Spin-dependent transport in a multifunctional spintronic device with graphene nanoribbon electrodes

We investigated the spin-dependent transport properties of a molecular device consisting of a phenanthrene molecule anchored via two carbon atoms to zigzag graphene nanoribbon electrodes, using density functional theory combined with the nonequilibrium Green’s function method. The results of the calculations show that the device exhibits perfect spin filtering and negative differential resistance effect in both parallel and antiparallel configuration, and perfect dual spin filtering and large spin rectification in antiparallel configuration. In addition, we changed the direction of the phenanthrene plane to be perpendicular to the two electrode planes, enabling molecular switching. The proposed structure combines interesting properties that enable its use in multifunctional nanoelectronic devices.     

Circularly polarized spin current assisted fast resonant switching in magnetic tunnel junctions with perpendicular anisotropy

We propose a mechanism to substantially enhance spin transfer torque induced switching in perpendicular anisotropy magnetic tunnel junctions. Our method is based on injecting an additional assisting DC current with circular spin polarization into a magnetic free layer at frequencies that are resonant with its normal ferromagnetic resonance frequencies. We observe 80 % reduction in switching delays at constant switching currents and 2× improvement in critical switching current density. Spin current polarization chirality and spin polarization efficiency dependencies are investigated. Further, a device structure to experimentally realize the mechanism proposed in this letter is presented.     

Coherent electron transport through freestanding graphene junctions with metal contacts: a materials approach

In this article we highlight recent work in which we computed the spin unpolarized coherent electron transport through two terminal nanoscale graphene/metal junctions using equilibrium Green’s functions coupled to Density functional theory, capturing in detail the important electronic effects created at metal/graphene interfaces. In those calculations the metal contacts may or may not bind covalently to graphene. Along the way, connections to other models for coherent transport on graphene junctions with metal contacts are given as well. As it may be known, the computation of the electronic dispersion at the interface between graphene and binding metals in a transport setup is extremely time-consuming, and it is perhaps for this reason that effects of metals are neglected or are captured only qualitatively in the theoretical and computational modeling of graphene devices. It thus seems to us that a methodology to go past this stumbling block may be well-received. We outline an approach to obtain tight-binding parameters describing the electronic dispersion at interfaces between titanium leads and graphene. The deployment of those tight-binding parameters is new, and it constitutes the main contribution on the present paper.     

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.     

Giant Magnetoresistance and Its Sensor Applications

Current status is addressed for spin polarized transport such as spin-dependent scattering and spin-dependent tunneling in artificial structured magnetic thin films. The main points discussed include the physical origin of the giant magnetoresistance (GMR) and various GMR materials. The applications of the GMR to magnetic sensors such as read heads for ultra high density magnetic recording are also discussed.     

Spin filtering in a $$\updelta $$-magnetic-barrier nanostructure modulated by Rashba and Dresselhaus spin–orbit couplings

We theoretically investigated the electron-spin transport properties of an antiparallel double \(\updelta \)-magnetic-barrier nanostructure modulated by spin–orbit coupling (SOC), which could be fabricated experimentally by depositing two ferromagnetic stripes with horizontal magnetization on the top and bottom of an InAs/Al\(_{x}\)In\(_{1-x}\)As semiconductor heterostructure. Both Rashba and Dresselhaus SOCs were taken into account, and the transmission coefficient, conductance, and spin polarization calculated analytically by means of the improved transfer matrix method. The electron-spin transport through this nanosystem is found to be strongly dependent on the SOC. The electron-spin polarization is also found to vary with the strength of the SOC, potentially enabling tunable spin filters for use in spintronic applications.     

Bias-induced destruction of ferromagnetism and disorder effects in GaMnAs heterostructures

The magneto-electric properties of resonant tunneling double barrier structures using GaMnAs for the quantum well is investigated within a self-consistent Green??s function approach and a tight-binding electronic structure model. The magnetic state of the well is determined self-consistently by the tunneling current which controls the hole spin density and, hence, the degree of exchange splitting of the subbands inside the well. Prompted by recent experiments we compare model systems of increasing defect concentration (substitutional disorder) regarding their I?CV curve, magnetic state, and spin polarization. We predict that, near resonance, the ferromagnetic order which may be present at zero bias in the GaMnAs well tends to be destroyed. Resonance peaks are found to be more sensitive to disorder than ferromagnetic ordering and spin polarization of the steady-state current.     

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.     

Comparative study of symmetrical and asymmetrical B40 molecular junctions

Journal of Computational Electronics - This work investigates quantum transport in symmetrical and asymmetrical borospherene-based molecular junctions with adenine. Adenine is one of the four...     

Fabrication of a conductive molecular wire using the ionization‐assisted evaporation method

Organic molecules are interesting materials with potential for use in next‐generation optical and electronic devices. It is important to prepare highly oriented molecular wires, since the optical and electrical properties of organic films strongly depend on their molecular orientation. The charge‐transfer‐complex wire of TTF‐TCNQ has been studied for application to molecular wires having quasi‐one‐dimensional conductivity. We have prepared highly oriented TTF‐TCNQ grains using the ionization and electric‐field‐assisted deposition method, and have investigated the growth mechanism of TTF‐TCNQ grains. These results demonstrate that needle‐like TTF‐TCNQ grain growth near the electrodes is controlled both by the electric field between the electrodes and by the ionization of evaporated molecules. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 140(3): 8–15, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10015     

Molecular interfacing for protein molecular devices andneurodevices

Several protein-based molecular biodevices have been made possible by the molecular interfacing technology that enables proteins to electronically communicate with the conventional electronic materials, such as metals or semiconductors. The molecular interfacing technology, however, should be extensively improved to allow for precise designs at the single-molecule level. The molecular-scale design of protein-based molecular biodevices may be accomplished by further innovation in molecular interfacing technology. It is also important to pursue the possibility of utilizing the self-organization of biomolecules for information processing, and to make it possible to create a molecular network-based biodevice without any preprogrammed wiring among its molecules. Major advances have recently been made in basic technology. Yet, it will take some time to construct a prototype neurodevice in which neurons are utilized as components. Further investigations should focus on the development of molecular interfaces that offer bilateral communication between neurons and conventional electronic materials     

Heat dissipation and non-equilibrium phonon distributions in molecular devices

Using a density functional approach we compute vibrations of a styrene molecule adsorbed on a Si(100) substrate and the electron-phonon coupling of these modes. A non-equilibrium Green’s function approach is used to compute the partially coherent transmission in molecular junctions due to electron-vibration scattering. The electronic power dissipated into molecular vibrations allows to set a rate equation for the phonon population in the vibrational degrees of freedom of the molecule. The rate of phonon decay is computed using a microscopic approach which includes a first-principle calculation of the coupling of the molecular modes with the vibrations of the contacts. In turn, the calculated phonon lifetime is used to correct the phonon propagator. A self consistent loop allows to compute the steady state non-equilibrium phonon population of the molecular junction under bias condition.     

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.     

电磁场对大功率直流电源pn结排布的影响

在大功率整流电源的制作设计中,一个很重要的问题是如何解决输电线上交变电流对pn结的电磁影响。交变电流会在空间产生交变电磁场,交变电磁场的分布直接影响着整流电源中pn结的排列设计。各输电线上的交流电流都会在空间产生电磁场,其中输入输出交流电的两输电线对空间电磁场的分布起着举足轻重的作用。从麦克斯韦方程组出发,以有限长的平行直导线为模型研究了载有交流电的两输电线周围的电磁场分布,可以直接用于指导整流电源的设计。     

First-principles quantum transport modeling of thermoelectricity in single-molecule nanojunctions with graphene nanoribbon electrodes

We overview the nonequilibrium Green function combined with density functional theory (NEGF-DFT) approach to modeling of independent electronic and phononic quantum transport in nanoscale thermoelectrics with examples focused on a new class of devices where a single organic molecule is attached to two metallic zigzag graphene nanoribbons (ZGNRs) via highly transparent contacts. Such contacts make possible injection of evanescent wavefunctions from the ZGNR electrodes, so that their overlap within the molecular region generates a peak in the electronic transmission around the Fermi energy of the device. Additionally, the spatial symmetry properties of the transverse propagating states in the semi-infinite ZGNR electrodes suppress hole-like contributions to the thermopower. Thus optimized thermopower, together with diminished phonon thermal conductance in a ZGNR|molecule|ZGNR inhomogeneous heterojunctions, yields the thermoelectric figure of merit ZT≃0.4 at room temperature with maximum ZT≃3 reached at very low temperatures T≃10 K (so that the latter feature could be exploited for thermoelectric cooling of, e.g., infrared sensors). The reliance on evanescent mode transport and symmetry of propagating states in the electrodes makes the electronic-transport-determined power factor in this class of devices largely insensitive to the type of sufficiently short organic molecule, which we demonstrate by showing that both 18-annulene and C10 molecule sandwiched by the two ZGNR electrodes yield similar thermopower. Thus, one can search for molecules that will further reduce the phonon thermal conductance (in the denominator of ZT) while keeping the electronic power factor (in the nominator of ZT) optimized. We also show how the often employed Brenner empirical interatomic potential for hydrocarbon systems fails to describe phonon transport in our single-molecule nanojunctions when contrasted with first-principles results obtained via NEGF-DFT methodology.     

Electronic transport across a layered structure of Fe/\upbeta -poly vinylidene fluoride/Fe using DFT calculations

Quantum electronic transport across a \(\upbeta \) -poly(vinylidene fluoride) ( \(\upbeta \) -PVDF) ferroelectric barrier structured between two ferromagnetic Fe layers is explored using DFT calculations. The multifunctional junction is organized in capacitor like structure, as FM (ferromagnetic metal)/FE (ferroelectric)/FM to understand the mechanism of electron transfer by controlling the spin polarization of the electrodes and also the ferroelectric polarization of the barrier. These studies are carried on a single bcc layer of Fe atoms in both the electrodes and two monomers of PVDF is utilized as a barrier. We investigated the dependence of total density of states (DOS), projected DOS, transmission coefficient and I–V characteristics on applied bias voltage using SIESTA & TRANSIESTA package.     

An active microelectronics device for multiplex DNA analysis

Nanogen's approach to DNA analysis involves the development of a unique active microelectronic device that provides electronic control over a variety of molecular biological affinity reactions. The core technology is an automated programmable electronic matrix (APEX), which has the ability to transport, bind and separate charged molecules in an electric field generated on the surface of the device. This broad-based platform technology is potentially applicable for multiplexed DNA hybridizations, immunoassays, receptor binding assays, cell typing assays, enzyme assays, combinatorial synthesis of oligonucleotides and peptides, and nanoparticle manipulations. Nanogen's initial developmental focus is in the area of DNA probe diagnostics. Our proof of concept and initial DNA hybridization results indicate great potential for Nanogen's active device technology. The multiplex hybridization array formats now being developed are directed at providing rapid simultaneous testing from a microliter volume of patient samples, with better sensitivity and specificity than current assays. Nanogen active device technology offers a number of distinct advantages versus current DNA diagnostic technology and differentiates itself from other chip technologies in the following way: (1) electronic addressing transports DNA molecules by charge; (2) the electronic hybridization's concentration effect improves the DNA hybridization rate; and (3) electronic stringency control improves selectivity and discrimination of hybrids     

Electroactive and photochromic molecular materials for wires,switches and memories

Properties of synthetic electroactive and photochromic molecular systems are described, with particular emphasis on microscopic processes controlling these properties. Both low-molecular weight and polymeric materials can be (and many have already been) used as conducting systems. The chemical flexibility of many molecular systems also allows for using them as information-processing materials, whose action is based on their photochromic properties. A relation between properties of individual molecules and those of macroscopic molecular systems allows one to envisage construction of molecular-scale devices. However, due to limitations associated with technological problems (construction of molecular-scale elements, their addressing, quantum-mechanical (size) limitations, etc.), the construction of such elements is still a problem that awaits solution     

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).