Pseudogap in the Optical Spectra of UPd_2Al_3

Abstract

The in-plane optical conductivity of UPd_2Al_3 was measured at temperatures $2 {\rm K}<T<300$ K in the spectral range from 1 cm^{-1} to 40 cm^{-1} (0.14 meV to 5 meV). As the temperature decreases below 25 K a well pronounced pseudogap of 0.2 meV develops in the optical response. In addition we observe a narrow conductivity peak at zero frequency which at 2 K is less than 1 cm^{-1} wide but which contains only a fraction of the delocalized carriers. The gap in the electronic excitations might be an inherent feature of the heavy fermioin ground state.