Superconductivity in magic-angle twisted graphene multilayers

5 problems

• Nodal or fully gapped superconducting order in magic-angle graphene

  Open

  The energy needed to break an electron pair may vanish along certain directions (nodes) or stay finite everywhere, and which case holds narrows the possible pairing mechanisms.

• Does magic-angle superconductivity require the neighbouring correlated insulator?

  Partially resolved

  Superconductivity in magic-angle graphene appears next to insulating states, as in cuprates, but devices in which a nearby metal gate screens the electron repulsion lose the insulators and keep the superconductivity, so the two may have different causes.

• Carbon isotope effect on the magic-angle superconducting transition temperature

  Open

  Replacing carbon-12 by heavier carbon-13 slows the lattice vibrations, and the resulting shift of the superconducting transition temperature, compared with what each pairing theory predicts, would test whether phonons bind the pairs.

• What binds electron pairs in magic-angle graphene superconductors?

  Contested

  Superconductivity needs electrons to pair up despite repelling each other, and in magic-angle graphene it is unknown whether lattice vibrations (phonons) or the electrons' own interactions supply the attraction.

• Spin state of Cooper pairs in magic-angle twisted trilayer graphene

  Open

  In magic-angle trilayer graphene, superconductivity survives in-plane magnetic fields two to three times larger than the field that breaks pairs of electrons with opposite spins, suggesting pairs with parallel spins (spin triplet).