Current Collapse in Tunneling Transport through Benzene

We investigate the electrical transport through a system of benzene coupled to metal electrodes by electron tunneling. Using electronic structure calculations, a semiquantitative model for the $\pi$ electrons of the benzene is derived that includes general two-body interactions. After exact diagonalization of the benzene model the transport is computed using perturbation theory for weak electrode-benzene coupling (golden rule approximation). We include the effect of an applied electric field on the molecular states, as well as radiative relaxation. We predict a current collapse and strong negative differential conductance due to a “blocking” state when the electrode is coupled to the para-position of benzene. In contrast, for coupling to the meta-position, a series of steps in the $I\mathrm{\text{−}}V$ curve is found.

Figure 1

$I\mathrm{\text{−}}V$ characteristics for symmetric bias. Left panel: $I\mathrm{\text{−}}V$ curve without (solid line) and with radiative relaxation (dashed line) without inclusion of bias effect. The current collapses at a bias when the antisymmetric anion state can become occupied by radiative relaxation from an excited anion state. For coupling of the right electrode at the meta-position (dash-dotted line) the “blocking state” has finite tunnel coupling to both electrodes and no current collapse is observed. Right panel: With inclusion of the bias effect (solid line) the onset of current is generally shifted to lower bias, but the current collapse remains and additional weak NDC occurs at larger bias.

Figure 2

Sketch of the energetics and symmetry of the relevant neutral and anion states. At a left bias of 2.1 eV the antisymmetric blocking state becomes occupied via radiative relaxation. At 3.4 eV electrons can escape the blocking state via tunneling to the antisymmetric state of neutral benzene.