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Â鶹ÒùÔºicists figure out how an electric field can switch off superconductivity

Transistors are fundamental to microchips and modern electronics. Invented by Bardeen and Brattain in 1947, their development is one of the 20th century's key scientific milestones. Transistors work by controlling electric current using an electric field, which requires semiconductors. Unlike metals, semiconductors have fewer free electrons and an energy band gap that makes it harder to excite electrons.

Doping introduces charge carriers, enabling current flow under an electric field. This allows for nonlinear current-voltage behavior, making signal amplification or switching possible, as in p–n junctions. Metals, by contrast, have too many free electrons that quickly redistribute to cancel external fields, preventing controlled current flow—hence, they can't be used as traditional transistors.

However, recent advances show promise in ultrathin superconducting metals as potential transistor materials. When cooled below a critical temperature, these materials carry current with zero resistance. This behavior arises from the formation of Cooper pairs—electrons bound by lattice vibrations—that condense into a coherent quantum state, immune to scattering and energy loss.

Application of a sufficiently high static electric field onto the film surface has been repeatedly shown to be able to suppress the superconducting current. However, the microscopic mechanism by which this current-suppression works has remained a mystery.

I started investigating this fascinating problem a couple of years ago, by first studying how the quantum confinement along the thin direction affects the superconductivity, and separately how an electric field, even if it partially penetrates the film, can effectively rupture the Cooper pairs, thus suppressing the superconductivity. While this brought some new insights, the question about how large the electric field should be to suppress the superconductivity in a chosen material has remained unanswered.

Working together with my fellow theoretical physicist colleagues Giovanni Ummarino and Alessandro Braggio and with experimentalist Francesco Giazotto (the first from Turin Polytechnic, the former two both at the Italian CNR in Pisa), I have finally figured out how the whole mechanism works.

A key aspect is that the metallic thin film has to be thin enough that the penetration depth of the applied electric field is comparable to the film thickness or at least no more than an order of magnitude smaller. For example, widely used niobium nitride thin films have an (anomalously large) penetration depth of 4–5 nm, comparable to the film thickness of about 10 to 30 nm. This ensures that the exponentially decaying electric field is never identically zero inside the film.

The non-vanishing electric field, in turn, promotes quantum tunneling of an electron bound in a Cooper pair, via a process that can be quantified using standard quantum mechanics to estimate the characteristic magnitude of the electric field to break the Cooper pair.

This is done by leveraging the most sophisticated computational description of superconductivity, known as the Eliashberg theory, to precisely determine the value of the electric field needed to suppress the electronic supercurrent, by also accounting for the fact that the electric field is strongly screened inside the film.

In this way, my colleagues and I have been able, for the first time, to correctly quantify the magnitude of the electric field needed to suppress the superconductivity in thin films, and this value, of the order of one hundred million volts per meter, agrees well with the experimentally determined values.

Our study is in the journal Â鶹ÒùÔºical Review B. Thanks to this new understanding and computational predictions, a whole range of quantum gate materials can be developed and optimized in future applications to implement the next-generation superconducting microelectronics and quantum computing.

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