Doping effects and spin correlations in C60: An unrestricted hartree-fock study

The p-bonded system of doped C60 is studied in the framework of the Hubbard model using the fully unrestricted Hartree-Fock (UHF) approximation, which allows noncollinear arrangements of local-spin polarizations. Ground-state properties of a single C60 molecule, such as density distribution, local-spin polarizations, and spin-spin correlation functions, are determined as a function of Coulomb repulsion strength U/t and for electron or hole dopings d close to half-band filling (δ≤3). For U>Uc(Uc/t=2.5–3.0) the competition between nearest-neighbor antiferromagnetic spin correlations and frustrations on pentagonal loops leads to remarkable noncollinear spin arrangements, which depend sensitively on δ and U/t. For a single extra particle (electron or hole) and U>Uc, the charge imbalance tends to concentrate with increasing U/t along one bond connecting two pentagons. At these atoms the antiparallel spin correlations are considerably weakened. Two extra particles tend to localize at opposite poles of the C60 sphere. Doping-induced changes in the noncollinear spin arrangements are analyzed. Goals and limitations of the UHF calculations are discussed by comparing them with exact numerical results in the case of a cagelike 12-atom cluster. © 1999 The American Physical Society.

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