Doping effects and spin correlations in C-60: An unrestricted Hartree-Fockstudy

The pi-bonded system of doped C-60 is studied in the framework of the Hubba rd model using the fully unrestricted Hartree-Fock (UHF) approximation, whi ch allows noncollinear arrangements of local-spin polarizations. Ground-sta te properties of a single C-60 molecule, such as density distribution, loca l-spin polarizations, and spin-spin correlation functions, are determined a s a function of Coulomb repulsion strength U/t and for electron or hole dop ings delta close to half-band filling (less than or equal to 3). For U>U-c (U-c/t=2.5-3.0) the competition between nearest-neighbor antiferromagnetic spin correlations and frustrations on pentagonal loops lends to remarkable noncollinear spin arrangements, which depend sensitively on delta and U/t. For a single extra particle (electron or hole) and U>U-c, the charge imbala nce tends to concentrate with increasing Lilt along one bond connecting two pentagons. At these atoms the antiparallel spin correlations are considera bly weakened. Two extra particles tend to localize at opposite poles of the C-60 sphere. Doping-induced changes in the noncollinear spin arrangements are analyzed. Goals and limitations of the UHF calculations are discussed b y comparing them with exact numerical results in the case of a cagelike 12- atom cluster.