Theoretical Study on Frenkel Excitons in Mott-Jahn-Teller Insulator A4C60

We study low-energy excitations in the Mott–Jahn–Teller insulator A 4 C 60 theoretically, where A is alkali metal. A model which takes account of both the electron–electron and electron–phonon interactions is employed and the low-energy excitations are calculated by using the Tamm–Dancoff approximation. It is found that the lowest excitation corresponds to the creation of the spin-singlet Frenkel excitons at about 0.3 eV and the next lowest excitation corresponds to the creation of the spin-triplet Frenkel excitons at about 0.6 eV. It is also found that the excitations of a pair of a free electron and a free hole lie over 0.8 eV. A remarkable point is that the spin-singlet Frenkel excitons are lower in energy than the spin-triplet Frenkel excitons in A 4 C 60 in contrast to usual insulators.