Far infrared reflectivity of narrow band materials

The application of a Fourier spectrometer to the study of the electronic structure in the far infrared of some intermediate valence or heavy fermion materials or of highT_c superconductors showed that FT-reflectivity is a powerful tool of investigation at very low energies. A very careful control of the experimental conditions and of the reference measurements is necessary to eliminate spurious reflectivities.The results show optical transitions at very low energy and permit to evaluate the electronic structures and the effective mass of the carriers in the vicinity of the Fermi level.