Anomalous Conductance Oscillations and Half-Metallicity in Atomic Ag-O Chains

Using spin density functional theory, we study the electronic and magnetic properties of atomically thin, suspended chains containing silver and oxygen atoms in an alternating sequence. Chains longer than 4 atoms develop a half-metallic ground state implying fully spin-polarized charge carriers. The conductances of the chains exhibit weak even-odd oscillations around an anomalously low value of $0.1G_0$ ($G_0=2e^2/h$) which coincide with the averaged experimental conductance in the long chain limit. The unusual conductance properties are explained in terms of a resonating-chain model, which takes the reflection probability and phase shift of a single bulk-chain interface as the only input. The model also explains the conductance oscillations for other metallic chains.

Figure 1

Calculated conductance as a function of chain length (blue squares). The length of the chain is given in units of 3.8 Å corresponding to the length of a Ag-O unit. Also shown is the result of the resonating-chain model (red triangles) and a fit to the experimental data of Ref. 10. The supercell used to model the suspended chains is also shown.

Figure 2

(a) Transmission functions for the majority spin (dashed line) and minority spin (solid line) for Ag-O chains containing 2–6 oxygen atoms. (b) Band structure of the infinite Ag-O chain. The band crossing the Fermi level is doubly degenerate, and the Bloch states have $m=\pm 1$ symmetry with respect to rotations around the chain axis.

Figure 3

(a) Conductance for Al as a function of the number of chain atoms $N$. The first-principles result (squares) is taken from Ref. 6, while the model result (triangles) is obtained from Eq. (1) with parameters derived using the same procedure as for the Ag-O system. (b) Conductance length dependence for Au with the first-principles result taken from Ref. 8.