Anomalous phonon peak in the superconducting state of ${\mathrm{ErNi}}_2^{11}{\mathrm{B}}_2\mathrm{C}$

An anomalous phonon peak that appears only in the superconducting state has been reported for nonmagnetic intermetallic superconductor $R{\mathrm{Ni}}_2^{11}{\mathrm{B}}_2\mathrm{C}$ $(R=\mathrm{Y},$ Lu). In the present paper, we shall demonstrate that another rare-earth borocarbide ${\mathrm{ErNi}}_2^{11}{\mathrm{B}}_2\mathrm{C}$ seems to show similar phonon anomalies. A spectral weight transfer from the above-lying TA phonon mode to the aforementioned peak was rather small, but it is consistent with the theoretical prediction for the system with smaller $2\Delta {/E}_{\mathrm{ph}}$ ratio. Combining the present results with those observed in the other borocarbide systems, we suggest that such a phonon anomaly might be a common phenomenon for superconductors with a strong nesting feature of the Fermi surface.