Heavy fermion materials have a low-temperature specific heat whose linear term is up to 1000 times larger than the value expected from the free electron model. The properties of the heavy fermion compounds often derive from the partly filled f-orbitals of rare-earth or actinide ions, which behave like localized magnetic moments. The name "heavy fermion" comes from the fact that the fermion behaves as if it has an effective mass greater than its rest mass.
Heavy fermion materials exhibit a low-temperature specific heat whose linear term is up to 1000 times greater than what would be predicted by the free electron model. The characteristics of heavy fermion compounds frequently arise from partially filled f-orbitals of rare-earth or actinide ions, which function as localized magnetic moments. The term "heavy fermion" stems from the fact that the fermions exhibit an effective mass exceeding their rest mass.