Atomic orbitals generally exhibit crystal field effect and orbital hybridization arising from the anisotropic environment in matter. Artificial atom, quantum dot with discrete energy levels, offers a unique platform for simulating the electronic features of real atoms. Very recently, the orbital hybridization was proposed and realized in artificial atoms, achieving the reorganization of orbitals of different types. However, the crystal field effect in artificial atoms—namely, the reorganization among orbitals of the same type—has so far remained unexplored. Here, we develop the theory of crystal field effect, based on graphene artificial atoms. We find that in monolayer graphene, the pseudo-time-reversal symmetry prohibits the crystal field effect. But such symmetry is broken in Bernal bilayer graphene, allowing the crystal field effect. Specifically, we propose that the elliptical anisotropy leads to reorganization between a pair of $p$ orbitals. Our numerical calculations and analytical derivations demonstrate the corresponding energy splitting and spatial distributions, confirming the crystal field effect. Our study marks a critical step towards designing quantum matter.

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• Atomic orbital
• Graphene
• Quantum dots
• Two-dimensional electron system
• Crystal-field theory
• Tight-binding model