Dielectric Fabry-Perot interferometer for temperature dependent far-infrared reflectivity measurements of heavy-fermion metals in high magnetic fields

A far-infrared reflectivity set up has been developed for spectroscopy on highly reflective materials in a 20 T-class resistive Bitter magnet. As a first application, far-infrared reflectivity measurements on the heavy-fermion compound URu2Si2 have been performed using a silicon reflection Fabry-Perot interferometer as a multiple reflection device. In a resonance, this Fabry-Perot technique is one order of magnitude more sensitive than a single reflection measurement. Changes in the reflectivity as a function of magnetic field are resolved with an accuracy of 0.2% and the absolute value of the reflectivity can be obtained with an accuracy of 0.5%. With this interferometer, an excitation at about 40 cm(-1) in the heavy-fermion system URu2Si2 is investigated at temperatures between 2 and 20 Kelvin and in magnetic fields up to 20 T. The excitation appears to extend to lower energies under influence of a large magnetic field. (C) 1996 American Institute of Physics.