Effects of spin-orbit coupling on the conductance of molecules contacted with gold electrodes

The effects of spin-orbit coupling on the conductance of molecular devices made with Au electrodes are investigated using a fully self-consistent ab initio approach, which combines the non-equilibrium Green's function formalism with density functional theory. In general, we find that the extent to which spin-orbit interaction affects the transport depends on the specific materials system investigated and on the dimensionality of the electrodes. For one-dimensional electrodes contacting benzene-dithiol molecules the spin-orbit coupling induces changes in the low-bias conductance up to about 20%. These originate mostly from changes in the electrode band structure. In contrast when three-dimensional electrodes are used, the bands near the Fermi level are only weakly modified by spin-orbit coupling and most of the variations are due to symmetry changes at the molecule-electrode interface. For this reason strongly coupled systems, such as Au atomic nanowires sandwiched between Au (100) surfaces and benzene-dithiol molecules bonded at the Au (111) hollow site, are rather insensitive to spin-orbit effects. In contrast, in junctions where the coupling between the molecule and the electrodes is weaker, as in the case of benzene-dithiol bonded to Au (111) at adatom positions, the transmission coefficient at the Fermi level can be modified by as much as 14%.