  The optical properties of the heavy-fermion compound UPd$_2$Al$_3$ have been
measured in the frequency range from 0.04 meV to 5 meV (0.3 to 40 cm$^{-1}$) at
temperatures $2 {\rm K}<T< 300$ K. Below the coherence temperature $T^*\approx
50$ K, the hybridization gap opens around 10 meV. As the temperature decreases
further ($T\leq 20$ K), a well pronounced pseudogap of approximately 0.2 meV
develops in the optical response; we relate this to the antiferromagnetic
ordering which occurs below $T_N\approx 14$ K. The frequency dependent mass and
scattering rate give evidence that the enhancement of the effective mass mainly
occurs below the energy which is associated to the magnetic correlations
between the itinerant and localized 5f electrons. In addition to this
correlation gap, we observe a narrow zero-frequency conductivity peak which at
2 K is less than 0.1 meV wide, and which contains only a fraction of the
delocalized carriers. The analysis of the spectral weight infers a loss of
kinetic energy associated with the superconducting transition.

Strongly Correlated Electrons