Strong reduction of quasiparticle scattering rate with gap formation in CeNiSn

We measured the temperature dependence of the surface impedance $Z_s{=R}_s{+iX}_s$ at 9.6 and 14 GHz in a single crystal of the Kondo lattice compound CeNiSn, from which microwave complex conductivity $\sigma \mathrm{}\left(T\right)={\sigma }_1\mathrm{}\left(T\right)\mathrm{}-i{\sigma }_2\mathrm{}\left(T\right)\mathrm{}$ was determined. Below about 10 K the imaginary part ${\sigma }_2$ increases rapidly and the real part ${\sigma }_1$ 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.