The majority of superconductors have more than one Fermi surface, on which the electrons pair below the critical temperature, yet their behavior can be well described by a single-band Bardeen-Cooper-Schrieffer theory. This is mostly due to interband scattering, especially in superconductors in the dirty limit, rigidly linking the pairing amplitude of the bands. This has limited experimental studies of the complex physics of multiband superconductivity. Here, we utilize the fact that elementary Pb—as a clean limit system—has two Fermi surfaces that are only weakly coupled by interband scattering, allowing the formation of two separate condensates. By studying stacking fault tetrahedra with a millikelvin scanning tunneling microscope, we show how to locally tune interband coupling ranging from weak to strong coupling and modify the superconducting order parameters from two well separated gaps to one merged gap around defects. The experiments critically test the theory of multiband superconductors and give a route to access a wide range of predicted quantum effects in these systems.
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