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 show a specific heat at low temperatures that is significantly higher than predicted by the free electron model. This behavior is linked to the partially filled f-orbitals of rare-earth or actinide ions, which function as localized magnetic moments. The term "heavy fermion" signifies that these particles have an effective mass greater than their rest mass.