Three-dimensional electronic instabilities in polymerized solidAC60

The low-temperature structure of A${\mathrm{C}}_{60}$ (A=K,Rb) is an ordered array of polymerized ${\mathrm{C}}_{60}$ chains, with magnetic properties that suggest a nonmetallic ground state. We study the paramagnetic state of this phase using first-principles electronic-structure methods, and examine the magnetic fluctuations around this state using a model Hamiltonian. The electronic and magnetic properties of even this polymerized phase remain strongly three dimensional, and the magnetic fluctuations favor an unusual three-dimensional antiferromagnetically ordered structure with a semimetallic electronic spectrum.