A dual tip STM for superconducting phase-difference detection*

We have built a dual tipped Pan-style STM, with each tip capable of independently scanning a sample. We have tested it at room temperature on graphite samples, and calibrated both scanners using atomic steps seen on graphite. We intend to use the STM at ultra-low (mK) temperatures to measure the spatial variation of the gauge-invariant phase difference in superconducting samples at the atomic scale. The two tips will function as the junctions of an asymmetric SQUID, with one tip acting as a reference while the other scans the sample. We will reduce fluctuations in the Josephson phase of the scanning tip by adding capacitance across the junctions and minimizing the geometric inductance.