Control of bulk superconductivity in a BCS superconductor by surface charge doping via electrochemical gating

The electrochemical gating technique is a powerful tool to tune the surface conduction properties of various materials by means of pure charge doping, but its efficiency is thought to be hampered in materials with a good electronic screening. We show that, if applied to a metallic superconductor (NbN thin films), this approach allows the observation of reversible enhancements or suppressions of the bulk superconducting transition temperature, which vary with the thickness of the films. These results are interpreted in terms of a proximity effect, and indicate that the effective screening length depends on the induced charge density, becoming much larger than that predicted by a standard screening theory at very high electric fields.