Rhombohedral multilayer graphene: anomalous Hall states and superconductivity

6 problems

• Is rhombohedral graphene a chiral topological superconductor

  Open

  Tetralayer and pentalayer graphene superconduct out of a state in which electron spins and valleys (the two inequivalent momentum regions where graphene's low-energy electrons are found) are aligned, and the superconductor shows magnetic hysteresis, suggesting pairs that rotate in one direction (chiral pairing).

• Nature of the extended quantum anomalous Hall state

  Contested

  At the lowest temperatures pentalayer graphene shows an integer quantized Hall resistance over a wide range of electron densities, replacing the fractional states, and the electronic structure behind it is unknown.

• Precision quantization of zero-field fractional Hall states in graphene

  Partially resolved

  In a true fractional quantum Hall state the sideways resistance equals an exact fraction of a natural constant and the forward resistance vanishes, and early graphene data fell short of both.

• Is the moire potential needed for fractional anomalous Hall states

  Contested

  Fractional zero-field Hall states were found in pentalayer graphene aligned with boron nitride, but the electrons are on the layer far from the moire interface, so it is unclear whether the moire pattern matters.

• Superconductivity from repulsion in a single-flavor graphene band

  Open

  In a band where all electrons share one spin and one valley (one of graphene's two momentum regions for low-energy electrons), pairs must have odd symmetry, and whether the electrons' own repulsion can produce such pairing at the observed temperature is unsettled.

• Phonon or electronic pairing in rhombohedral trilayer and Bernal bilayer graphene

  Contested

  Graphene with three layers in staircase stacking, and ordinary two-layer graphene, superconduct at very low electron densities without any moire pattern, and it is disputed whether lattice vibrations or electron interactions cause the pairing.