Fig. 4: Influence of strong and weak tip interaction on charge rectification.

a Schematic illustration of the electronic coupling strength of Vac_Se to the STM tip at different tip height. b LW-STM orbital images with \({V}_{{{{\rm{THz}}}}}^{{{{\rm{pk}}}}}=0.49\) V and at V_dc = 0.7 V (\(\Delta {V}_{{z}_{0}}=0.0\) V). Images left to right at increasing tip—sample distance show a transition from the saturated (Δz = −3.5 Å) to the unsaturated (Δz = −1 Å) orbital shape. Reduced charge rectification at the center of Vac_Se at Δz = −3.5 Å is related to strong back tunneling, i.e., efficient charge depletion by the tip. The dotted line marks the position for cross sections in Fig. S5b. c Q_LW point spectrum (black curve) as a function of tip--sample distance measured on the orbital lobe of Vac_Se. The maximum at −2.5 Å emphasizes the interplay of forward and backward tunneling, modeled by a simulation (red) that assumes a single vibrational mode with an energy of 8 meV and a Huang-Rhys factor of 2.2 (Methods). The data for b and d were obtained on \({{{{\rm{Vac}}}}}_{{{{\rm{Se}}}}}^{2.2}\) and \({{{{\rm{Vac}}}}}_{{{{\rm{Se}}}}}^{2.4}\) respectively (Fig. S1). Black crosses in b mark the equivalent position of the point spectrum in (c). Integration time per data point is 100 ms for z-spectroscopy and 15 ms for LW-STM, corresponding to 4.1 M and 0.6 M THz pulses at 41 MHz repetition rate.