Microwave properties of borocarbide superconductors R${\mathrm{Ni}}_2$${\mathrm{B}}_2$C (R=Y,Er,Tm,Ho)

We report measurements of the microwave surface impedance of the borocarbide family of superconductors R${\mathrm{Ni}}_2$${\mathrm{B}}_2$C (R=Y,Er,Tm,Ho). The experiments enable direct measurements of the superfluid density, and are particularly sensitive to the influence of magnetic pair breaking. In ${\mathrm{HoNi}}_2$${\mathrm{B}}_2$C 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 ${\mathrm{ErNi}}_2$${\mathrm{B}}_2$C the antiferromagnetic transition is not seen in zero-field data. Magnetic effects are responsible for anomalies in the low-temperature surface impedance below approximately 4 K in ${\mathrm{HoNi}}_2$${\mathrm{B}}_2$C and ${\mathrm{TmNi}}_2$${\mathrm{B}}_2$C. The temperature dependence of the microwave impedance disagrees with BCS calculations.