The Bardeen-Cooper-Schrieffer supercurrent field-effect transistor.

In their original formulation of superconductivity, the London brothers predicted more than eighty years ago the exponential suppression of an $electrostatic$ field inside a superconductor over the so-called London penetration depth, $\lambda_L$, in analogy to the Meissner-Ochsenfeld effect. $\lambda_L$ ranges from a few tens to several hundreds of nanometers for Bardeen-Cooper-Schrieffer (BCS) superconductors. Conventional BCS predictions based on Fermi liquid estimate a sub-atomic electrical penetration depth which coincides with the Thomas-Fermi screening length. Yet, more recent theories have suggested that, albeit being localized at the surface, electrostatic fields may manifest themselves non-locally deep inside the superconductor, i.e., at distances of the order of the coherence length or larger. Despite a few experiments indicating hints of perturbation induced by electrostatic fields, no clue has been provided so far on the possibility to manipulate conventional superconductors via field-effect. Here we report the evidence of full field-effect control of the supercurrent in $all$-metallic transistors made of different BCS superconducting thin films. At low temperature, our field-effect transistors (FETs) show a monotonic decay of the critical current under increasing electrostatic field up to total quenching for gate voltage values as large as $\pm 40$V in titanium-based devices. This $bipolar$ field effect persists up to $\sim 85\%$ of the critical temperature ($\sim 0.41$K), and in the presence of sizable magnetic fields. A similar behavior, though less pronounced, was observed in aluminum thin film FETs. A phenomenological theory accounts for our observations, and provides a description compatible with an electric-field-induced non-local perturbation propagating deeply inside the superconducting film.