Fig. 3: Suppression of back tunneling via the Franck–Condon blockade.
From: Ultrafast Coulomb blockade in an atomic-scale quantum dot
a Schematic illustration of reduced back tunneling rate at elevated Vdc. b LW-STM orbital images of VacSe/2 ML showing a strong nonlinear increase of net rectified charge and orbital sensitivity as ΔV = VLUMO − Vdc → 0 V. Measurement obtained at \({V}_{{{{\rm{THz}}}}}^{{{{\rm{pk}}}}}=0.49\) V, VLUMO = 0.7 V, and z = z0 −3 Å. c dI/dV (green shaded curve) and I(V) spectrum (black, upper panel) and rectified charge (black, lower panel) as a function of Vdc measured with \({V}_{{{{\rm{THz}}}}}^{{{{\rm{pk}}}}}=0.32\) V and z = z0 − 3 Å. The simulation (red curve) assumes a single vibrational mode ℏΩ = 8 meV, a Huang-Rhys factor of 2.2, and experiment values for THz waveform, LDOS and z (Methods). The data for (b) and (c) were obtained on \({{{{\rm{Vac}}}}}_{{{{\rm{Se}}}}}^{2.2}\) and \({{{{\rm{Vac}}}}}_{{{{\rm{Se}}}}}^{2.4}\) respectively (Fig. S1). Black crosses in panel b indicate an equivalent position of the point spectrum. Integration time per data point is 100 ms for LW-STS and 15 ms for LW-STM, corresponding to 4.1 M and 0.6 M THz pulses at 41 MHz repetition rate. d Transitions between vibronic states of the charged and neutral VacSe LUMO in the Franck–Condon picture at high Vdc(ΔV → 0 V) are limited to ground state transitions. e At reduced Vdc transitions to higher vibronic modes of the neutral state become available, enhancing the effect of back tunneling.
