Superconducting energy gap and antiferromagnetic spin fluctuations in the superconductor ${\mathrm{YNi}}_2$${\mathrm{B}}_2$C: An NMR study

Pulsed NMR studies of ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ have been carried out in the recently discovered quaternary borocarbide superconductor ${\mathrm{YNi}}_2$${\mathrm{B}}_2$C (${\mathit{T}}_{\mathit{c}}$=15.5 K). These results are as follows: (a) Superconducting state: Though there is only one crystallographic B site in this structure, we have observed two ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ resonances in the superconducting state. One signal comes from a superconducting region, and the other from a normal-metal region. As for the former signal, the nuclear relaxation rate, ${\mathit{T}}_1^{\mathrm{-}1}$ decreases drastically with further decrease of temperature with no appreciable enhancement just below ${\mathit{T}}_{\mathit{c}}$, and it starts to saturate below 6 K. (b) Normal state: (${\mathit{T}}_1$T${)}^{\mathrm{-}1}$ increases as temperature decreases. We interpret this temperature dependence as arising due to two-dimensional or three-dimensional antiferromagnetic spin fluctuations (structure of ${\mathrm{YNi}}_2$${\mathrm{B}}_2$C is tetragonal and highly anisotropic).