Nanoscale physics

Superexchange nonresonant tunneling mediated by 
a molecular nanowire
Petrov E.G.
Bogolyubov Institute for Theoretical Physics of the NAS of Ukraine. Metrologichna, 14-B, Kyiv-03680, Ukraine. 
E-mail: epetrov @bitp.kiev.ua

Tight binding model for a molecular nanowire embedded in between the metallic electrodes is used to derive the analytic expressions for a nonresonant tunneling current mediated by the localized and delocalized molecular orbitals associated with the terminal and regular interior units, respectively. The specific regimes of charge transmission controlled by resonant and nonresonant participation of terminal’s energy levels, are studied and the role of energy position of delocalized orbitals in formation of a distant superexchange electrode-electrode coupling , is clarified. The criteria for reduction of superexchange model  of charge tunneling tothe flat barrier model are formulated  and the parameters of barrier model  (energy gap and effective electron mass) are specified in the terms of inter-site couplings  and energy distance from the Fermi level to the delocalized  wire’s HOMO level. 
Special attention is payed to a derivation of analytic  expressions  for the tunneling current with use of different approximations. It has been shown that at passive participation  of terminal units in a charge transmission process, the best correspondence  with the observed current-voltage characteristics is achieved with use of the simplest Gauss and the mean-value  analytic forms. This is supported by comparison of the theory  with experimental results concerning  the dependence of nonresonant tunneling current on the voltage bias  and the number of chain units  in alkanedithiol chains
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