Gapless magnetic excitations in the kagome antiferromagnet Ca-kapellasite probed by Cl-35 NMR spectroscopy

The low-energy magnetic excitations of the spin-$1/2$ kagome antiferromagnet ${\mathrm{CaCu}}_{3}{(\mathrm{OH})}_{6}{\mathrm{Cl}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}0.6{\mathrm{H}}_{2}\mathrm{O}$ (Ca-kapellasite) have been investigated by a $^{35}\mathrm{Cl}$ NMR experiment in fields up to 18.9 T. Recently, Ca-kapellasite was found to be one of the most promising candidates to explore the intrinsic magnetic properties in kagome antiferromagnets, because magnetic defects, which deteriorate the intrinsic magnetic properties, are minimized by choosing Ca ions with a large ionic radius as the counterions. From our nuclear spin-lattice relaxation rate $1/{T}_{1}$ measurement, we found a power-law temperature dependence below the magnetic ordering temperature ${T}_{M}=7.2$ K, which leads us to suggest that gapless magnetic excitations survive even in the ordered state. This power-law behavior is suppressed in high magnetic fields. We discuss a possible magnetic state that can generate gapless excitations at low fields.