Interplay between electron-electron and electron-vibration interactions on the thermoelectric properties of molecular junctions

The linear thermoelectric properties of molecular junctions are theoretically studied close to room temperature within a model including electron-electron and electron-vibration interactions on the molecule. A nonequilibrium adiabatic approach is generalized to include large Coulomb repulsion through a self-consistent procedure and applied to the investigation of large molecules, such as fullerenes, within the Coulomb blockade regime. The focus is on the phonon thermal conductance which is quite sensitive to the effects of strong electron-electron interactions within the intermediate electron-vibration coupling regime. The electron-vibration interaction enhances the phonon and electron thermal conductance, and it reduces the charge conductance and the thermopower inducing a decrease of the thermoelectric figure of merit. For realistic values of junction parameters, the peak values of the thermoelectric figure of merit are still of the order of unity since the phonon thermal conductance can be even smaller than the electron counterpart.Comment: 8 pages, 1 Appendix, 12 pages. arXiv admin note: substantial text overlap with arXiv:1406.377

Electron-vibration effects on the thermoelectric efficiency of molecular junctions

The thermoelectric properties of a molecular junction model, appropriate for large molecules such as fullerenes, are studied within a non-equilibrium adiabatic approach in the linear regime at room temperature. A self-consistent calculation is implemented for electron and phonon thermal conductance showing that both increase with the inclusion of the electron-vibration coupling. Moreover, we show that the deviations from the Wiedemann-Franz law are progressively reduced upon increasing the interaction between electronic and vibrational degrees of freedom. Consequently, the junction thermoelectric efficiency is substantially reduced by the electron-vibration coupling. Even so, for realistic parameters values, the thermoelectric figure of merit can still have peaks of the order of unity. Finally, in the off-resonant electronic regime, our results are compared with those of an approach which is exact for low molecular electron densities. We give evidence that in this case additional quantum effects, not included in the first part of this work, do not affect significantly the junction thermoelectric properties in any temperature regime.Comment: 15 pages, 11 figures, 2 Appendice

Ground state features of the Frohlich model

Following the ideas behind the Feynman approach, a variational wave function is proposed for the Fr\"ohlich model. It is shown that it provides, for any value of the electron-phonon coupling constant, an estimate of the polaron ground state energy better than the Feynman method based on path integrals. The mean number of phonons, the average electronic kinetic and interaction energies, the ground state spectral weight and the electron-lattice correlation function are calculated and successfully compared with the best available results.Comment: 6 figure

Interplay between electron-phonon couplings and disorder strength on the transport properties of organic semiconductors

The combined effect of bulk and interface electron-phonon couplings on the transport properties is investigated in a model for organic semiconductors gated with polarizable dielectrics. While the bulk electron-phonon interaction affects the behavior of mobility in the coherent regime below room temperature, the interface coupling is dominant for the activated high $T$ contribution of localized polarons. In order to improve the description of the transport properties, the presence of disorder is needed in addition to electron-phonon couplings. The effects of a weak disorder largely enhance the activation energies of mobility and induce the small polaron formation at lower values of electron-phonon couplings in the experimentally relevant window $150 K<T<300 K$. The results are discussed in connection with experimental data of rubrene organic field-effect transistors.Comment: 4 pages, 3 figure

Effects of electron coupling to intra- and inter-molecular vibrational modes on the transport properties of single crystal organic semiconductors

Electron coupling to intra- and inter-molecular vibrational modes is investigated in models appropriate to single crystal organic semiconductors, such as oligoacenes. Focus is on spectral and transport properties of these systems beyond perturbative approaches. The interplay between different couplings strongly affects the temperature band renormalization that is the result of a subtle equilibrium between opposite tendencies: band narrowing due to interaction with local modes, band widening due to electron coupling to non local modes. The model provides an accurate description of the mobility as function of temperature: indeed, it has the correct order of magnitude, at low temperatures, it scales as a power-law $T^{-\delta}$ with the exponent $\delta$ larger than unity, and, at high temperatures, shows an hopping behavior with a small activation energy.Comment: 3 Figures, Submitte

Electronic transport within a quasi two-dimensional model for rubrene single-crystal field effect transistors

Spectral and transport properties of the quasi two-dimensional adiabatic Su-Schrieffer-Heeger model are studied adjusting the parameters in order to model rubrene single-crystal field effect transistors with small but finite density of injected charge carriers. We show that, with increasing temperature $T$, the chemical potential moves into the tail of the density of states corresponding to localized states, but this is not enough to drive the system into an insulating state. The mobility along different crystallographic directions is calculated including vertex corrections which give rise to a transport lifetime one order of magnitude smaller than spectral lifetime of the states involved in the transport mechanism. With increasing temperature, the transport properties reach the Ioffe-Regel limit which is ascribed to less and less appreciable contribution of itinerant states to the conduction process. The model provides features of the mobility in close agreement with experiments: right order of magnitude, scaling as a power law $T^{-\gamma}$, with $\gamma$ close or larger than two, and correct anisotropy ratio between different in-plane directions. Due to a realistic high dimensional model, the results are not biased by uncontrolled approximations.Comment: 10 pages, 9 figures, Submitte

Spectral, optical and transport properties of the adiabatic anisotropic Holstein model: Application to slightly doped organic semiconductors

Spectral, optical and transport properties of an anisotropic three-dimensional Holstein model are studied within the adiabatic approximation. The parameter regime is appropriate for organic semiconductors used in single crystal based field effect transistors. Different approaches have been used to solve the model: self-consistent Born approximation valid for weak electron-phonon coupling, coherent potential approximation exact for infinite dimensions, and numerical diagonalization for finite lattices. With increasing temperature, the width of the spectral functions gets larger and larger making the approximation of quasi-particle less accurate. On the contrary, their peak positions are never strongly renormalized in comparison with the bare ones. As expected, the density of states is characterized by an exponential tail corresponding to localized states at low temperature. For weak electron-lattice coupling, the optical conductivity follows a Drude behavior, while, for intermediate electron-lattice coupling, a temperature dependent peak is present at low frequency. For high temperatures and low particle densities, the mobility always exhibits a power-law behavior as function of temperature. With decreasing the particle density, at low temperature, the mobility shows a transition from metallic to insulating behavior. Results are discussed in connection with available experimental data.Comment: 9 pages, 7 figures, submitted to Phys. Rev.

Rashba quantum wire: exact solution and ballistic transport

The effect of Rashba spin-orbit interaction in quantum wires with hard-wall boundaries is discussed. The exact wave function and eigenvalue equation are worked out pointing out the mixing between the spin and spatial parts. The spectral properties are also studied within the perturbation theory with respect to the strength of the spin-orbit interaction and diagonalization procedure. A comparison is done with the results of a simple model, the two-band model, that takes account only of the first two sub-bands of the wire. Finally, the transport properties within the ballistic regime are analytically calculated for the two-band model and through a tight-binding Green function for the entire system. Single and double interfaces separating regions with different strengths of spin-orbit interaction are analyzed injecting carriers into the first and the second sub-band. It is shown that in the case of a single interface the spin polarization in the Rashba region is different from zero, and in the case of two interfaces the spin polarization shows oscillations due to spin selective bound states

Spin polarization of electrons with Rashba double-refraction

We demonstrate how the Rashba spin-orbit coupling in semiconductor heterostructures can produce and control a spin-polarized current without ferromagnetic leads. Key idea is to use spin-double refraction of an electronic beam with a nonzero incidence angle. A region where the spin-orbit coupling is present separates the source and the drain without spin-orbit coupling. We show how the transmission and the beam spin-polarization critically depend on the incidence angle. The transmission halves when the incidence angle is greater than a limit angle and a significant spin-polarization appears. Increasing the spin-orbit coupling one can obtain the modulation of the intensity and of the spin-polarization of the output electronic current when the input current is unpolarized. Our analysis shows the possibility to realize a spin-field-effect transistor based on the propagation of only one mode with the region with spin-orbit coupling. Where the original Datta and Das device [Appl.Phys.Lett. {\bf 56}, 665 (1990)] use the spin-precession that originates from the interference between two modes with orthogonal spin.Comment: 12 pages with 7 figure

Bloch's theory in periodic structures with Rashba's spin-orbit interaction

We consider a two-dimensional electron gas with Rashba's spin-orbit interaction and two in-plane potentials superimposed along directions perpendicular to each other. The first of these potentials is assumed to be a general periodic potential while the second one is totally arbitrary. A general form for Bloch's amplitude is found and an eigen-value problem for the band structure of the system is derived. We apply the general result to the two particular cases in which either the second potential represents a harmonic in-plane confinement or it is zero. We find that for a harmonic confinement regions of the Brillouin zone with high polarizations are associated with the ones of large group velocity.Comment: 6 pages, 5 figure