Effect of gold electrode crystallographic orientations on charge transport through aromatic molecular junctions

ABSTRACT

We examined the electrical conduction through single-molecular junctions comprising of anthracenedithiol molecule coupled to two gold electrodes having ?1,0,1?, ?1,1,0? and ?1,1,1? crystallographic orientations. Owing to this jellium model, we evaluated the values of current and conductance using non-equilibrium Green's functions combined with extended Huckel theory. This data was further interpreted in terms of transmission spectra, density of states and their molecular orbital analysis for zero bias. We evinced the oscillating conductance in all three cases, due to the oscillation of orbital energy relative to Fermi level. Our detailed analysis suggested that electrode orientation can tune the molecule–electrode coupling and hence conduction. Anthracene molecular junction with ?1,1,0? orientation displayed favourable conduction, when compared to the other two orientations, thus can provide us an insight while designing futuristic molecular electronic devices.     

In-chain tunneling through charge-density-wave nanoconstrictions and break junctions

We have fabricated longitudinal nanoconstrictions in the charge-density wave conductor (CDW) NbSe3 using a focused ion beam and using a mechanically controlled break-junction technique. Conductance peaks are observed below the TP1=145 K and TP2=59 K CDW transitions, which correspond closely with previous values of the full CDW gaps 2Delta1 and 2Delta2 obtained from photoemission. These results can be explained by assuming CDW-CDW tunneling in the presence of an energy gap corrugation epsilon2 comparable to Delta2, which eliminates expected peaks at +/-|Delta1+Delta2|. The nanometer length scales our experiments imply indicate that an alternative explanation based on tunneling through back-to-back CDW-normal-conductor junctions is unlikely.     

Electron tunneling through superconducting A1/monolayer/Pb junctions

The current-voltage characteristic of Al/adsorbed monolayer/Pb junctions was measured at 77, 4.2 and 1.8K at applied voltages from 1 to 3 mV. At 77K the current changes linearly with voltage whereas at 4.2 and 1.8 K the relationship becomes nonlinear. From the results at 1.8 K we obtain an approximate band gap for Pb equal to 2.6 meV. The observation of a nonlinear current-voltage characteristic at temperatures where Pb becomes superconducting is strong evidence that the observed current through the insulator is a tunneling current.     

Quantum coherent effects and Zener tunneling in superconducting tunnel junctions

We calculate the current-voltage characteristics of a small capacitance underdamped superconducting tunnel junction and find the value of the critical current which corresponds to switching between coherent voltage oscillations and uncorrelated single electron tunneling. Both Zener tunneling and dissipative relaxation are important in this context.     

Quantum mechanical tunneling phenomena in metal–semiconductor junctions

The impact of the quantum mechanical tunneling effect on the operation of MESFET device structure has been investigated. Due to the presence of a Schottky barrier in a highly doped semiconductor, the depletion region is so narrow that electrons can tunnel through the barrier and contribute to the gate leakage current. This, in turn, facilitates current gain of the Schottky junction transistor (SJT) in the subthreshold region. In a simulation of a SJT we have used 2D Monte Carlo particle-based simulations. Quantum mechanical tunneling effects have been accounted for by using the Airy function transfer matrix approach, valid for piecewise linear potential barriers.     

Resonant Josephson tunneling through S-I-S junctions of arbitrary size

The Josephson tunneling current in S-I-S structures where the main current transport channel is resonant tunneling through an isolated localized state is calculated using the Bogolyubov-de Gennes equations. It is shown that the efficiency of equilibrium Josephson resonant tunneling is determined only by the ratio of the width of the resonance level to the absolute value of the order parameter for the superconducting electrodes with arbitrary relationships among the system parameters. Zh. éksp. Teor. Fiz. 112, 342–352 (July 1997)     

Sequential tunneling through molecular spin rings

We consider electrical transport through molecules with Heisenberg-coupled spins arranged in a ring structure in the presence of an easy-axis anisotropy. The molecules are coupled to two metallic leads and a gate. In the charged state of the ring, a Zener double-exchange mechanism links transport properties to the underlying spin structure. This leads to a remarkable contact-site dependence of the current, which for an antiferromagnetic coupling of the spins can lead to a total suppression of the zero-bias conductance when the molecule is contacted at adjacent sites.     

Spin-flip assisted tunneling through quantum dot based magnetic tunnel junctions

We theoretically study the spin-polarized transport through double barrier magnetic tunnel junction (DBMTJ) consisting of the quantum dot sandwiched by two ferromagnetic (FM) leads. The tunneling current through the DBMTJ is evaluated based on the Keldysh nonequilibrium Green’s function approach. The self-energy and Green’s function of the dot are analytically obtained via the equation of motion method, by systematically incorporating two spin-flip phenomena, namely, intra-dot spin-flip, and spin-flip coupling between the lead and the central dot region. The effects of both spin-flip processes on the spectral functions, tunneling current and tunnel magnetoresistance (TMR) are analyzed. The spin-flip effects result in spin mixing, thus contributing to the spectral function of the off-diagonal Green’s function components ( G_s[`(s)] ^r )\left( {G_{\sigma \bar \sigma }^r } \right). Interestingly, the spin-flip coupling between the lead and dot enhances both the tunneling current and the TMR for applied bias above the threshold voltage V _th . On the other hand, the intra-dot spin-flip results in an additional step in the I-V characteristics near V _th . Additionally, it suppresses the tunneling current but enhances the TMR. The opposing effects of the two types of spin-flip on the tunneling current means that one spin-flip mechanism can be engineered to counteract the other, so as to maintain the tunneling current without reducing the TMR. Their additive effect on the TMR enables the DBMTJ to attain a large tunneling current and high TMR for above threshold bias values.     

分子磁体中的量子隧穿及宏观量子效应

文章介绍了分子磁体中的量子隧穿和宏观量子效应理论和实验研究的新进展．分子磁体既有宏观磁体特性也呈现纯量子行为，例如磁化矢量的量子隧穿．文章作者解释了如何通过量子隧穿实现宏观量子相干(即薛定谔猫态的相干叠加)和量子态位相干涉．对隧穿率计算的瞬子方法，特别是有限温度隧穿理论及其在分子磁体量子隧穿中的应用也做了简要的阐述．     

Anomalous surface enhanced molecular Raman scattering from inelastic tunneling spectroscopy junctions

Anomalously strong Raman spectra have been obtained from molecular monolayers adsorbed on the insulator in metal-insulator-metal tunnel junctions. We show unambiguously that Raman spectroscopy can readily detect molecular monolayers and consider the effects of surface roughness, the molecule-metal interface and the metal on our results.     

Origin of discrepancies in inelastic electron tunneling spectra of molecular junctions

We report inelastic electron tunneling spectroscopy (IETS) of multilayer molecular junctions with and without incorporated metal nanoparticles. The incorporation of metal nanoparticles into our devices leads to enhanced IET intensity and a modified line shape for some vibrational modes. The enhancement and line-shape modification are both the result of a low lying hybrid metal nanoparticle-molecule electronic level. These observations explain the apparent discrepancy between earlier IETS measurements of alkane thiolate junctions by Kushmerick et al. [Nano Lett. 4, 639 (2004)] and Wang et al. [Nano Lett. 4, 643 (2004)].     

Resonant tunneling through electronic trapping states in thin MgO magnetic junctions

We report an inelastic electron tunneling spectroscopy study on MgO magnetic junctions with thin barriers (0.85-1.35 nm). Inelastic electron tunneling spectroscopy reveals resonant electronic trapping within the barrier for voltages V>0.15 V. These trapping features are associated with defects in the barrier crystalline structure, as confirmed by high-resolution transmission electron microscopy. Such defects are responsible for resonant tunneling due to energy levels that are formed in the barrier. A model was applied to determine the average location and energy level of the traps, indicating that they are mostly located in the middle of the MgO barrier, in accordance with the high-resolution transmission electron microscopy data and trap-assisted tunneling conductance theory. Evidence of the influence of trapping on the voltage dependence of tunnel magnetoresistance is shown.     

Quantum oscillation of the tunneling conductance in fully epitaxial double barrier magnetic tunnel junctions

We investigated spin-dependent tunneling conductance properties in fully epitaxial double MgO barrier magnetic tunnel junctions with layered nanoscale Fe islands as a middle layer. Clear oscillations of the tunneling conductance were observed as a function of the bias voltage. The oscillation, which depends on the middle layer thickness and the magnetization configuration, is interpreted by the modulation of tunneling conductance due to the spin-polarized quantum well states created in the middle Fe layer. This first observation of the quantum size effect in the fully epitaxial double barrier magnetic tunnel junction indicates great potential for the development of the spin-dependent resonant tunneling effect in coherent tunneling regime.     

Complex band structure and tunneling through ferromagnet /Insulator /Ferromagnet junctions

We investigate the importance of metal-induced gap states for the tunneling of metal electrons through epitaxial insulator films. By introducing an imaginary part kappa to the wave vector in order to describe the decay of the wave function in the insulator, we obtain the complex band structure in the gap region. The spectrum of the decay parameters kappa is calculated for the semiconductors Si, Ge, GaAs, and ZnSe. In most cases, for large enough film thicknesses the tunneling is dominated by states of normal incidence on the interface. Possible exceptions are considered. Based on our conclusions, we discuss the spin-dependent tunneling in Fe/semiconductor/Fe (001) junctions.     

The inelastic electron tunneling spectroscopy of edge-modified graphene nanoribbon-based molecular devices

The inelastic electron tunneling spectroscopy(IETS) of four edge-modified finite-size grapheme nanoribbon(GNR)-based molecular devices has been studied by using the density functional theory and Green's function method. The effects of atomic structures and connection types on inelastic transport properties of the junctions have been studied. The IETS is sensitive to the electrode connection types and modification types. Comparing with the pure hydrogen edge passivation systems, we conclude that the IETS for the lower energy region increases obviously when using donor–acceptor functional groups as the edge modification types of the central scattering area. When using donor–acceptor as the electrode connection groups, the intensity of IETS increases several orders of magnitude than that of the pure ones. The effects of temperature on the inelastic electron tunneling spectroscopy also have been discussed. The IETS curves show significant fine structures at lower temperatures. With the increasing of temperature, peak broadening covers many fine structures of the IETS curves.The changes of IETS in the low-frequency region are caused by the introduction of the donor–acceptor groups and the population distribution of thermal particles. The effect of Fermi distribution on the tunneling current is persistent.     

Shot noise in magnetic tunnel junctions: evidence for sequential tunneling

We report the experimental observation of sub-Poissonian shot noise in single magnetic tunnel junctions, indicating the importance of tunneling via impurity levels inside the tunnel barrier. For junctions with weak zero-bias anomaly in conductance, the Fano factor (normalized shot noise) depends on the magnetic configuration being enhanced for antiparallel alignment of the ferromagnetic electrodes. We propose a model of sequential tunneling through nonmagnetic and paramagnetic impurity levels inside the tunnel barrier to qualitatively explain the observations.     

Quantum groups: A review

The recent development of quantum groups is summarized from the point of view of quantum physics. The emphasis is on the ideas, concepts, and motivation of these new developments.Invited paper at the Quantum Structures 92 Conference, Castiglioncello, Italy, September 21–26, 1992.