Anomalous Phonon Scattering Below ${\mathit{T}}_{\mathit{c}}$ in $Y{\mathrm{Ni}}_2{}_{}{}^{11}{}_{}{}^{}B_2^{}{}_{}{}^{}C$

Inelastic neutron scattering measurements were performed on large volume $\left(\sim 2{\mathrm{cm}}^3\right)$ single crystals of ${}^{11}\mathrm{B}$ substituted intermetallic superconductor ${\mathrm{YNi}}_2^{11}{\mathrm{B}}_2\mathrm{C}$. With decreasing temperature, a gradual softening of a phonon spectrum is observed at around $Q=(0.525,0,8)$, similar to the case of ${\mathrm{LuNi}}_2{\mathrm{B}}_2\mathrm{C}$ reported by Dervenagas et al. [Phys. Rev. B 52, R9839 (1995)]. In addition, we discovered that an intense new peak appears below $T_c=14.2\mathrm{K}$. With application of magnetic field, however, it is easily suppressed and disappears by ${H=H}_{c2\mathrm{}}$ ( $\sim 4.7\mathrm{T}$ at $T=5.5\mathrm{K}$). These results strongly indicate that this new peak is undoubtedly associated with its superconducting state.