Dynamical exchange-correlation effects in electronic and thermal transport

MEC y MICINN

Status: ongoing project
Contract Number:
FIS2013-43130-P

Ongoing miniaturization of electronic devices has led over the last decade to the first experimental steps towards the vision of Molecular Electronics which aims at using single molecules as active electronic devices. As a consequence, an improved theoretical understanding of electronic and thermal transport at the atomistic or nanoscale level is of paramount importance. Static density functional theory (DFT) is the standard theory to describe molecular electronic structure in equilibrium. In combination with the Landauer-Büttiker formalism (LB-DFT) it is often used to describe electronic and thermal transport through single molecules. On the one hand, this formalism has proven to be highly useful in simulating electronic and thermal transport at the molecular scale. On the other hand, the approach is also known to be formally incomplete in general since static density functional theory is not applicable to non-equilibrium situations such as transport. For these situations time-dependent density functional theory is a more suitable approach and leads to a correction of the LB-DFT formalism due to dynamical exchange-correlation effects. The investigation of these effects is one of the central topics of the present project. Recently an approximation for this correction in the zero-bias electrical conductance has been proposed leading to promising first results which merit further investigation. In particular, these corrections have been shown to be extremely important qualitatively and quantitatively in the case of molecules weakly coupled to leads where effects of the electron-electron interaction become dominant such as in the Coulomb blockade regime. The description of this effect in a DFT framework requires (i) the inclusion of the dynamical corrections and (ii) an approximation to the effective single-particle potential which includes the step structure at integer particle number due to charge quantization. The construction of approximations which include this property is therefore also an important aim of this project. Furthermore, due to the importance of the dynamical corrections in electronic transport, one may also expect these effects to play a significant role in the electronic contribution to thermal transport, and therefore to the thermoelectric energy conversion. We aim at establishing this connection and at generalizing the dynamical corrections also to thermal transport. A final aim of the project is the investigation of a recently suggested approximate potential including the step structure in the context of truly time-dependent quantum transport where one starts from the system in equilibrium and follows its time evolution upon application of an external bias.

Objectives

* Study transport through nanoscale systems with multiple correlated levels in the DFT framework using newly developed xc functionals both for the zero-bias limit and the finite-bias regime. Study the importance of the dynamical exchange-correlation corrections.

* Derive the dynamical exchange-correlation corrections for the electronic contribution to thermal transport. Investigate their importance for model systems.

* Implement the dynamical exchange-correlation corrections to both the electrical and thermal transport coefficients into existing electronic structure codes.

* Implementation and investigation of time-dependent and steady-state quantum transport within the approximation of strictly correlated electrons. Study of the consequences of the high non-locality of this functional.

Personal

Stefan Kurth
Roberto D'Agosta
Yann Pouillon
Victor Morón Tejero
Kaike Yang
Robert Biele

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