Skutterudite results shed light on heavy fermion physics

Only few selected examples among the great diversity of anomalous rare earth skutterudite are reviewed. Focus is first given on PrFe₄P ₁₂ in comparison with URu₂Si₂. For PrFe ₄P₁₂, great progress has been made on determining the nature of the order parameter (OP). A non magnetic order parameter with a multipolar component emerges here while for URu₂Si₂ the nature of the so-called hidden order remains mysterious. The two systems have several similarities in their temperature-pressure (T, P) and magnetic field-temperature (H, T) phase diagrams, in their spin dynamics, in their nesting character and in their high sensitivity to impurities. Advances on one side must stimulate new views on the other. Besides general considerations on the choice of the OP, a simple basic problem is the treatment of the Kondo coupling in a system with low charge carrier number for the cases of uncompensated and compensated semi-metal. An interesting problem is also the possible decoupling between exciton modes and itinerant carriers.