We present a ${}^{35}\mathrm{Cl}$ NMR study for the spin $S=1/2$ perfect kagome antiferromagnet Ca-kapellasite $[{\mathrm{CaCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2·0.6{\mathrm{H}}_2\mathrm{O}]$ with a magnetic transition at $T^{*}=7.2\mathrm{K}$. The static magnetic structure in the ground state has been determined to be a chirality-ordered $Q=0$ state, which is selected by a finite Dzyaloshinskii-Moriya interaction. The low-energy magnetic excitations in the ordered state are investigated by a nuclear spin-lattice relaxation rate measurement. We detect a weakly temperature-dependent contribution in the magnetic fluctuations perpendicular to the kagome plane in addition to the dispersive spin-wave contribution in the kagome plane. The low-energy magnetic excitations from the coplanar spin structure are attributed to the zero mode originating from the flat band in this kagome antiferromagnet.

• Received 17 September 2019
• Accepted 16 April 2020

DOI:https://doi.org/10.1103/PhysRevResearch.2.023269

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Published by the American Physical Society