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Superconducting field-effect transistors go metal

November 26, 2018 - 11:00am
Francesco Giazotto
Scuola Normale Superiore di Pisa



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, λL, in analogy to the Meissner-Ochsenfeld effect. 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. In this talk, I will report the evidence of full field-effect control of the supercurrent in all-metallic transistors made of different BCS superconducting thin films [1]. 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 ±40V in titanium-based devices. This bipolar field effect persists up to ∼85% of the critical temperature (∼0.41K), and in the presence of sizable magnetic fields. A similar behavior, though less pronounced, was observed in aluminum thin film FETs [1]. 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. In our interpretation, this affects the pairing potential, and quenches the supercurrent. 
Moreover, I will show the experimental realization of Ti-based Dayem bridge field-effect transistors (DB − FETs) [2, 3] able to control the Josephson critical current (IC) of the superconducting channel. Our easy fabrication process DB − FETs show symmetric full suppression of IC  for an applied critical gate voltage as low as VGC±8V at temperatures reaching about the 85% of the record critical temperature 550mK for titanium. Our devices show extremely high values of transconductance (up to 15µA/V) and variations of Josephson kinetic inductance with gate voltage of two orders of magnitude. 
Finally, I will show the behavior of mesoscopic superconductor-normal metal-superconductor (SNS) Josephson field-effect transistors which will reveal as well the impact of intense electrostatic fields even on proximity metals. All this seems to suggest that the field effect is universal, i.e., it can affect either genuine or proximity fully-metallic superconductors.
Besides shedding light on a key issue in physics, these results represent a groundbreaking asset for the realization of an all-metallic superconducting field-effect electronics and leading edge quantum information architectures based on Josephson FETs. Possible electronic and circuital schemes based on this all-metallic technology will be furthermore discussed.
[1] G. De Simoni, F. Paolucci, P. Solinas, E. Strambini, and F. Giazotto, Nat. Nanotechnol.13, 802 (2018).
[2] F. Paolucci, G. De Simoni, E. Strambini, P. Solinas, and F. Giazotto, NanoLett. 18, 4195 (2018).
[3] F. Paolucci, G. De Simoni, P. Solinas, E. Strambini, N. Ligato, P. Virtanen, A. Braggio, and F. Giazotto, arXiv:1808.00353.


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