Three-dimensional electronic instabilities in polymerized solid [ital A]C[sub 60]

The low-temperature structure of [ital A]C[sub 60] ([ital A]=K,Rb) is an ordered array of polymerized C[sub 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.