▎ 摘　　要

We study a triangular graphene quantum dot with two electrons filled in the degenerate shell. The electronic structures of the dot are calculated as a function of an electric field using a combination of tight-binding, Hartree-Fock and exact-diagonalization methods. To use the total spin (-S) of the dot as a qubit, we pay attention to the energy difference between the lowest S = 0 and S = 1 states. The energy difference increases unexpectedly up to a rather large value before a crossing between the lowest S = 0 and S = 1 states occurs. To understand the unexpected change, we propose an approach to analyze the total interaction energy of the two-electron states. As the electric field changes, the total interaction energies of the lowest S = 0 and S = 1 states exhibit three characteristics in turn: oscillation, transition and stability. Our findings support the possibility to use a triangular graphene quantum dot as a qubit of a carbon-based quantum computer.