The two ways graphene sheets usually sit on one another in graphite. In Bernal, or AB, stacking every second layer lines up with the one two below it; in rhombohedral, or ABC, stacking each layer is shifted the same way as the one before, so the pattern repeats only every third layer. Natural graphite is mostly Bernal with some rhombohedral regions, and the difference matters: a few rhombohedral layers have very flat electron bands, and such have turned out to superconduct and to hold unusual kinds of magnetism.
As the site uses it
The number counts the layers in one repeat; the letter gives the symmetry of the whole stack – H for hexagonal, R for rhombohedral, T for trigonal.
Bernal stacking alternates two positions, rhombohedral stacking steps through three. In rhombohedral graphene the bands flatten with every added layer, which is why thin rhombohedral flakes show strongly interacting electrons without any twist.
Two ways to stack a honeycomb
Each graphene layer can sit in one of three positions relative to its neighbours, called A, B and C, each shifted from the next by one carbon–carbon bond. Neighbouring layers do not sit directly on top of one another, because that costs energy, so a crystal is built by stepping from one position to another. Bernal graphite steps back and forth, ABAB; rhombohedral graphite keeps stepping the same way, ABCABC.
The two cost almost the same energy, so both occur. Natural graphite is mostly Bernal with a minority of rhombohedral regions; grinding it raises the rhombohedral share, and heating converts it back.
Flat bands without a twist
In rhombohedral graphene the electrons at the lowest energies live on the two outermost layers, and in the simplest model their energy rises with the Nth power of their momentum, N being the number of layers. The bands therefore flatten as layers are added, the electrons slow down and their mutual repulsion takes over – as in , but without twisting anything. A voltage across the stack opens a gap and tunes how flat the bands are.
In 2021 rhombohedral trilayer graphene showed , next to metals in which the electrons all choose the same and . In 2024 five rhombohedral layers aligned with hBN showed the effect, with no magnetic field applied.
Finding it and keeping it
Exfoliated graphite flakes contain rhombohedral domains here and there. They are found by , where the takes a different shape, or by infrared imaging, and then cut out with an tip or a laser before they relax to Bernal stacking – which heat, and the forces of transfer can all set off. Unless someone has looked for rhombohedral domains on purpose, few-layer graphene devices are Bernal-stacked.
For specialists
The two common stacking sequences of graphene layers: Bernal (AB, hexagonal graphite) and rhombohedral (ABC), which differ in the lateral position of every third layer; follow the same rhombohedral sequence. In the simplest model the low-energy bands of N-layer ABC graphene disperse as the Nth power of momentum and sit on the two outer layers, so they flatten with thickness and a perpendicular opens a tunable gap; Bernal multilayers instead decompose into bilayer-like bands plus, for odd N, a monolayer-like one. Rhombohedral stacking is metastable and relaxes to Bernal under heat, strain or processing.