Fig. 2: Modulation experiments on para-NP using the alternating current scanning tunneling microscopy break-junction (AC-STM-BJ) technique.

a AC-STM-BJ set-up, in which substrate-tip distance modulation experiments of conductance and thermopower response are recorded simultaneously. b Results from modulation experiments using the AC-STM-BJ. Top panel: 3.5 Hz triangular voltage wave with a peak-to-peak amplitude of 15 mV applied to the piezo-electric element (orange), connected to the STM tip, resulting in a tip-substrate modulation distance of 0.3 nm peak-to-peak. Center panel: double frequency conductance response of the molecular junction (blue). Bottom panel: corresponding thermopower signal with T_h−T_c = 38 K (red). c Superimposed individual conductance dips obtained from multiple modulation experiments, where the molecular junction exhibits double frequency conductance response (blue) and their calculated average (black). d Corresponding thermopower response (red) and its average (black).