Structural, superconducting, and magnetic properties of ${\mathrm{YNi}}_2$${\mathrm{B}}_2$C and ${\mathrm{ErNi}}_2$${\mathrm{B}}_2$C

Discovery of a quaternary superconducting system Y-Ni-B-C has been reported recently. Our structural studies on ${\mathrm{YNi}}_2$ ${\mathrm{B}}_2$C (${\mathit{T}}_{\mathit{c}}$≊15.5 K) reveal large and anisotropic thermal vibrations of C atoms in the Y-C plane of the structure. No crystallographic phase transition is observed down to 50 K. Our specific-heat data suggest that ${\mathrm{YNi}}_2$ ${\mathrm{B}}_2$C is a strong-coupling superconductor. Results of our resistivity, magnetic, and specific-heat data clearly suggest coexistence of superconductivity and magnetism (of Er spins) in ${\mathrm{ErNi}}_2$ ${\mathrm{B}}_2$C (${\mathit{T}}_{\mathit{c}}$≊10.5 K) below ≊7 K.