Role of electronic structure in photoassisted transport through atomic-sized contacts

We study theoretically quantum transport through laser-irradiated metallic atomic-sized contacts. The radiation field is treated classically, assuming its effect to be the generation of an ac voltage over the contact. We derive an expression for the dc current and compute the linear conductance in one-atom-thick contacts as a function of the ac frequency, concentrating on the role played by electronic structure. In particular, we present results for three materials (Al, Pt, and Au) with very different electronic structures. It is shown that, depending on the frequency and the metal, the radiation can either enhance or diminish the conductance. This can be intuitively understood in terms of the energy dependence of the transmission of the contacts in the absence of radiation.

Figure 1

(Color online) (a) Two infinite fcc [001] surfaces contacted with pyramids of ten atoms in each, forming a “dimer” contact. The tip-to-tip distance is denoted by $D$; all other interatomic distances are the same as in the bulk. (b) Two model ac voltage profiles $U_i^{ac}$, where $i$ labels atomic sites and orbitals in the $C$ region: a double step $\left(A\right)$ and a linear ramp $\left(B\right)$ between the lead values $U_L^{ac}$ and $U_R^{ac}$.

Figure 2

(Color online) Top panel: Equilibrium transmission $T^{eq}$ and its decomposition into conduction channels $T_{1,2,3,4}$ for an Al dimer contact. The position of ${ϵ}_F$ is indicated by a vertical dotted line. Lower panels: Zero-temperature photoconductance for several values of $\alpha$ as a function of frequency $\omega$ using the voltage profile $A$ (a), profile $B$ (b), and Eq. (9) (c). In (b) wavelengths $\lambda$ with a tick spacing of $400\mathrm{nm}$ are shown. The range of visible light is indicated by vertical dotted lines.

Figure 3

(Color online) Same as Fig. 2, but for Pt.

Figure 4

(Color online) Same as Fig. 2, but for Au.

Figure 5

(Color online) Tunneling limit for Al contacts. Top left panel: Zero-frequency conductance $G_{dc}(\omega =0)$ as a function of tip distance $D$. The vertical lines indicate the distances where the examples with corresponding line styles in the other panels are computed. Top right panel: transmission $T^{eq}\left(ϵ\right)$ for the example distances. Lower panels: the left-hand (a), (d), central (b), (e), and right-hand (c), (d) panels are for profile $A$, profile $B$, and Eq. (9), respectively. The panels (a)–(c) are for $\alpha =0.5$ and (d)–(f) for $\alpha =1.0$.

Figure 6

(Color online) Same as Fig. 5, but for Pt.

Figure 7

(Color online) Same as Fig. 5, but for Au.