STRONG REDUCTION OF QUASI-PARTICLE SCATTERING RATE WITH GAP FORMATIONIN 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 lattic e compound CeNiSn, from which microwave complex conductivity sigma(T) = sigma(1)(T) - i sigma(2)(T) was determined. Below about 10 K the ima ginary part sigma(2) increases rapidly and the real part sigma(1) is m uch lower than the de conductivity. These results indicate a rapid dec rease in the scattering rate of the quasiparticles, whose excitation s pectrum is drastically changed with the coherence gap formation at low temperatures.