THE PHYSICS OF HEAVY FERMIONS WITH A HYBRIDIZATION GAP

Optical measurements in the far infrared performed at low temperatures on heavy fermion compounds have given surprising results and important information about the electronic structure, complementary to the one obtained by the other techniques (e.g. specific heat, magnetization). The existence of a narrow gap a few meV below (above) the Fermi level has been observed as a common feature of heavy fermions, together with the evidence of a coherent hybridization of f- and conduction-electrons. The implications of the hybridization gap on other physical properties such as the temperature dependence of the resistivity, the specific heat and the susceptibility will be discussed. Low plasma frequencies, due to heavy effective masses, have been measured. The effect of alloying has also been studied and discussed.