Shot-noise measurements of single-atom junctions

The ability to control and manipulate single atoms or a few atomic layers resulted in many new and exotic quasiparticle states. Yet, not all properties of these states are accessible using traditional measurement techniques. For example, detecting a quasiparticle at the end of a one-dimensional atomic chain is possible, while the desired information regarding the charge and quasiparticle correlations of such a state remain inaccessible. This is because the most direct way to measure correlations is by studying current fluctuations in small constrictions, i.e. shot noise, which is technically very difficult. The challenge becomes even greater when high spatial resolution is required. 

Measuring and manipulating single atoms, first envisioned by Feynman already in 1960, is possible today using a scanning tunneling microscope (STM). In the talk, I will describe an STM-based measurement system we developed in order to measure shot noise with subatomic spatial resolution. I will present noise data from both metallic and superconducting single-atom junctions. For the metallic junctions we find very good agreement between our experimental results and the theory. The situation is more complex when studying superconducting junctions. Multiple transmission channels and the appearance of competing tunneling processes, e.g.  Andreev reflections result in a rich noise signature.