Tomasz Kostyrko

Current rectification in molecular junctions produced by local potential fields

The transport properties of an octane-dithiol (ODT) molecule coupled to Au(001) leads are analyzed using density-functional theory (DFT) and nonequilibrium Greens functions. It is shown that a symmetric molecule can turn into a diode under influence of a local electric field created by an external charged probe. The origin of the asymmetry of the current-voltage (I-V) dependence is traced back to the appearance of a probe-induced quasilocal state in the pseudogap of the ODT molecule. The induced state affects electron transport provided it is close to the Fermi level of the leads. An asymmetric placement of the charged probe along the alkane chain makes the induced quasilocal state in the energy gap very sensitive to the bias voltage and results in rectification of the current. The results based on DFT are supported by independent calculations using a simple one-particle model Hamiltonian.

Linear and nonlinear Stark effect in a triangular molecule

We analyze changes of the electronic structure of a triangular molecule under the influence of an electric field (i.e., the Stark effect). The effects of the field are shown to be anisotropic and include both a linear and a nonlinear part. For strong electron correlations, we explicitly derive exchange couplings in an effective spin Hamiltonian. For some conditions one can find a dark spin state, for which one of the spins is decoupled from the others. The model is also applied for studying electronic transport through a system of three coherently coupled quantum dots. Since electron transfer rates are anisotropic, the current characteristics are anisotropic as well, differing for small and large electric field.

Spin-Dependent Electronic Transport through Molecular Devices

A role of interference and electronic correlations in a transport through molecular devices is considered within an extended Anderson model. In magnetic systems correlations reduce the value of the magnetoresistance, which can even change its sign in some cases. Transport studies through a two-atomic molecule showed a series of voltage ranges with characteristic current dependences. For strong Coulomb interactions the studies predict bistable current solutions.