Gapless magnetic excitations in the kagome antiferromagnet Ca-kapellasite probed by ${}^{35}\mathrm{Cl}$ NMR spectroscopy

The low-energy magnetic excitations of the spin-$1/2$ kagome antiferromagnet ${\mathrm{CaCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2·0.6{\mathrm{H}}_2\mathrm{O}$ (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.

Figure 1

${}^{35}\mathrm{Cl}$ NMR spectrum of the powder sample obtained at 100 K. The insets show the close view near (a) the central peak and (b) the satellite peak. The double-horn structure for the central peak and the broad satellites are consistently explained by the simulation, which is shown as the thin red line. From the fitting we found that the anisotropy of the Knight shift is small.

Figure 2

The $T$ dependence of the Knight shift determined by the peak positions of the center line. The $T$ dependence of $K$ (triangles) scales to that of $\chi$ (solid line) with a negative coefficient. The inset shows the $K\text{−}\chi$ plot. From the linear relationship we determined the hyperfine coupling constant $A_{\mathrm{hf}}$.

Figure 3

Temperature variation of the central peak near $T_M$. The double-horn spectrum at high $T$ becomes a single broad peak at 3 K. The horizontal arrow represents the internal field sensed by the ${}^{35}\mathrm{Cl}$ nuclei when $0.1{\mu }_B$ of Cu spins are ordered. The inset shows the $T$ dependence of $\sqrt{M_2}$ (left scale) and the intensity multiplied by $T$ (right scale). At high $T$, $\sqrt{M_2}$ originates from the electric quadrupole effect on the powder spectrum. The onset $T$ for the spectrum broadening is higher than $T_M$.

Figure 4

Temperature dependence of $1/T_1$ measured at 2.0, 5.6, and 18.9 T. At high $T$, $1/T_1$ is independent of $T$ and fields. We observed two peaks at $T^{*}=20$ K, and $T_M=7.2$ K. The peak at $T^{*}$ is ascribed to the short-range spin dynamics. Below $T_M$, a power-law $T$ dependence was observed. This behavior is fitted to a two-term power function $A(BT+T^3)$ (see text), and the results for each field are shown as dashed lines. The inset is the field dependence of the coefficient $A$. The extrapolation to high fields (dotted line) suggests that the low-energy spin excitations are completely suppressed above 22 T.