First-principles study on the differences between the equilibrium conductance of carbon and silicon atomic wires

We perform first-principles calculations of the equilibrium conductance of carbon and silicon atomic wires coupled to two Al(100) nanoscale electrodes using the nonequilibrium Green formalism combined with the density functional theory. The conductance of atomic wires with quite a large range of number of atoms from N = 3-20 is considered. Our calculations show that, for the carbon atomic wire, the equilibrium conductance as a function of the number of atoms (G-N relation) exhibits evident oscillatory behavior when the wire is not very long, and there is a big difference in conductance between the even-numbered wire and the odd-numbered wire. This difference becomes smaller and smaller with increasing wire length, until finally the conductance saturates to a constant value. The G-N relation for the Si atomic wires shows similar behavior when the number of atoms N < 9, but great differences appear when N >= 9. Compared with the cases with numbers of atoms N = 5-9 where the conductance of even-numbered wires is larger than that of the odd-numbered wires, the opposite result is obtained with N = 10-14. As a whole, the conductance for Si wires shows an even-odd oscillatory behavior in a period of 'M' shape. The above behavior is analyzed via the charge transfer and the projected density of states (PDOS) and reasonable explanations are presented.