Induced DC Signals due to RF Excitation of the Scanning Tunneling Microscope

Experimental data demonstrated that a direct current (DC) bias between the tip and sample in a scanning tunneling microscope (STM) could be induced by a radio frequency (RF) signal passed through a copper coil placed around the tip of the STM. A resonant response to the RF signal, indicated by an increase in the measured current, was discovered at certain frequencies, including 800MHz, 1.3GHz, and 2.0GHz. This project intended to verify the experimental data by modeling the STM, including the scanner and radiation shields, in Ansoft’s High Frequency Structure Simulator (HFSS), and then testing this model for resonance at excitation frequencies from 500MHz to 2GHz in 100MHz steps. Results indicated that resonance is due strongly to the radiation shields (inner and outer) in the model, as the model showed insignificant resonance in the absence of the radiation shields. Further, plots of the electric field across the sample were made in relation to the distance between the tip and sample, varying from 10PP in to 10PP in, which indicated a strong relation between the magnitude of the electric field and the tip-sample distance as well as an extremely strong localization around the tip. Trends seen in results were generally consistent with expectations, but certain inconsistencies did exist in resonant frequencies and electric field magnitudes on the tip, and could be tied to various approximations made in the construction of the model.