A Dual Tip STM for Imaging the Superconducting Phase Difference*

We have built a dual tipped millikelvin STM with each tip capable of independently scanning a sample. We will use the STM to measure spatial variations of the gauge-invariant phase difference at the atomic scale. The two tips along with the superconducting sample constitute a SQUID. This configuration is designed to minimize fluctuations in the superconducting phase of one of the tips as it scans the sample, hence inhibiting supercurrent suppression. We are currently developing a technique to fabricate superconducting Niobium tips for use with this system.