First-principles calculation on the conductance of ruthenium-quasi cumulene-ruthenium molecular junctions

The conductance of a family of ruthenium-quasi cumulene-ruthenium molecular junctions including different numbers of carbon atoms, both in even numbers and odd numbers, are investigated using a fully self-consistent ab initio approach which combines the non-equilibrium Green’s function formalism with density functional theory. Our calculations demonstrate that although the overall transport properties of the Ru-quasi cumulene-Ru junctions with an even number of carbon atoms are different from those of the junctions with an odd number of carbon atoms, the difference between the corresponding current-voltage (I–V) characteristics of these molecular junctions declines to lesser than 16% when the voltage goes up. In each group, the molecular junctions give a large transmission around the Fermi level since the Ru-C π bonds can extend the π conjugation of the carbon chains into the Ru electrodes, and their I–V characteristics are almost linear and independent of the chain length, illustrating potential applications as conducting molecular wires in future molecular electronic devices and circuits.      

A comparative study of electron transport in benzene molecule covalently bonded to gold and silicon electrodes for pioneering the electron transport properties of silicon quantum dot-molecule hybrid polymers

In order to pioneer the electron transport properties of silicon (Si) quantum dot-molecule hybrid polymers, we investigate the electron transport properties of the benzene molecule in silicon (Si) semiconductor electrodes, based on nonequilibrium Green's function (NEGF) method coupled with density functional theory (DFT), in comparison with conventional gold (Au) metal electrodes, with three different anchoring linker groups: thiol for dithiol-benzene (DTB), methylene for dimethyl-benzene (DMB), and direct bonding for benzene (Ph). It is interestingly found that, due to band gap nature of the Si semiconductor electrodes, the molecular junctions with the Si electrodes show no current up to the bias voltage of around 0.8 V. In addition, the DTB molecular junctions in the Si semiconductor electrodes connected with Si–S bond show higher conducting properties than other DMB and Ph molecular junctions directly coupled to the electrodes with the Si–C bonds (DMB < Ph < DTB). The electron transport properties of the molecules in the two different electrodes are analyzed on the basis of the understanding transmission spectra, projected density of states (PDOS), and molecular orbitals. We believe that the use of thiol linker may open new possibility in the molecular electronics with the Si semiconductor electrodes and the Si QD-molecule hybrid polymers concept.     

Ferromagnetic 0–<Emphasis Type="BoldItalic">π</Emphasis> Josephson junctions

We present a study on low-T_c superconductor-insulator-ferromagnet-superconductor (SIFS) Josephson junctions. SIFS junctions have gained considerable interest in recent years because they show a number of interesting properties for future classical and quantum computing devices. We optimized the fabrication process of these junctions to achieve a homogeneous current transport, ending up with high-quality samples. Depending on the thickness of the ferromagnetic layer and on temperature, the SIFS junctions are in the ground state with a phase drop of either 0 or π. By using a ferromagnetic layer with variable step-like thickness along the junction, we obtained a so-called 0–π Josephson junction, in which 0 and π ground states compete with each other. At a certain temperature the 0 and π parts of the junction are perfectly symmetric, i.e. the absolute critical current densities are equal. In this case the degenerate ground state corresponds to a vortex of supercurrent circulating clock- or counterclockwise and creating a magnetic flux which carries a fraction of the magnetic flux quantum Φ₀. PACS  74.50.+r; 74.78.Fk; 74.81.-g; 85.25.Cp     

Study of the transport mechanism in molecular self-assembling devices

This paper focuses on the intrinsic charge transport in self-assembled monolayers (SAMs) and on the nature of transport in organic systems, in which surface and bulk properties are undistinguishable due to scale of consistent materials. Developed SAM-OFETs and photovoltaic (SAM-PVC) devices are characterized independently to study a role of charge delocalization both in electrical and optical manifold. The dynamics of charge transport are determined and used to clarify a transport mechanism. Taken together, these SAM devices provide a unique tool to study the fundamentals of polaronic transport on organic surfaces and to discuss the SAM-OFET and SAM PVC performance. Vapor phase molecular self-assembly of 1, 4, 5, 8-naphthalene-tetracarboxylic diphenylimide (NTCDI) having a rich π-stacking charge delivery system is used to enhance the performance of SAM-OFET and SAM PVC devices. Charge mobility in SAM-OFET could achieve values of more than 30 cm² V⁻¹ s⁻¹. The dynamics of charge transport in NTCDI-derived SAM-OFETs were probed using time-resolved measurements in an NTCDI-derived photovoltaic cell device. Time-resolved photovoltaic studies allow us to separate the charge annihilation kinetics in the conductive NTCDI channel from the overall charge kinetic in a SAM-OFET device. It has been demonstrated that tuning of the type of conductivity in NTCDI SAM-OFET devices is possible by changing Si substrate doping. In addition, the possibility of measuring transport in highly ordered SAM structures shines light on the polaron charge transfer in organic materials. Our study proposes that a cation-radical exchange (redox) mechanism is the major transport mechanism in SAM nanodevices. The role and contribution of the transport through delocalized states of redox active surface molecular aggregates of NTCDI are exposed and investigated in this report.     

Subgap anomaly and above-energy-gap structure in chains of diffusive SNS junctions

We present the results of low-temperature transport measurements on chains of superconductor-normal constriction-superconductor (SNS) junctions fabricated on the basis of superconducting PtSi film. A comparative study of the properties of the chains, consisting of 3 and 20 SNS junctions in series, and single SNS junctions reveals essential distinctions in the behavior of the current-voltage characteristics of the systems: (i) a gradual decrease of the effective suppression voltage for the excess conductivity observed at zero bias as the quantity of the SNS junctions increases; (ii) a rich fine structure on the dependences dV/dI-V at dc bias voltages higher than the superconducting gap and corresponding to some multiples of 2Δ/e. A model explaining this above-energy-gap structure based on the energy relaxation of electrons via Cooper-pair-breaking in the superconducting island connecting normal metal electrodes is proposed.     

Coexistence of superconductivity and ferromagnetism in ferromagnet/superconductor proximity structures

The Nambu spinor Green's function approach is applied to calculating the density of states (DOS) and superconducting order parameter in normal-metal/insulator/ferromagnet/superconductor (NM/I/FM/SC) junctions. It is found that the s-wave superconductivity and ferromagnetism can coexist near the FM/SC interface, which is induced by proximity effect. On the SC side, the spin-dependent DOS appears both within and without the energy gap. On the FM side, the superconducting order parameter displays a damped oscillation and the DOS exhibits some superconducting behavior. The calculated result for the DOS in FM for “0 state” and “π state” can reproduce recent tunneling spectra in Al/Al₂O₃/PdNi/Nb tunnel junctions. Received 1st July 2002 Published online 19 November 2002     

Theoretical design of single-molecule NOR and XNOR logic gates by using transition metal dibenzotetraaza[14]annulenes

The idea of replacing traditional silicon-based electronic components with the ones assembled by organic molecules to further scale down the electric circuits has been attracting extensive research focuses. Among the molecularly assembled components, the design of molecular logic gates with simple structure and high Boolean computing speed remains a great challenge. Here, by using the state-of-the-art nonequilibrium Green's function theory in conjugation with first-principles method, the spin transport properties of single-molecule junctions comprised of two serially connected transition metal dibenzotetraaza[14]annulenes(TM(DBTAA), TM = Fe, Co) sandwiched between two single-walled carbon nanotube electrodes are theoretically investigated. The numerical results show a close dependence of the spin-resolved current-voltage characteristics on spin configurations between the left and right molecular kernels and the kind of TM atom in TM(DBTAA)molecule. By taking advantage of spin degree of freedom of electrons, NOR or XNOR Boolean logic gates can be realized in Fe(DBTAA) and Co(DBTAA) junctions depending on the definitions of input and output signals. This work proposes a new kind of molecular logic gates and hence is helpful for further miniaturization of the electric circuits.     

0-π oscillations in nanostructured Nb/Fe/Nb Josephson junctions

The physics of the π phase shift in ferromagnetic Josephson junctions may enable a range of applications for spin-electronic devices and quantum computing. We investigate transitions from “0” to “π” states in Nb/Fe/Nb Josephson junctions by varying the Fe barrier thickness from 0.5 nm to 5.5 nm. From magnetic measurements we estimate for Fe a magnetic dead layer of about 1.1 nm. By fitting the characteristic voltage oscillations with existing theoretical models we extrapolate an exchange energy of 256 meV, a Fermi velocity of 1.98 ×10⁵ m/s and an electron mean free path of 6.2 nm, in agreement with other reported values. From the temperature dependence of the I_CR_N product we show that its decay rate exhibits a nonmonotonic oscillatory behavior with the Fe barrier thickness.     

门电场对有机单分子器件的影响 (cited: 3)

基于第一性原理和弹性散射格林函数方法,从理论上研究了门电场对一系列有机单分子结的电输运性质的影响. 结果显示,含有氧化还原中心并且在门电场方向具有较大电偶极矩的分子结能够对门电场有显著响应. 2,5-二甲基噻吩二硫醇的伏安特性显示了类似N-沟道金属氧化物半导体管性质. 这一独特的伏安特性可以从能级、耦合能以及原子电荷随门电场的演化来理解.     

Transverse dispersion and interfacial dephasing effects on the shape and amplitude of the ballistic-electron-emission spectroscopy of nanographenes

We investigate charge transport across metal-molecule-metal junctions, i.e. hexagonal and triangular nanographene molecular layers sandwiched between Pt and Pd thin films, as measured by ballistic-electron-emission spectroscopy (BEEM). The measured shape of current-voltage curves cannot be explained in the framework of existing BEEM theories of bulk inorganic semiconductors. We develop a tight-binding model for the BEEM process and propose that the energetic dispersion of molecular layers and the dephasing effect due to the interface states account for the anomalous BEEM current-voltage behavior and play an important role in determining the shape of the curve. The electron-phonon scattering can also affect the shape of current-voltage curves.     

Manifestations of phase-coherent transport in graphene

The electronic transport properties of graphene exhibit pronounced differences from those of conventional two dimensional electron systems investigated in the past. As a consequence, well established phenomena such as the integer quantum Hall effect and weak localization manifest themselves differently in graphene. Here we present an overview of recent experiments that we have performed to probe phase coherent transport. In particular, we have investigated in great detail Josephson supercurrent and superconducting proximity effect in junctions consisting of a graphene layer in between superconducting electrodes. We have also used the same devices to measure aperiodic conductance fluctuations and weak localization. The experimental results clearly indicate that low-temperature transport in graphene is phase coherent on a ∼ 1μm length scale, irrespective of the position of the Fermi level. We discuss the different behavior of Josephson supercurrent and weak localization in terms of the unusual properties of the electronic states in graphene upon time reversal symmetry.     

The electrical properties of semiconductor/metal,semiconductor/liquid,and semiconductor/conducting polymer contacts

Critical comparisons are drawn between the basic electrical properties of semiconductor/metal, semiconductor/liquid, and semiconductor/conducting polymer junctions. A theoretical model is developed to describe the basic current-voltage properties of semiconductor contacts, with emphasis on the contrasts between ideal and observed behavior. Using the concepts from this model, the characteristics of a variety of semiconductor contacts are evaluated. The discussion focuses on the following semiconductors: Si, GaAs, InP, and II-VI compounds based on the Cd-(chalcogenide) materials.     

Understanding the newly observed Y(4008) by Belle

Very recently a new enhancement around 4.05 GeV was observed by the Belle experiment. We discuss some possible assignments for this enhancement, i.e. the ψ(3S) and D^*D̄^* molecular states. In these two assignments, Y(4008) can decay into J/ψπ⁰π⁰ with a branching ratio comparable with that of Y(4008)→J/ψπ⁺π^-. Thus, one suggests high energy experimentalists to look for Y(4008) in J/ψπ⁰π⁰ channel. Furthermore one proposes further experiments to search for the missing channels DD̄, DD̄^*+h.c. and especially χ_cJπ⁺π^-π⁰ and η_cπ⁺π^-π⁰, which will be helpful to distinguish the ψ(3S) and D^*D̄^* molecular state assignments for this new enhancement. PACS 13.30.Eg 13.75.Lb     

Fluorescence spectroscopy of ultrathin molecular organic films on surfaces

We review our recent experiments by linear optical spectroscopy in the visible spectral range on interface controlled monolayers and thin films of π-conjugated organic molecules on well-defined surfaces. In particular, cw fluorescence and fluorescence excitation spectroscopy are considered. We discuss two interesting aspects: the possibility to prepare interface controlled films with non-bulk like structures, and consequently different and possibly superior optical properties, and the use of optical spectroscopy as an analytical tool to investigate properties of interfacial organic films.     

Formation of Schottky-type metal/SrTiO3 junctions and their resistive properties

Motivated by the successful use of strontium titanate with different doping metals for memory cells on the basis of resistive switching and the recent findings on the major importance of oxygen vacancy redistribution in this compound, the present work shows the possibility of a non-volatile resistance change memory based on vacancy-doped SrTiO₃. The formation of corresponding metal/SrTiO_3−δ junctions (δ>0) in an electric field will be discussed as well as the switching between ohmic and Schottky-type contact behavior. A notable hysteresis in the current–voltage characteristics is used to carry out Write, Read, and Erase operations exemplifying the memory cell properties of such junctions. But whereas the electric field-induced formation of Schottky-type junctions is explainable by oxygen vacancy redistribution, the resistive switching needs to be discussed in terms of vacancies serving as electron trap states at the metal/oxide interface.     

Transport properties and electronic structure of epitaxial tunnel junctions

We present ab initio calculations for the electronic ground-state and transport properties of epitaxial Fe/semiconductor/Fe (0 0 1) tunnel junctions. The ground state properties are determined by the ab initio Screened KKR Green's function method and the transport properties by a Green's function formulation of the Landauer–Büttiker formalism. We focus on tunnel junctions with a semiconducting ZnSe barrier and compare them to results for junctions with Si and GaAs barriers. We comment on the presence of metal-induced gap states (MIGS) in the semiconductor, the spin polarization of which strongly depends on the nature of the barrier. We investigate furthermore the influence of one atomic layer at the interface of a non-magnetic metal (Cu, Ag, Al) and of a magnetic 3d transition metal.     

ππ S-wave phase shifts and non-perturbative chiral approach

We extend a recent non-perturbative chiral approach to ππ S-wave scattering of Oller and Oset by including the couplings to the ηη-channel. We find that the isospin-zero and isospin-two ππ S-wave phase shifts of the model deviate considerably from a recent solution of the Roy-equations. Including the couplings with the ηη-channel does not improve the situation. In particular, no σ-meson like enhancement structure shows up in the Roy-equation solution. We also consider the ππ-scattering lengths in this approach. Received: 17 August 1998 / Revised version: 21 September 1998     

Negative differential resistance in a molecular junction of carbon nanotube and benzene

We propose a novel molecular junction with single-walled carbon nanotubes as electrodes bridged by a benzene molecule, in which the electrodes are saturated by different terminations (C-, H- and N-). It is found that the different terminations at the carbon nanotube ends strongly affect the electronic transport properties of the junction. The current-voltage (I-V) curve of the N-terminated carbon nanotube junction shows a more striking nonlinear feature than that of the C- and H-terminated junctions at smal...     

The effect of base thickness on the reverse volt–ampere characteristic of an S-diode

The effect of base thickness (π-layer thickness d _π) on the reverse current-voltage characteristic and the switching voltage U _sw in a diffusion avalanche S-diode is studied. It is shown that the current-voltage characteristic shape is independent of d _π, whereas the switching voltage U _sw weakly decreases (to 40%) with a significant (4–5-fold) decrease in d _π. It is assumed that the results obtained can be explained, taking into account electron injection from the forward-biased contact to the π-layer. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 48–53, February, 2009.