MICROWAVE PROPERTIES OF BOROCARBIDE SUPERCONDUCTORS RNI(2)B(2)C (R=Y,ER,TM,HO)

We report measurements of the microwave surface impedance of the boroc arbide family of superconductors RNi(2)B(2)C(R=Y,Er,Tm,Ho). The experi ments enable direct measurements of the superfluid density, and are pa rticularly sensitive to the influence of magnetic pair breaking. In Ho Ni2B2C the antiferromagnetic transition is clearly observed at zero fi eld, and leads to a drastic reduction of the superfluid density, which recovers at lower temperatures. In ErNi2B2C the antiferromagnetic tra nsition is not seen in zero-field data. Magnetic effects are responsib le for anomalies in the low-temperature surface impedance below approx imately 4 K in HoNi2B2C and TmNi2B2C. The temperature dependence of th e microwave impedance disagrees with BCS calculations.