Recommendable! A nice overview article!
"... Two twisted bilayer graphene (TBG) ...
Is it conventional superconductivity, such as that first identified in mercury more than a century ago, or is it unconventional superconductivity, such as that discovered in the 1980s in high-temperature superconductors? A new study by Julien Barrier of the University of Manchester in the UK and colleagues may help to provide a resolution to this conundrum ...
To help settle this debate, Barrier and colleagues stacked two TBGs, one with a superconductivity-inducing twist close to the magic angle, the other with a smaller (not magic) twist angle (Fig. 1). This second bilayer acted as a normal metal. By gating each bilayer separately, the researchers could induce different amounts of charge in each, allowing the metallic layer to screen electrons in the superconducting layer. To preserve their distinct electronic properties, the two TBGs had to be electronically decoupled so that their electronic states would not mix. In previous experiments, researchers achieved this decoupling using dielectric hexagonal boron nitride (h-BN) spacers, but these several-nanometer-thick layers weakened the screening effect. Instead, Barrier and colleagues introduced a 10° twist between the two TBGs. This ingenious trick decouples the two moiré patterns and suffices to keep their electronic states separate, avoiding the need for physical separation.
Engineering such screening is a good way to probe the pairing mechanism. In the Eliashberg theory of conventional superconductivity—a refinement of BCS theory—electrons experience an effective Coulomb repulsion μ * that must be overcome by an attraction mediated through the exchange of phonons. Increasing the screening between electrons lowers the repulsion μ* while leaving their phonon-mediated attraction unchanged.
If TBG is a conventional superconductor, this screening effect should make phonon-induced electron pairing easier and raise the material’s T c.
But this is exactly the opposite of what Barrier and colleagues observed. They found that increasing the screening between electrons in the superconducting TBG caused the superconducting phase to disappear.
After ruling out trivial explanations, the researchers concluded that this disappearance offers strong evidence that TBG’s superconductivity arises from some type of correlated electron motion that is, at most, very weakly dependent on phonon-mediated attraction. ...
To model the observed behavior, Barrier and colleagues assumed that TBG’s superconductivity arises from an unconventional plasmon-mediated mechanism. Their basic argument is that, for the electronic states involved in superconductivity, screening efficiency decays exponentially on a scale of ∼
2 nm. Because their device uses a large twist angle to decouple the two TBGs instead of a dielectric spacer, the interlayer separation is of subnanometer size, making screening maximally efficient, and much stronger than in earlier experiments with 3–10-nm h-BN spacer layers. Besides explaining the disappearance of the superconducting phase, this plasmon model could also explain an observed disappearance of correlated insulating phases at temperatures below 20 K. ..."
From the abstract:
"The origin of superconductivity in magic-angle twisted bilayer graphene has been a subject of intense debate. While some experimental evidence indicated an unconventional pairing mechanism which should be sensitive to Coulomb screening, experimental attempts to tune the critical temperature by screening Coulomb interactions so far have remained unsuccessful, possibly indicating a conventional phonon-mediated pairing.
Here we study a double-layer electronic system consisting of two twisted graphene bilayers in immediate proximity of each other but remaining electronically decoupled.
By increasing the carrier density in one bilayer, we completely suppressed both the superconductivity and the correlated-insulator state in the adjacent magic-angle graphene. The observation of such an effect from screening offers support for an unconventional mechanism of Cooper pairing in magic-angle twisted bilayer graphene, shedding new light on the underlying physics governing their properties."
Fig. 1 Characterization of twisted tetralayer graphene device
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