Thermoelectric efficiency of single-molecule junctions with long molecular linkers

We report results of theoretical studies of thermoelectric efficiency of single-molecule junctions with long molecular linkers. The linker is simulated by a chain of identical sites described using a tight-binding model. It is shown that thermoelectric figure of merit ZT strongly depends on the bridge length, being controlled by the lineshape of electron transmission function within the tunnel energy range corresponding to HOMO/LUMO transport channel. Using the adopted model we demonstrate that ZT may significantly increase as the linker lengthens, and that gateway states on the bridge (if any) may noticeably affect the length-dependent ZT. Temperature dependences of ZT for various bridge lengths are analyzed. It is shown that broad minima emerge in ZT versus temperature curves whose positions are controlled by the bridge lengths.Comment: 6 pages, 5 figure

Inelastic electron transport through molecular junctions

Currently, molecular tunnel junctions are recognized as important active elements of various nanodevices. This gives a strong motivation to study physical mechanisms controlling electron transport through molecules. Electron motion through a molecular bridge is always somewhat affected by the environment, and the interactions with the invironment could change the energy of the traveling electron. Under certain conditions these inelastic effects may significantly modify electron transport characteristics. In the present work we describe inelastic and dissipative effects in the electron transport occurring due to the molecular bridge vibrations and stochastic thermally activated ion motions. We intentionally use simple models and computational techniques to keep a reader focused on the physics of inelastic electron transport in molecular tunnel junctions. We consider electron-vibron interactions and their manifestations in the inelastic tunneling spectra, polaronic effects and dissipative electron transport. Also, we briefly discuss long-range electron transfer reactions in macromolecules and their relation to the electron transport through molecular junctions.Comment: 18 pages, 12 figures, In Handbook of Nanophysics, (Vol. VI, Nanoelectronics and Nanophotonics, Ch. 10). Ed. by K. Sattler (Taylor and Francis, New York, 2010). http://www.crcnetbase.com/isbn/978142007551

The effect of Coulomb interactions on nonlinear thermovoltage and thermocurrent in quantum dots

In the present work, we theoretically study the nonlinear regime of charge transport through a quantum dot coupled to the source and drain reservoirs. The investigation is carried out using a nonequilibrium Green's functions formalism beyond the Hartree-Fock approximation. Employed approximations for the relevant Green's functions allow to trace a transition from Coulomb blockade regime to Kondo regime in the thermoelectric transport. Effects arising when electrons move in response to thermal gradient applied across the system are discussed, including experimentally observed thermovoltage zeros.Comment: 9 pages, 10 figure

Local Features of the Fermi Surface Curvature and the Anomalous Skin Effect in Metals

In this paper we present a theoretical analysis of the effect of local geometrical structure of the Fermi surface on the surface impedance of a metal at the anomalous skin effect. We show that when the Fermi surface includes nearly cylindrical and/or flattened segments it may significantly change both magnitude and frequency dependence of the surface impedance. Being observed in experiments these unusual frequency dependencies could bring additional information concerning fine geometrical features of the Fermi surfaces of metals.Comment: 7 pages, 3 figures, text added, accepted for publication in J. Phys: Condens. Matte

Electron transport through asymmetric ferroelectric tunnel junctions: current-voltage characteristics

We have carried out calculations of current-voltage characteristics for the electron tunnel current through a junction with a thin insulating ferroelectric barrier assuming that interface transmissions for the left and right interfaces noticeably differ due to dissimilarity of the interfaces. Obtained conductance versus voltage and current versus voltage curves exhibit well distinguishable asymmetric hysteresis. We show that the asymmetry in the hysteretic effects could originate from the asymmetric bias voltage profile inside the junction. In particular, we analyze the hysteresis asymmetries occurring when the bias voltage distribution is low sensitive to the spontaneous polarization reversal.Comment: 4 pages, 2 figure

The effect of Coulomb interactions on thermoelectric properties of quantum dots

Thermoelectric effects in a quantum dot coupled to the source and drain charge reservoirs are explored using a nonequilibrium Green's functions formalism beyond the Hartree-Fock approximation. Thermal transport is analyzed within a linear response regime. A transition from Coulomb blockade regime to Kondo regime in thermoelectric transport through a single-level quantum dot is traced using unified approximations for the relevant Green's functions.Comment: 6 pages, 3 figure

Specific features of electric charge screening in few-layer graphene films

We present a non-linear Thomas-Fermi theory which describes the electric charge screening in the system including two charged substrate layers separated by a few-layered graphene film. We show that by increasing the charge at the interfaces, the system could be turned from the weak screening regime where the whole film responds to the external charge, to the strong screening regime where the external charge is screened by a surface charge distribution confined to the bounding graphene layers. The transition from weak to strong screening is shown to turn on relatively quickly, and it happens when the applied external charge/external field reaches a certain crossover magnitude. The possibilities for experimental observation of the predicted crossover are discussed.Comment: 7 pages, 3 figure

The effect of dephasing on thermoelectric efficiency of molecular junctions

In this work we report the results of theoretical analysis of the effect of thermal environment on the thermoelectric efficiency of molecular junctions. The environment is represented by two thermal phonon baths associated with the electrodes which are kept at different temperatures. The analysis is carried out using the Buttiker model within the scattering matrix formalism to compute electron transmission through the system. This approach is further developed, so that the dephasing parameters are expressed in terms of relevant energies including the thermal energy, strengths of coupling between the molecular bridge and the electrodes and characteristic energies of electron-phonon interactions. It is shown that the latter significantly affect thermoelectric efficiency by destroying coherency of the electron transport through the considered system.Comment: 9 pages, 6 figures, text added, Fig.2 is chenge

Length-dependent Seebeck effect in single-molecule junctions beyond linear response regime

In the present work we theoretically study characteristics of nonlinear Seebeck effect in a single-molecule junction with chain-like bridge of an arbitrary length. We have employed tight-binding models to compute electron transmission trough the system. We concentrate on analysis of dependences of thermovoltage $V_{th}$ and differential thermopower $S$ on the bridge length. It is shown that $V_{th}$ becomes stronger and $S$ grows as the bridge lengthens. We discuss the effects of the bridge coupling to the electrodes and of specific characteristics of terminal sites on the bridge on the length-dependent $V_{th}$ and $S$ which appear when the system operates beyond linear response regime.Comment: 7 pages, 4 figure

On the electron transport in conducting polymer nanofibers

Recent advances in synthesis and electrical characterization of nanofibers and nanotubes made out of various conjugated polymers attract attention of the research community to studies of transport properties of these materials. In this work we present a theoretical analysis of electron transport in polymer nanofibers assuming them to be in conducting state. We treat a conducting polymer as a network of metallic-like grains embedded in poorly conducting environment, which consists of randomly distributed polymeric chains. We analyze the contribution from intergrain electron resonance tunneling via intermediate states localized on the polymeric chains between the grains. Correspondingly, we apply the quantum theory of conduction in mesoscopic systems to analyze this transport mechanism. We show that the contribution of resonance electron tunneling to the intergrain electron transport may be predominating, as follows from experiments on the electrical characterization of single polyaniline nanofibers. We study the effect of temperature on the transport characteristics. We represent the thermal environment as a phonon bath coupled to the intermediate state, which provides electron tunneling between the metallic-like grains. Using the Buttiker model within the scattering matrix formalism combined with the nonequilibrium Green's functions technique, we show that temperature dependencies of both current and conductance associated with the intergrain electron tunneling, differ from those typical for other conduction mechanisms in conducting polymers. Also, we demonstrate that under certain conditions the phonon bath may cause suppression of the original intermediate state accompanied by emergence of new states for electron tunneling. The temperature dependencies of the magnitudes of the peaks in the transmission corresponding to these new states are analyzed.Comment: 15 pages, 7 figures, in Collection "Nanofibers", ed. A. KUmar (INTECH, Vienna, 2010) http://www.intechopen.com/books/nanofiber