mK Scanning Tunneling Microscopy
STM has evolved into one of the most versatile tools at the atomic scale. Reducing the temperature to the lowest Millikelvin regime opens entirely new perspectives not only for the study of smallest energy scales, such as Zeeman energy, Josephson energy, elementary superconductivity, but also new insight for the tunneling process itself. With an energy resolution of 3.5µeV (Josephson current peak to peak distance) our Millikelvin-STM currently holds the world record.
An important breakthrough was the study of the Josephson effect with the Milliklevin-STM using a superconducting sample and a superconducting tip. Here, we could perform first STM measurements of such nanoscopic Josephson junctions for which the thermal energy is small compared to the energies in the Hamiltonian (i.e. charging energy EC and Josephson energy EJ). We have demonstrated that under these conditions we are able to quantitatively model the Josephson current in the dynamical Coulomb blockade regime. This opens new possibilities to study Cooper pair tunneling in a dissipative environment. For example, we have built the smallest superconducting quantum interference device in a tunnel junction and demonstrated a supercurrent reversal across the quantum phase transition of a Yu-Shiba-Rusinov state. In addition, we have demonstrated the smallest tunnel junction by tunneling between two single energy levels, i.e. two Yu-Shiba-Rusinov states protected inside a superconducting gap.