Heavy fermion charge fluctuations

In the limit of weak hybridization of the 4f and conduction electrons, the charge fluctuations are strongly suppressed and there remain only spin fluctuations. In this Kondo limit the f-electron level width is small compared to the (negative) f-electron energy Sf as well as small compared to the Coulomb energy U. At low temperatures these systems exhibit an unusually high electronic specific heat coefficient y or a large effective mass m and are therefore called heavy-fermion systems. 

---