Impact of Exchange-Correlation Effects on the $IV$ Characteristics of a Molecular Junction

The role of exchange-correlation effects in nonequilibrium quantum transport through molecular junctions is assessed by analyzing the $IV$ curve of a generic two-level model using self-consistent many-body perturbation theory (second Born and $GW$ approximations) on the Keldysh contour. It is demonstrated how the variation of the molecule’s energy levels with the bias voltage can produce anomalous peaks in the $dI/dV$ curve. This effect is suppressed by electronic self-interactions and is therefore underestimated in standard transport calculations based on density functional theory. Inclusion of dynamic correlations introduces quasiparticle (QP) scattering which in turn broadens the molecular resonances. The broadening increases strongly with bias and can have a large impact on the calculated $IV$ characteristic.

Figure 1

(a) Density of states for a molecular junction under zero and nonzero bias voltage. As indicated, the bias changes the molecule’s DOS which in turn affects the $dI/dV$. (b) Diagrams for the second Born (2B) and $GW$ self-energies. Full lines represent the Green’s function of the molecule with coupling to leads. Wiggly lines represent the interaction.

Figure 2

$dI/dV$ curves for different values of the tunneling strength $\Gamma$ and occupation of the molecule, $N_{\mathrm{el}}$. The curves are calculated using different approximations for the xc self-energy.

Figure 3

Position of the HOMO and LUMO levels as a function of the bias voltage for the Hartree (crosses), HF (triangles), and $GW$ (circles) approximations. The horizontal lines show the FWHM of the $GW$ resonances. The FWHM of the Hartree and HF resonances is $2\Gamma$ independently of $V$. Notice the differences in the way the levels enter the bias window: The Hartree gap opens while the HF and $GW$ gaps close. In the upper graph $\Gamma =0.25$, $N_{\mathrm{el}}=2.0$ (symmetric case). In the lower graph $\Gamma =0.25$, $N_{\mathrm{el}}=2.1$.