Spin Fluctuations in Frustrated Kagomé Lattice System Sr${\mathrm{Cr}}_8$${\mathrm{Ga}}_4$${\mathrm{O}}_{19}$ Studied by Muon Spin Relaxation

We have performed muon spin relaxation measurements of the frustrated kagomé lattice spin system Sr${\mathrm{Cr}}_8$ ${\mathrm{Ga}}_4$ ${\mathrm{O}}_{19}$. Our results demonstrate the slowing down of the Cr spin fluctuations when cooling toward the susceptibility-cusp temperature $T_g\sim 3.5$ K. The saturation of the relaxation rate below $T_g$, together with its weak dependence on longitudinal field (LF) between 0 and 2 kG, indicates the presence of dynamic spin fluctuations persisting even at $T\sim 100$ mK without static order parameter. We propose a spin-liquid type ground state to explain an undecouplable Gaussian shape of the relaxation function observed at $T\le T_g$.