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Register a new userTransport properties of molecular junctions from many-body perturbation theory
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Added by
Tonatiuh Rangel Gordillo
Publication date
2011
fields
Physics
and research's language is
English
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The conductance of single molecule junctions is calculated using a Landauer approach combined to many-body perturbation theory MBPT) to account for electron correlation. The mere correction of the density-functional theory eigenvalues, which is the standard procedure for quasiparticle calculations within MBPT, is found not to affect noticeably the zero-bias conductance. To reduce it and so improve the agreement with the experiments, the wavefunctions also need to be updated by including the non-diagonal elements of the self-energy operator.
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We present a novel hybrid quantum/classical (QM/MM) approach to the calculation of charged excitations in molecular solids based on the many-body Greens function $GW$ formalism. Molecules described at the $GW$ level are embedded into the crystalline environment modeled with an accurate classical polarizable scheme. This allows the calculation of electron addition and removal energies in the bulk and at crystal surfaces where charged excitations are probed in photoelectron experiments. By considering the paradigmatic case of pentacene and perfluoropentacene crystals, we discuss the different contributions from intermolecular interactions to electronic energy levels, distinguishing between polarization, which is accounted for combining quantum and classical polarizabilities, and crystal field effects, that can impact energy levels by up to $pm0.6$ eV. After introducing band dispersion, we achieve quantitative agreement (within 0.2 eV) on the ionization potential and electron affinity measured at pentacene and perfluoropentacene crystal surfaces characterized by standing molecules.
We present a comprehensive study of the properties of the off-resonant conductance spectrum in oligomer nanojunctions between graphitic electrodes. By employing first-principle-based methods and the Landauer approach of quantum transport, we identify how the electronic structure of the molecular junction components is reflected in electron transport across such systems. For virtually all energies within the conduction gap of the corresponding idealised polymer chain, we show that: a) the inverse decay length of the tunnelling conductance is intrinsically defined by the complex-band structure of the molecular wire despite ultrashort oligomer lengths of few monomer units, and b) the contact conductance crucially depends on both the local density of states on the metal side and the realised interfacial contact.
We develop a numerically exact scheme for resumming certain classes of Feynman diagrams in the self-consistent perturbation expansion for the electron and magnon self-energies in the nonequilibrium Green function formalism applied to a coupled electron-magnon (mbox{e-m}) system which is driven out of equilibrium by the applied finite bias voltage. Our scheme operates with the electronic and magnonic GFs and the corresponding self-energies viewed as matrices in the Keldysh space, rather than conventionally extracting their retarded and lesser components. This is employed to understand the effect of inelastic mbox{e-m} scattering on charge and spin current vs. bias voltage $V_b$ in F/I/F magnetic tunnel junctions (MTJs), which are modeled on a one-dimensional (1D) tight-binding lattice for the electronic subsystem and 1D Heisenberg model for the magnonic subsystem. For this purpose, we evaluate Fock diagram for the electronic self-energy and the electron-hole polarization bubble diagram for the magnonic self-energy. The respective electronic and magnonic GF lines within these diagrams are the fully interacting ones, thereby requiring to solve the ensuing coupled system of nonlinear integral equations self-consistently. Despite using the 1D model and treating mbox{e-m} interaction in many-body fashion only within a small active region consisting of few lattice sites around the F/I interface, our analysis captures essential features of the so-called zero-bias anomaly observed in both MgO- and AlO$_x$-based realistic 3D MTJs where the second derivative $d^2 I/dV_b^2$ (i.e., inelastic electron tunneling spectrum) of charge current exhibits sharp peaks of opposite sign on either side of the zero bias voltage.
We study the electronic contribution to the main thermoelectric properties of a molecular junction consisting of a single quantum dot coupled to graphene external leads. The system electrical conductivity (G), Seebeck coefficient ($S$), and the thermal conductivity ($kappa$), are numerically calculated based on a Greens function formalism that includes contributions up to the Hartree-Fock level. We consider the system leads to be made either of pure or gapped-graphene. To describe the free electrons in the gapped-graphene electrodes we used two possible scenarios, the massive gap scenario, and the massless gap scenario, respectively. In all cases, the Fano effect is responsible for a strong violation of the Wiedemann-Franz law and we found a substantial increase of the system figure of merit $ZT$ due to a drastic reduction of the system thermal coefficient. In the case of gapped-graphene electrodes, the system figure of merit presents a maximum at an optimal value of the energy gap of the order of $Delta/Dsim$ 0.002 (massive gap scenario) and $Delta/Dsim$ 0.0026 (massless gap scenario). Additionally, for all cases, the system figure of merit is temperature dependent.
The bandstructure of gold is calculated using many-body perturbation theory (MBPT). Different approximations within the GW approach are considered. Standard single shot G0W0 corrections shift the unoccupied bands up by ~0.2 eV and the first sp-like occupied band down by ~0.4 eV, while leaving unchanged the 5d occupied bands. Beyond G0W0, quasiparticle self-consistency on the wavefunctions lowers the occupied 5d bands by 0.35 eV. Globally, many-body effects achieve an opening of the interband gap (5d-6sp gap) of 0.35 to 0.75 eV approaching the experimental results. Finally, the quasiparticle bandstructure is compared to the one obtained by the widely used HSE (Heyd, Scuseria, and Ernzerhof) hybrid functional.
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