Simultaneously scanning two connected tips in a scanning tunneling microscope

We have modified a dual-tip scanning tunneling microscope (STM) by electrically connecting the tips together with a short (3 mm) strip of flexible 25 mu m thick Nb foil. For simultaneous topographic imaging with both tips, we moved each tip to within tunneling distance z of a surface and modulated one tip's z-piezo at 5 kHz and the other at 10 kHz. The resulting combined tunneling current has modulation at both frequencies which we detect using individual lock-in amplifiers. Each lock-in output is fed back to its corresponding tip's individual STM z-position controller to maintain a stable current in both junctions. During the tests at room temperature, simultaneous imaging was performed with both tips made of Pt-Ir on Au/mica and highly oriented pyrolytic graphite (HOPG) samples, where a small tip-to-tip mechanical coupling was observed. We describe the system's performance, show results from simultaneous imaging, and discuss the potential application of the system to imaging superconducting phase differences. Published by AIP Publishing.