Conduction channels at finite bias in single-atom gold contacts

We consider the effect of a finite voltage bias on the conductance of single-atom gold contacts. We employ a nonorthogonal $\mathrm{spd}$-tight-binding Hamiltonian combined with a local charge neutrality assumption. The conductance and charge distributions for finite bias are calculated using the nonequilibrium-Green-function formalism. We calculate the voltage drop through the contacts and find the main drop located near the negative electrode. We argue that this is due to the filled d-state resonances. The conduction is analyzed in terms of transmission eigenchannels and density of states of the eigenchannels projected onto tight-binding orbitals. We find a single almost fully transmitting channel with mainly s character for low bias while for high bias this channel becomes less transmitting and additional channels involving only d orbitals start to conduct.