Microwave Photon-Assisted Incoherent Cooper-Pair Tunneling in a Josephson STM

A scanning tunneling microscope (STM) reveals a sample's electronic structure at the atomic scale. Knowing the charge of the carriers that are actually tunneling is an important clue to the origin of a conductance peak, i.e. the phenomenon actually being observed. The authors use a specialized STM with simultaneous microwave irradiation at very low temperatures to determine that charge, which in their test case is the 2e of a Cooper pair of electrons in superconducting niobium. The agreement of their data with theory is striking, and this technique promises to be a great boon to superconductivity research, including for example the hunt for Majorana fermions.