Currents In Molecular Junctions Research Articles

Hierarchical quantum master equation approach to charge transport in molecular junctions with time-dependent molecule-lead coupling strengths

Time-dependent currents in molecular junctions can be caused by structural fluctuations or interaction with external fields. In this publication, we demonstrate how the hierarchical quantum master equation approach can be used to study time-dependent transport in a molecular junction. This reduced density matrix methodology provides a numerically exact solution to the transport problem including time-dependent energy levels, molecule-lead coupling strengths and transitions between electronic states of the molecular bridge. Based on a representative model, the influence of a time-dependent molecule-lead coupling on the electronic current is analyzed in some detail.

Controlling local currents in molecular junctions

The effects of nonequilibrium constraints and dephasing on the circulating currents in molecular junctions are analyzed. Circulating currents are manifestations of quantum effects and can be induced either by externally applied bias or an external magnetic field through the molecular system. In a symmetric Aharonov-Bohm ring, bond currents have two contributions, bias driven and magnetic field driven. We analyze the competition between these two contributions and show that, as a consequence, current through one of the branches can be completely suppressed. We then study the effect of asymmetry (as a result of chemical substitution) on the current pathways inside the molecule and study asymmetry-induced circulating currents (without magnetic field) by tuning the coupling strength of the substituent (at finite bias).

Perfect spin filtering by symmetry in molecular junctions

Obtaining highly spin-polarized currents in molecular junctions is crucial and desirable for nanoscale spintronics devices. Motivated by our recent symmetry-based theoretical argument for complete blocking of one spin conductance channel in atomic-scale junctions [A. Smogunov and Y. J. Dappe, Nano Lett. 15, 3552 (2015)], we explore the generality of the proposed mechanism and the degree of achieved spin-polarized current for various ferromagnetic electrodes (Ni, Co, Fe) and two different molecules, quaterthiophene and p-quaterphenyl. A simple analysis of the spin-resolved local density of states of a free electrode allowed us to identify the Fe(110) as the most optimal electrode, providing perfect spin filtering and high conductance at the same time. These results are confirmed by $ab$ $initio$ quantum transport calculations and are similar to those reported previously for model junctions. It is found, moreover, that the distortion of the p-quaterphenyl molecule plays an important role, reducing significantly the overall conductance.

Metal-free molecular junctions on ITO via amino-silane binding—towards optoelectronic molecular junctions

Light control over currents in molecular junctions is desirable as a non-contact input with high spectral and spatial resolution provided by the photonic input and the molecular electronics element, respectively. Expanding the study of molecular junctions to non-metallic transparent substrates, such as indium tin oxide (ITO), is vital for the observation of molecular optoelectronic effects. Non-metallic electrodes are expected to decrease the probability of quenching of molecular photo-excited states, light-induced plasmonic effects, or significant electrode expansion under visible light. We have developed micron-sized, metal free, optically addressable ITO molecular junctions with a conductive polymer serving as the counter-electrode. The electrical transport was shown to be dominated by the nature of the self-assembled monolayer (SAM). The use of amino-silane (APTMS) as the chemical binding scheme to ITO was found to be significant in determining the transport properties of the junctions. APTMS allows high junction yields and the formation of dense molecular layers preventing electrical short. However, polar amino-silane binding to the ITO significantly decreased the conductance compared to thiol-bound SAMs, and caused tilted geometry and disorder in the molecular layer. As the effect of the molecular structure on transport properties is clearly observed in our junctions, such metal-free junctions are suitable for characterizing the optoelectronic properties of molecular junctions.

Spin inelastic currents in molecular ring junctions

Within a simple model, we discuss spin inelastic currents in molecular ring junctions. We generalize considerations of the spin-flip inelastic electron tunneling spectroscopy (IETS) to the case of multisite molecular system, and formulate a conserving approximation, which takes into account renormalization of elastic channel. We also extend recent studies of circular currents in molecular junctions beyond scattering theory formulation. We demonstrate control of the spin-flip IETS signal and discuss spin polarization of total and circular currents in a benzene ring junction.

Transient switch-on/off currents in molecular junctions

Based on the nonequilibrium density matrix theory we put forward a unified description of the transient and the steady state current formation through a molecular junction. It is demonstrated that the current follows the time evolution of the populations of those molecular charged states which participate in the inter-electrode charge transmission. As an example, the formation of switch-on/switch-off currents is analyzed for a junction where the molecule has two active terminal sites. It is shown that just after a sudden voltage switch-on or switch-off, the resulting transient currents can significantly exceed their steady state value. This feature is caused by molecular charging or discharging processes, which are fast compared to those processes responsible for establishing the steady state current in the junction. The largest transient currents appear if the coupling of the molecule to the adjacent electrodes is asymmetric, or if the applied voltage causes a transformation of extended molecular states into localized ones.

Ultrafast stop and go

Theoretical physicists have predicted that ultrashort laser pulses can be used to drive electrical currents through single molecules, and also to stop currents in molecular junctions.

Nonequilibrium superoperator Green’s function approach to inelastic resonances in STM currents

Closed expressions for tunneling currents in molecular junctions are derived to the fourth-order in electron-phonon coupling. The Keldysh-Schwinger formalism is recast in terms of density matrices in Liouville space, and the calculation only involves forward propagation in real time and is represented by the double sided Feynman diagrams commonly used for computing optical response functions. The current is related to the decay of coherences in Fock space between many-body molecular states with $N$ and $N\ifmmode\pm\else\textpm\fi{}1$ electrons. Application is made to the fundamental and overtone vibrational resonances of methane.

Resonant inelastic tunneling in molecular junctions

Within a phonon-assisted resonance level model we develop a self-consistent procedure for calculating electron transport currents in molecular junctions with intermediate to strong electron-phonon interaction. The scheme takes into account the mutual influence of the electron and phonon subsystems. It is based on the second order cumulant expansion, used to express the correlation function of the phonon shift generator in terms of the phonon momentum Green function. Equation of motion (EOM) method is used to obtain an approximate analog of the Dyson equation for the electron and phonon Green functions in the case of many-particle operators present in the Hamiltonian. To zero order it is similar in particular cases (empty or filled bridge level) to approaches proposed earlier. The importance of self-consistency in resonance tunneling situations (partially filled bridge level) is stressed. We confirm, even for strong vibronic coupling, a previous suggestion concerning the absence of phonon sidebands in the current versus gate voltage plot when the source-drain voltage is small [Mitra et al., Phys. Rev. B 69, 245302 (2004)].