Optical Properties of UBe_13 in the Normal and Superconducting States
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M. Reedyk, A.A Brown Holden, and G.M. Wardlaw
Dept. Physics, Brock University, St. Catharines, ON, L2S 3A1, Canada

J. L. Smith
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA


The far-infrared reflectance of heavy-fermion superconductor UBe_13 has 
been measured as a function of temperature.   For the first time 
measurements are carried out below the superconducting transition temperature.  
At low temperatures the coherent state is characterized by an optical 
conductivity that exhibits a renormalized Drude peak at low frequencies 
superimposed on a broad background giving rise to a minimum near 75 cm^-1.  
Invoking an extended Drude analysis gives rise to  the emergence of a peak in 
the frequency-dependent scattering rate which, in analogy with other 
heavy-fermion materials, can be associated with the characteristic energy 
separating coherent behaviour from  single impurity scattering.    Both the 
energy of the peak and the temperature at which the behaviour manifests itself 
are  found  to correspond to a temperature of ~45 K although the resistivity 
of UBe_13 continues to increase with decreasing temperature down to ~2.5 K.  
The superconducting state is characterized by a noticeable decrease in spectral 
weight over a wide frequency range and a scattering rate which is depressed 
at low frequencies, suggesting that the formation of the condensate alters 
dramatically the interaction between the heavy electrons.

Work at Brock was supported by the Natural Sciences and Engineering Research
Council of Canada and that at Los Alamos under the auspices of the U.S. Dept. 
of Energy.