Manchester team switches off graphene superconductivity by screening its electrons

The discriminating power here is in the shape of the response rather than the fact of an off switch. As carrier density in the neighbouring bilayer rose, superconductivity weakened step by step, and only past a high enough density did it disappear ^[10]. A dose-response curve is much harder to explain away than a single on/off result: it says the dial the experimenters turned is genuinely connected to the thing they were watching.

That dial is the whole point of the device, which is published in Physical Review X ^[5]. Two twisted bilayers sit less than a nanometre apart and stay electronically decoupled, so one screens Coulomb interactions in the other without sharing carriers with it ^[6]. Julien Barrier, the lead author, described two problems they had to solve: getting the screening layer a fraction of a nanometre from the superconducting graphene while keeping it electronically separate, and making that layer tunable, which they did with a twisted bilayer in atomic contact ^[7]. The team reports the effect was substantially stronger than in earlier screening experiments, and attributes that to the unusually small separation ^[13]. Alexey Berdyugin of the National University of Singapore, a corresponding author, said the complete suppression surprised them, and reads it as clear evidence that superconductivity in this system comes from strong electron-electron interactions ^[8].

There are limits on that reading. The correlated insulating states vanished under the same conditions ^[11]. Screening removed the correlated phases together with the pairing, so what the measurement bounds is the interaction-driven physics as a whole rather than the pairing glue in isolation. The suppression of the critical temperature is also quoted as a factor, more than an order of magnitude ^[12], which puts the residual transition below a tenth of its unscreened value, better than 90 percent of it gone ^[1]. That ratio is not an absolute temperature. The only temperature marker offered is Sir Andre Geim's, and it is qualitative: cold enough that helium itself turns liquid ^[9].

What the evidence earns is a claim of necessity. In this device, weaken the Coulomb interaction and superconductivity does not survive ^[1], which the authors present as helping settle a question left open since superconductivity was first found in the material, where competing accounts put the pairing either on the electrons themselves or on vibrations of the atomic lattice ^{[4] [2]}. The phonon contribution goes unmeasured here, which squeezes the lattice-driven account rather than excluding it. Geim calls the study a tiny step towards nailing down the mechanism, and adds that Rome was not built in a day ^[9]. For an argument that has run since researchers began twisting two carbon sheets 1.1 degrees out of register ^[3], the electronic side has just acquired the better piece of evidence, and the burden of explanation has moved with it.