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R. Stadler:
"Coherent tunneling and electron hopping in molecules with redox centers";
Vortrag: Seminarvortrag am Center of Atomic-Scale Materials Design, Department of Physics, Technical University of Denmark, Lyngby, Denmark (eingeladen); 14.09.2015.

For defining the conductance of single molecule junctions with a redox functionality in an electrochemical cell, two conceptually different electron transport mechanisms, namely coherent tunnelling and vibrationally induced hopping compete with each other, where implicit parameters of the setup such as the length of the molecule and the applied gate voltage decide which mechanism is the dominant one. Although coherent tunnelling is most efficiently described within Landauer theory, while the common theoretical treatment of electron hopping is based on Marcus theory, both theories are adequate for the processes they describe without introducing accuracy limiting approximations. For a direct comparison, however, it has to be ensured that the crucial quantities obtained from electronic structure calculations, i.e. the transmission function T(E) [1], in Landauer theory, and the transfer integral [2], the reorganisation energy and the driving force in Marcus theory, are derived from similar grounds [3]. In this contribution our framework is a single particle picture, where we perform density functional theory calculations for the conductance corresponding to both transport mechanisms for junctions with the central molecule containing one, two or three Ruthenium centers, respectively, and we extrapolate our results in order to define the critical length for the transition point of the two regimes.

In a joint experimental and theoretical investigation of the transport properties of organometallic molecules containing either Fe, Ru or Mo centers, hysteretic transport properties with continuous transitions were found for all compounds, and additionally an abrupt switching for Mo [4]. The latter could be modelled by taking into account bias-driven charging and explained by an oxidation/reduction mechanism mediated by a localized MO with weak coupling to the electrodes that is unique to the Mo compound because of the latter´s spin-polarized ground state. Di-nuclear Fe compounds with various different anchor groups were also investigated, where excellent agreement of the trends in conductance between theory and experiment were obtained [5].

[1] G. Kastlunger and R. Stadler, Phys. Rev. B 88, 035418 (2013).
[2] G. Kastlunger and R. Stadler, Phys. Rev. B 89, 115412 (2014).
[3] G. Kastlunger and R. Stadler, Phys. Rev. B 91, 125410 (2015).
[4] F. Schwarz, G. Kastlunger, F. Lissel, C. Egler-Lucas, S. N. Semenov, K. Venkatesan, H. Berke, R. Stadler and E. Lörtscher, submitted (2015).
[5] F. Schwarz, G. Kastlunger, F. Lissel, H. Riel, K. Venkatesan, H. Berke, R. Stadler and E. Lörtscher, Nano Lett. 14, 5932-5940 (2014).

Projektleitung Robert Stadler:
Elektrochemische Interferenz