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Register a new userAn ab initio non-equilibrium Greens function approach to charge transport: dithiolethine
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We present a novel ab initio non-equilibrium approach to calculate the current across a molecular junction. The method rests on a wave function based full ab initio description of the central region of the junction combined with a tight binding approximation for the electrodes in the frame of the Keldysh Greens function formalism. Our procedure is demonstrated for a dithiolethine molecule between silver electrodes. The main conducting channel is identified and the full current-voltage characteristic is calculated.
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We present a novel ab initio non-equilibrium approach to calculate the current across a molecular junction. The method rests on a wave function based description of the central region of the junction combined with a tight binding approximation for the electrodes in the frame of the Keldysh Greens function formalism. In addition we present an extension so as to include effects of the two-particle propagator. Our procedure is demonstrated for a dithiolbenzene molecule between silver electrodes. The full current-voltage characteristic is calculated. Specific conclusions for the contribution of correlation and two-particle effects are derived. The latter are found to contribute about 5% to the current. The order of magnitude of the current coincides with experiments.
The electronic structure of organic-inorganic interfaces often feature resonances originating from discrete molecular orbitals coupled to continuum lead states. An example are molecular junctions, individual molecules bridging electrodes, where the shape and peak energy of such resonances dictate junction conductance, thermopower, I-V characteristics and related transport properties. In molecular junctions where off-resonance coherent tunneling dominates transport, resonance peaks in the transmission function are often assumed to be Lorentzian functions with an energy-independent broadening parameter $Gamma$. Here we define a new energy-dependent resonance broadening function, $Gamma(E)$, based on diagonalization of non-Hermitian matrices, which can describe resonances of a more complex, non-Lorentzian nature and can be decomposed into components associated with the left and right lead, respectively. We compute this quantity via an emph{ab initio} non-equilibrium Greens function approach based on density functional theory for both symmetric and asymmetric molecular junctions, and show that our definition of $Gamma(E)$, when combined with Breit-Wigner formula, reproduces the transmission calculated from DFT-NEGF. Through a series of examples, we illustrate how this approach can shed new light on experiments and understanding of junction transport properties in terms of molecular orbitals.
We have combined the Boltzmann transport equation with an {it ab initio} approach to compute the thermoelectric coefficients of semiconductors. Electron-phonon, ionized impurity, and electron-plasmon scattering rates have been taken into account. The electronic band structure and average intervalley deformation potentials for the electron-phonon coupling are obtained from the density functional theory. The linearized Boltzmann equation has then been solved numerically beyond the relaxation time approximation. Our approach has been applied to crystalline silicon. We present results for the mobility, Seebeck coefficient, and electronic contribution to the thermal conductivity, as a function of the carrier concentration and temperature. The calculated coefficients are in good quantitative agreement with experimental results.
In this work, we propose an efficient computational scheme for first-principle quantum transport simulations to evaluate the open-boundary conditions. Its partitioning differentiates from conventional methods in that the contact self-energy matrices are constructed on smaller building blocks, principal layers (PL), while conventionally it was restricted to have the same lateral dimensions of the adjoining atoms in a channel region. Here, we obtain the properties of bulk electrodes through non-equilibrium Greens function (NEGF) approach with significant improvements in the computational efficiency without sacrificing the accuracy of results. To exemplify the merits of the proposed method we investigate the carrier density dependency of contact resistances in silicon nanowire devices connected to bulk metallic contacts.
A procedure is presented that combines density functional theory computations of bulk semiconductor alloys with the semiconductor Bloch equations, in order to achieve an ab initio based prediction of the optical properties of semiconductor alloy heterostructures. The parameters of an eight-band kp-Hamiltonian are fitted to the effective band structure of an appropriate alloy. The envelope function approach is applied to model the quantum well using the kp-wave functions and eigenvalues as starting point for calculating the optical properties of the heterostructure. It is shown that Luttinger parameters derived from band structures computed with the TB09 density functional reproduce extrapolated values. The procedure is illustrated by computing the absorption spectra for a (AlGa)As/Ga(AsP)/(AlGa)As quantum well system with varying phosphide content in the active layer.
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