Lattice Instabilities in Heavy Fermion Superconductors

Abstract

Heavy Fermion superconductors are a subclass of the socalled Heavy Fermion (HF) systems, a class of intermetallic compounds with rare earth or actinide component characterized by an enormous value of the electronic specific heat coefficient γ(T) = C (T)/T in the Helium temperature range and below, where it can become larger than 1 J/mole K2 1. A somewhat arbitrary limit of γ > 0.4J/mole K2 distinguishes these systems1 from a much larger number of compounds with similar constituents whose specific heat is also still very large compared with ordinary metals like copper (γ = 0.65 mJ/mole K2). For the following we shall call such compounds with say 0. 4 J/mole K2 > γ > 0.04 J/mole K2 Bantam Fermion (BF) systems. The large γ is thought to be a measure of the inverse width wf of an extremely narrow band of f states, which intersects the Fermi energy at T = 0. Interest in HF systems became intense after superconductivity was discovered in some of them2,3,4, because contrary to all previous experience, according to which narrow f states at the Fermi level strongly suppress superconductivity, in the HF superconductors they seemed to cause it.