Doping effects and spin correlations inC60:An unrestricted Hartree-Fock study

Abstract

The \ensuremath{\pi}-bonded system of doped ${\mathrm{C}}_{60}$ 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 ${\mathrm{C}}_{60}$ 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 \ensuremath{\delta} close to half-band filling (|\ensuremath{\delta}|\ensuremath{\leqslant}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 leads to remarkable noncollinear spin arrangements, which depend sensitively on \ensuremath{\delta} and $U/t.$ For a single extra particle (electron or hole) and $U>{U}_{c},$ 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 ${\mathrm{C}}_{60}$ 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.