Quantum Interference and Decoherence in Single-Molecule Junctions: How Vibrations Induce Electrical Current

Quantum interference and decoherence in single-molecule junctions is analyzed employing a nonequilibrium Green’s function approach. Electrons tunneling through quasidegenerate states of a molecular junction exhibit interference effects. We show that electronic-vibrational coupling, inherent to any molecular junction, strongly quenches such interference effects. This decoherence mechanism may cause significantly larger electrical currents and is particularly pronounced if the junction is vibrationally highly excited, e.g., due to inelastic processes in the resonant transport regime.

Figure 1

(a): Scheme of a SMJ at a finite bias voltage $\Phi$ that exhibits destructive interference. The gray shaded areas depict the continuum of occupied electronic states in the left ($L$) and the right ($R$) lead. Electronic states located at the molecular bridge ($M$) are represented by black bars. The blue and red arrows depict the coupling of these states to the leads, which due to the different symmetry of the corresponding orbitals, gerade and ungerade, differ by sign (example orbitals are given in the insets). (b) and (c): Example processes for vibrational excitation (red wiggly line) and deexcitation (blue wiggly line) upon inelastic tunneling of an electron through the SMJ.

Figure 2

IV for a generic model SMJ exhibiting destructive quantum interference effects (see Fig. 1). The inset shows the corresponding vibrational excitation, $⟨a^{†}a⟩$.

Figure 3

Transmission functions $t(ϵ)$ (dashed black lines) corresponding to the currents depicted by the solid gray, blue, and red lines in Fig. 2 at a high bias voltage. For clarity, the graphs for these three different cases are separated by offsets. In addition, the incoherent sum of transmission channels $t_{\mathrm{sum}}(ϵ)$ (orange lines) and the interference term $t_{\mathrm{int}}(ϵ)$ (green lines) are depicted [cf. Eqs. (3)].

Figure 4

Electron transport in an $o$-biphenylacetylenedithiolate SMJ. (a): relevant molecular orbitals. (b): IVs for different scenarios (cf. legend).