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 borocarbide family of superconductors RNi(2)B(2)C(R=Y,Er,Tm,Ho). The experiments enable direct measurements of the superfluid density, and are particularly sensitive to the influence of magnetic pair breaking. In HoNi2B2C the antiferromagnetic transition is clearly observed at zero field, and leads to a drastic reduction of the superfluid density, which recovers at lower temperatures. In ErNi2B2C the antiferromagnetic transition is not seen in zero-field data. Magnetic effects are responsible for anomalies in the low-temperature surface impedance below approximately 4 K in HoNi2B2C and TmNi2B2C. The temperature dependence of the microwave impedance disagrees with BCS calculations.