Vibrationally Induced Two-Level Systems in Single-Molecule Junctions

Single-molecule junctions are found to show anomalous spikes in $dI/dV$ spectra. The position in energy of the spikes is related to local vibration mode energies. A model of vibrationally induced two-level systems reproduces the data very well. This mechanism is expected to be quite general for single-molecule junctions. It acts as an intrinsic amplification mechanism for local vibration mode features and may be exploited as a new spectroscopic tool.

Figure 1

(a) $dI/dV$ spectrum on a CO molecule bridging a Pt contact displaying symmetric positive spikes. The red (gray) curve shows a fit by the model discussed in the text [21]; (b) $I\mathrm{\text{−}}V$ measurement on a CO molecule bridging a Pt contact.

Figure 2

Panels (a)–(c) display the gradual change of $dI/dV$ spectra of a Pt-D-D-Pt contact; panels (b) and (c) show spectra taken for the same junction as (a) after stretching by 0.1 and 0.2 Å, respectively. Panels (d) and (e) show histograms of peak positions in spectra of junctions for $\mathrm{Pt}\mathrm{\text{−}}{\mathrm{H}}_2$ (d), $\mathrm{Au}\mathrm{\text{−}}{\mathrm{H}}_2$ (e) (solid bars) and $\mathrm{Pt}\mathrm{\text{−}}{\mathrm{D}}_2$ (d), $\mathrm{Au}\mathrm{\text{−}}{\mathrm{D}}_2$ (e) (hatched bars). All data were obtained at $T=4.2\mathrm{K}$.

Figure 3

Energy landscape of a molecule in an atomic contact. The molecule can be vibrationally excited and relax into the other energy minimum.

Figure 4

Several fits of VITLS for single-molecule junction contacts and atomic chains: (a) ${\mathrm{H}}_2$ in a Pt contact; (b) evolution of occupation numbers $n_0$, $n_1$, $n_e$ for the fit of (a); (c) ${\mathrm{H}}_2$ in a Au atomic chain; (d) ${\mathrm{H}}_2$ in a Au atomic chain; (e) ${\mathrm{O}}_2$ in a Ni contact. The bath temperature was 5 K in all cases except in (d), where the contact was heated to 20 K. For fit parameters, see 21.