Emergence of Frustrated Short-Range Order above Long-Range Order in the $S=1\mathrm{/}2$ Kagome Antiferromagnet CaCu${}_3$(OD)${}_6$Cl${}_2\cdot 0.6$D${}_2$O

We report on the low-energy dynamics in the kagome antiferromagnet CaCu${}_3$(OD)${}_6$Cl${}_2\cdot 0.6$D${}_2$O (Ca-kapellasite) as studied by use of ${}^2$D-NMR measurements. Previous ${}^{35}$Cl-NMR measurements revealed that the nuclear spin-lattice relaxation rate ($1\mathrm{/}{T}_1$) shows two peaks at temperatures, $T^{\ast }=7.2$ K and $T_s\simeq 25$ K. While the low-temperature peak at $T^{\ast }$ is ascribed to the critical fluctuations near the long-range magnetic ordering, the origin of the high-temperature peak has not been fully understood. From the $1\mathrm{/}{T}_1$ measurements on the D sites at the OD groups (D${}_{\mathrm{O}\mathrm{D}}$), we find no peak at $T_s$, evidencing that the high-temperature peak is not related to the molecular dynamics of the OD groups. We discuss the possibility of a frustration-induced short-range ordered state below $T_s$ before the long-range order is stabilized by the Dzyaloshinskii-Moriya interaction. We also observed static internal fields at the D${}_{\mathrm{O}\mathrm{D}}$ site in the long-range ordered state below $T^{\ast }$, and confirm the previously proposed negative-chirality $q=0$ magnetic structure.