The ionic gating technique used to realize electric-double-layer field-effect (EDL-FET) devices for the study of the fundamental physics of electronic transport in various materials is becoming increasingly popular. This is related to the unique possibility of this technique to induce huge electrostatic surface charge densities n2D in excess of 1014 cm−2. This large modification of n2D can even induce superconductivity in insulating or semiconducting materials, and was recently employed for demonstrating the true 2D nature of superconductivity in gated MoS2. Starting from our experience in EDL gating of metallic thin films of Au, we apply the same technique in thin films of conventional superconductors.
In NbN thin films with thickness d between 10 and 40 nm small positive and negative shifts in the critical temperature (|dTc| < 0.1 K) have been observed at the change of gate-voltage polarity. The Tc shift increases at the decrease of the film thickness t. All these findings indicate that the perturbation to the superconducting properties extends in a region much larger than a single unit cell. Indeed, the dependence of dTc on n2D and t can be explained by means of ab-initio DFT and the strong-coupling McMillan equation, only if the thin surface layer where electron accumulation takes place is coupled to the underlying, unperturbed bulk via proximity effect. Moreover, the thickness of this surface layer (i.e. the penetration length of the electric field) strongly increases for n2D > 7-8x1014 cm−2, reaching values of the order of 3 nm at the highest doping.
Furthermore, we consider from a theoretical point of view the effects of the applied electric field on the superconducting transition temperature of Pb thin films, where both the electronic and phononic properties can be obtained via ab-initio DFT calculations. From this approach we obtain results entirely consistent with our experimental observation in NbN. Hence, the proximity-effect-induced transformation of the quasi-2D perturbation to the electron density into a 3D bulk modification of the superconducting properties appears to be a general behaviour in gated superconductors that could hinder the possibility to obtain large Tc shifts in films thicker than the electrostatic screening length.