s function simulations of Graphene nano-ribbons in the quantum hall regime

The quantum Hall effect in Graphene nano-ribbons (GNR) is investigated with the non-equilibrium Green’s function (NEGF) based quantum transport model in the ballistic regime. The nearest neighbor tight-binding model based on p _z orbital constructs the device Hamiltonian. GNRs of different edge geometries (Zigzag and Armchair) are considered. The magnetic field is included in both the channels and contact through Peierls substitution. Efficient algorithms for calculating the surface Green function are used to reduce computation time to enable simulating realistically large dimensions comparable to those used in experiments. Hall resistance calculations exactly reproduce the quantum Hall plateaus observed in the experiments. Use of large dimensions in the simulation is crucial in order to capture the quantum Hall effect within experimentally magnetic fields relevant 10–20 T. R. Golizadeh-Mojarad and A.N.M. Zainuddin contributed equally to this work.     

Atomistic deconstruction of current flow in graphene based hetero-junctions

We describe the numerical modeling of current flow in graphene heterojunctions, within the Keldysh Landauer Non-equilibrium Green’s function (NEGF) formalism. By implementing a k-space approach along the transverse modes, coupled with partial matrix inversion using the Recursive Green’s function Algorithm (RGFA), we can simulate on an atomistic scale current flow across devices approaching experimental dimensions. We use the numerical platform to deconstruct current flow in graphene, compare with experimental results on conductance, conductivity and quantum Hall, and deconstruct the physics of electron ‘optics’ and pseudospintronics in graphene pn junctions. We also demonstrate how to impose exact open boundary conditions along the edges to minimize spurious edge reflections.     

Exact analytical solution to the electron density for monolayer and bilayer graphene

This article presents a mathematical method for analytically evaluating the electron density for both monolayer and bilayer graphene. Once an appropriate probability function is chosen for finding the electronic density of states, a complete analytical solution is found for the electronic density without recourse to any analytical approximations or asymptotic formulations. The form of the solution is in terms of a convergent series involving the complementary error function. The analytical solution developed in this paper is valid for all temperatures and for all Fermi energies greater than or equal to zero. The method is easily extended to include Fermi energies less than zero.     

Majorana bound states in a quantum dot device coupled with a superconductor zigzag chain

Research in condensed matter physics on topological insulators and superconductors has contributed greatly to the characterization of the surface properties and zero modes of nanowires. In this work we investigated theoretically, using the recursive Green’s function approach, electron transport through a T-shaped single-level spinless quantum dot, connected to a zigzag chain and coupled to a p-wave superconductor. This model is an extension of the Kitaev chain for a triangular network of finite size with three, four, and five sites. We found that the Majorana zero modes can be tuned through the coupling parameters of the device and that the linear conductance shows Majorana bound states (MBS) in the topological phase, being maximally robust in the general topological phase. This more realistic model permits the detection of MBS via control of the parameters governing the electronic tunneling and could be helpful for relevant experiments.     

Magnetic properties of bilayer graphene: a Monte Carlo study

The lattice structure of bilayer graphene atoms within the same layer is studied by Monte Carlo simulations. The ground-state phase diagrams of mixed spin-2 and spin-3/2 with Ising model on a bilayer graphene are investigated using the Monte Carlo simulations. The reduced transition temperatures for the plane and interplane exchange interactions and for different crystal fields have been obtained. The total magnetization and magnetic susceptibility with the crystal field have been established for different plane exchange interactions. We have also given the magnetic hysteresis cycle for different plane exchange interactions, different temperatures, and with different crystal fields in bilayer graphene. Finally, the system exhibits the superparamagnetic phase at the reduced transition temperature and for a fixed crystal field.     

Probing quantum coherent states in bilayer graphene

An active area of post-CMOS device research is to study the possibility of realizing and exploiting exotic quantum states in nanostructures. In this paper we consider one such system, two layers of graphene separated by an oxide insulator. This system has been predicted to have an excitonic condensate that survives above room temperature. We describe a computational technique—path integral quantum Monte Carlo (PIMC)—that directly simulates many-body quantum phenomena, including excitonic condensation. Starting from a simplified quasiparticle model, the many-body PIMC simulations show excitonic pairing and a confirm a superfluid phase that persists above room temperature. We then present an atomistic PIMC model that captures more details of graphene than our quasiparticle model, and discuss how to extract parameters for a non-equilibrium Green’s function calculation.     

Towards understanding the superfluid behavior in double layer graphene nanostructures

The unique electronic properties that are found in graphene layers have been touted as an attractive means to not only study fundamental physical principles but to design new types of electronic and optical information processing technologies. Of the physical observables present in graphene which may be exploited for device technologies, the proposed superfluid phase transition of indirectly bound excitons in closely spaced layers of graphene is one of the most exciting. Nevertheless, the superfluid phase of double layer graphene remains a poorly understood quantity. In this work, we theoretically investigate the properties of the superfluid phase in double layer graphene systems via two disparate methods: path-integral quantum Monte Carlo and non-equilibrium Green’s functions. We show that the superfluid phase in double layer graphene persists up to ambient temperatures in spinless systems. When we increase the number of degrees of freedom in the system to include spin, we find that the screening effectiveness is suppressed by intralayer correlations resulting in higher transition temperatures than previously predicted. Furthermore, we estimate the magnitude of the interlayer currents that the superfluid can sustain under non-ideal conditions by considering the effects of layer disorder and the electron-phonon interaction. We show that the superfluid dynamics is significantly affected not only by the total amount of disorder but also depends very heavily on the location of the disorder in the layers. When the electron-phonon interaction is included, we demonstrate that for high layer carrier densities the electron-phonon interaction does not affect superfluid flow but degrades the transport properties significantly as the layer carrier concentration decreases.     

An extended Hückel theory based atomistic model for graphene nanoelectronics

An atomistic model based on the spin-restricted extended Hückel theory (EHT) is presented for simulating electronic structure and I–V characteristics of graphene devices. The model is applied to zigzag and armchair graphene nano-ribbons (GNR) with and without hydrogen passivation, as well as for bilayer graphene. Further calculations are presented for electric fields in the nano-ribbon width direction and in the bilayer direction to show electronic structure modification. Finally, the EHT Hamiltonian and NEGF (Nonequilibrium Green’s function) formalism are used for a paramagnetic zigzag GNR to show 2e ²/h quantum conductance.     

The recursive Green’s function method for graphene

We describe how to apply the recursive Green’s function method to the computation of electronic transport properties of graphene sheets and nanoribbons in the linear response regime. This method allows for an amenable inclusion of several disorder mechanisms at the microscopic level, as well as inhomogeneous gating, finite temperature, and, to some extend, dephasing. We present algorithms for computing the conductance, density of states, and current densities for armchair and zigzag atomic edge alignments. Several numerical results are presented to illustrate the usefulness of the method.     

Phonon bottleneck effects in rectangular graphene quantum dots

For a graphene sheet with confining structures in the orthogonal directions of zigzag- and armchair-edge, the confined carrier states are determined. These wavefunctions and eigenvalues are used to study carrier-longitudinal optical (LO)-phonon interactions in these graphene quantum dots. The optical deformation potential is derived for these graphene quantum dots as the basis for the study of these carrier-LO-phonon interactions. Phonon bottleneck effects are identified and the Fermi golden rule transition rates are formulated.     

铁磁/重金属双层膜结构中电流驱动的铁磁共振

在理论上研究了具有垂直磁各向异性铁磁/重金属双层膜结构中电流驱动的铁磁共振。通过线性展开包含自旋霍尔效应自旋矩项的Landau-Lifshitz-Gilbert方程,获得了交流电流频率和直流电流密度调节的铁磁共振谱。发现平衡位置、共振位置和共振线宽都可以通过改变直流电流密度和外磁场的大小进行调节。联合自旋霍尔效应感应的交流和直流自旋矩,可以减小电流感应磁矩反转的临界电流密度。     

Light scattering in semiconductor quantum-dot cavity systems

Scattering theory is used to investigate the interaction of a coherent light beam with two distant one-sided semiconductor double micro-cavities. Each cavity contains a single quantum dot charged by one extra electron. The polarization and phase shift of the scattered light is studied as a function of the initial spin state of the two electrons, as well as differences in structural properties of the two cavities. It is shown that the Faraday rotation of the transmitted light is sensitive to electronic and structural properties of the cavities making careful calibration mandatory, when entanglement generation between the electron spin states is to be achieved.     

Theoretical Evidence of Spontaneous Spin Polarization in GaAs/AlGaAs Split-Gate Heterostructures

The spontaneous spin polarization of a quantum point contact (QPC) formed by the lateral confinement of a high-mobility two-dimensional electron gas in a GaAs/AlGaAs heterostructure is investigated. We present self consistent calculations of the electronic structure of the QPC using the spin-polarized density functional formalism of Kohn and Sham. Spin polarization occurs at low electron densities and exchange potential is found to be the dominant mechanism driving the local polarization within the QPC. We compute the conductance using the cascading scattering matrix approach and observe the conductance anomaly at ∼ 0.7 (2e²/h).