Local evidence for collective spin excitations in the distorted kagome antiferromagnet Pr 3 BWO 9

We report a local probe investigation of the frustrated antiferromagnet ${\mathrm{Pr}}_{3}{\mathrm{BWO}}_{9}$ with a distorted kagome lattice. The absence of magnetic order or spin-freezing is indicated by a spectral analysis down to 0.3 K and by specific heat measurements down to 0.09 K. The Knight shifts show an upturn behavior with the sample cooling down, which is further suppressed by the external field. For the spin dynamics, gapped spin excitation is observed from the temperature dependence of spin-lattice relaxation rates, with the gap size proportional to the applied magnetic field intensity. Comparatively, an unexpected sharp peak is observed in the nuclear spin-spin relaxation rate data at ${T}^{*}\ensuremath{\sim}4\text{--}5$ K. These results indicate an unconventional persistent fluctuating paramagnetic ground state with antiferromagnetic collective spin excitations in the strongly frustrated spin system.