Imaging doubled shot noise in a Josephson scanning tunneling microscope

In this talk I will show how we have imaged the current noise with atomic resolution in a Josephson scanning tunneling microscope [1]. By measuring the current noise as a function of applied bias, we reveal the step from single-electron tunneling above the superconducting gap energy to double-electron charge transfer below the gap energy when Andreev processes become dominant. By spatially resolving the noise on a superconducting Pb(111) surface we demonstrate that this doubling of the noise occurs homogeneously on the sample surface, also on impurity locations, indicating that indeed the charge pairing is not influenced by disruptions in the superconductor smaller than the superconducting coherence length. In addition, I will show how we have used this spectroscopic noise STM technique on a disordered film of TiN to discover Cooper pairing without a spectral gap in a disordered superconductor above Tc.
[1] Bastiaans, K.M. et al. Phys. Rev. B 100, 104506 (2019)