Abstract
Density functional theory has been used to investigate the behavior of the electrons in bilayer graphene and graphite under compression along the axis. We have studied both conventional Bernal (A-B) and A-A stackings of the graphene layers. In bilayer graphene, only about 0.5% of the -electron density is squeezed through the network for a compression of 20%, regardless of the stacking order. However, this has a major effect, resulting in bilayer graphene being about six times softer than graphite along the axis. Under compression along the axis, the heavily deformed electron orbitals (mainly those of the electrons) increase the interlayer interaction between the graphene layers as expected, but, surprisingly, to a similar extent for A-A and Bernal stackings. On the other hand, this compression shifts the in-plane phonon frequencies of A-A stacked graphene layers significantly and very differently from the Bernal stacked layers. We attribute these results to some electrons in A-A stacking escaping the graphene plane and filling lower charge-density regions when under compression, hence, resulting in a nonmonotonic change in the -bond stiffness.
- Received 9 October 2019
- Revised 13 February 2020
- Accepted 2 March 2020
DOI:https://doi.org/10.1103/PhysRevB.101.125421
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