Abstract
We consider a model of a quantum dot (QD) in a semiconducting carbon nanotube, created by using a spatially limited electric potential, produced by a system of a few external elementary charges. Our main interest is in correlated states of a pair of electrons within the dot described in terms of the π-electron Hamiltonian supplemented with electron Coulomb repulsion (the extended Hubbard Hamiltonian). We determine the value of the potential field necessary to keep two extra electrons within the dot and determine the spin symmetry of the two-electron wavefunction. It has been found that the ground state of the two electrons localized in the QD is a triplet for moderate values of the potential and it changes to the singlet state for strong enough potential. Additionally, ab initio (SIESTA) computations have been performed for the doubly charged nanotube with a QD due to the presence of two positive ions placed nearby. Remarkably, both the approaches yield consistent results concerning the robustness of the two-electron states within the QD.
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