Spatially Indirect Exciton Condensates in Double Bilayer Graphene
Jung-Jung Su1*
1電物系, 國立交通大學, 新竹, Taiwan
* presenting author:Jung-Jung Su,
Many-body interaction effects have a strong influence on the low-energy electronic properties of graphene bilayers because of the nearly quadratic dispersion at the K/K’ band-crossing points. In the single graphene bilayer systems, the ground state has an energy gap thought to be a consequence of spin-density wave order and other competing ordered states are believed to be nearby in energy. In systems with two closely spacing bilayer, spatially indirect exciton states are expected in neutral systems with a small amount of inter-bilayer charge transfer. This transfer can be achieved by applying either a vertical electrical displacement fields or an interbilayer potential bias, or both. In this talk we report that the different combinations of displacement field and potential bias can give rise to different types of indirect exciton condensate states that are distinguished by the two-dimensional momentum dependence of the associated spontaneous inter-bilayer coherence. In general a displacement field prefers an excitonic condensate in which the phase coherence between the inner two layers of the four layer
system, while the potential bias prefers momentum-independent coherence between remote layers. A complete phase diagram of many-body ordered states is mapped out in detail using mean-field-theory and is predicted to exhibit the two types of excitonic coherence mentioned above, and even more interestingly, various mixtures of the two.

Keywords: graphene bilayer, exciton