Strong reduction of quasiparticle scattering rate with gap formation in CeNiSn

We measured the temperature dependence of the surface impedance Z{sub s}=R{sub s}+iX{sub s} at 9.6 and 14 GHz in a single crystal of the Kondo lattice compound CeNiSn, from which microwave complex conductivity {sigma}(T)={sigma}{sub 1}(T){minus}i{sigma}{sub 2}(T) was determined. Below about 10 K the imaginary part {sigma}{sub 2} increases rapidly and the real part {sigma}{sub 1} is much lower than the dc conductivity. These results indicate a rapid decrease in the scattering rate of the quasiparticles, whose excitation spectrum is drastically changed with the coherence gap formation at low temperatures. {copyright} {ital 1997} {ital The American Physical Society}