Radiofrequency-Induced Shifts of Field Emission Resonances in Scanning Tunneling Microscopy

Resonant tunneling phenomena are of crucial importance to understanding electrical transport in devices such as quantum-cascade lasers and resonant tunneling diodes. Tuning of these resonances enables enhanced control of electron transport at the nanoscale, and has significance for a wide range of quantum electronic devices. Field emission resonances (FER) emerge when a strong electric field is applied across a tunnel junction such as the vacuum gap in a scanning tunneling microscope (STM). When the applied bias becomes larger than the tip and sample work-functions, a triangular well forms leading to a series of quasi-bound electron states above the sample surface. These states manifest as sharp increases in the Fowler-Nordheim tunneling current at specific biases corresponding to the resonance conditions. While FERs have been extensively studied in the context of STM, the influence of time-varying electric fields on these resonances has remained unexplored. Here, we introduce a radiofrequency (RF) modulation in addition to the DC bias, and track the evolution of FERs with variable RF amplitude. Our experimental results show that the FERs shift to lower energy by an amount nearly equal to the RF voltage at the junction. Additionally, the resonance linewidth remains relatively constant even for large RF amplitudes. First-principles calculations reveal that this shift can be explained by the dynamical modulation of the tunneling barrier, causing an altered time-averaged tunneling probability. This work represents a novel approach for tuning tunneling resonances, and points to a versatile new method for characterizing the strongly frequency-dependent RF transmission in STM.